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<body>
<header><h1>Zig Language Reference</h1></header>
<div id="main-wrapper">
<div id="navigation">
<nav aria-labelledby="zig-version">
<h2 id="zig-version">Zig Version</h2>
<a href="https://ziglang.org/documentation/0.1.1/">0.1.1</a> |
<a href="https://ziglang.org/documentation/0.2.0/">0.2.0</a> |
<a href="https://ziglang.org/documentation/0.3.0/">0.3.0</a> |
<a href="https://ziglang.org/documentation/0.4.0/">0.4.0</a> |
<a href="https://ziglang.org/documentation/0.5.0/">0.5.0</a> |
<a href="https://ziglang.org/documentation/0.6.0/">0.6.0</a> |
<a href="https://ziglang.org/documentation/0.7.1/">0.7.1</a> |
<a href="https://ziglang.org/documentation/0.8.1/">0.8.1</a> |
<a href="https://ziglang.org/documentation/0.9.1/">0.9.1</a> |
<a href="https://ziglang.org/documentation/0.10.1/">0.10.1</a> |
<a href="https://ziglang.org/documentation/0.11.0/">0.11.0</a> |
master
</nav>
<nav aria-labelledby="table-of-contents">
<h2 id="table-of-contents">Table of Contents</h2>
{#nav#}
</nav>
</div>
<div id="contents-wrapper"><main id="contents">
{#header_open|Introduction#}
<p>
<a href="https://ziglang.org">Zig</a> is a general-purpose programming language and toolchain for maintaining
<strong>robust</strong>, <strong>optimal</strong>, and <strong>reusable</strong> software.
</p>
<dl>
<dt>Robust</dt><dd>Behavior is correct even for edge cases such as out of memory.</dd>
<dt>Optimal</dt><dd>Write programs the best way they can behave and perform.</dd>
<dt>Reusable</dt><dd>The same code works in many environments which have different
constraints.</dd>
<dt>Maintainable</dt><dd>Precisely communicate intent to the compiler and
other programmers. The language imposes a low overhead to reading code and is
resilient to changing requirements and environments.</dd>
</dl>
<p>
Often the most efficient way to learn something new is to see examples, so
this documentation shows how to use each of Zig's features. It is
all on one page so you can search with your browser's search tool.
</p>
<p>
The code samples in this document are compiled and tested as part of the main test suite of Zig.
</p>
<p>
This HTML document depends on no external files, so you can use it offline.
</p>
{#header_close#}
{#header_open|Zig Standard Library#}
<p>
The <a href="https://ziglang.org/documentation/master/std/">Zig Standard Library</a> has its own documentation.
</p>
<p>
Zig's Standard Library contains commonly used algorithms, data structures, and definitions to help you build programs or libraries.
You will see many examples of Zig's Standard Library used in this documentation. To learn more about the Zig Standard Library,
visit the link above.
</p>
{#header_close#}
{#header_open|Hello World#}
{#code_begin|exe|hello#}
const std = @import("std");
pub fn main() !void {
const stdout = std.io.getStdOut().writer();
try stdout.print("Hello, {s}!\n", .{"world"});
}
{#code_end#}
<p>
Most of the time, it is more appropriate to write to stderr rather than stdout, and
whether or not the message is successfully written to the stream is irrelevant.
For this common case, there is a simpler API:
</p>
{#code_begin|exe|hello_again#}
const std = @import("std");
pub fn main() void {
std.debug.print("Hello, world!\n", .{});
}
{#code_end#}
<p>
In this case, the {#syntax#}!{#endsyntax#} may be omitted from the return
type because no errors are returned from the function.
</p>
{#see_also|Values|@import|Errors|Root Source File|Source Encoding#}
{#header_close#}
{#header_open|Comments#}
<p>
Zig supports 3 types of comments. Normal comments are ignored, but doc comments
and top-level doc comments are used by the compiler to generate the package documentation.
</p>
<p>
The generated documentation is still experimental, and can be produced with:
</p>
{#shell_samp#}zig test -femit-docs main.zig{#end_shell_samp#}
{#code_begin|exe|comments#}
const print = @import("std").debug.print;
pub fn main() void {
// Comments in Zig start with "//" and end at the next LF byte (end of line).
// The line below is a comment and won't be executed.
//print("Hello?", .{});
print("Hello, world!\n", .{}); // another comment
}
{#code_end#}
<p>
There are no multiline comments in Zig (e.g. like <code class="c">/* */</code>
comments in C). This allows Zig to have the property that each line
of code can be tokenized out of context.
</p>
{#header_open|Doc Comments#}
<p>
A doc comment is one that begins with exactly three slashes (i.e.
{#syntax#}///{#endsyntax#} but not {#syntax#}////{#endsyntax#});
multiple doc comments in a row are merged together to form a multiline
doc comment. The doc comment documents whatever immediately follows it.
</p>
{#code_begin|syntax|doc_comments#}
/// A structure for storing a timestamp, with nanosecond precision (this is a
/// multiline doc comment).
const Timestamp = struct {
/// The number of seconds since the epoch (this is also a doc comment).
seconds: i64, // signed so we can represent pre-1970 (not a doc comment)
/// The number of nanoseconds past the second (doc comment again).
nanos: u32,
/// Returns a `Timestamp` struct representing the Unix epoch; that is, the
/// moment of 1970 Jan 1 00:00:00 UTC (this is a doc comment too).
pub fn unixEpoch() Timestamp {
return Timestamp{
.seconds = 0,
.nanos = 0,
};
}
};
{#code_end#}
<p>
Doc comments are only allowed in certain places; it is a compile error to
have a doc comment in an unexpected place, such as in the middle of an expression,
or just before a non-doc comment.
</p>
{#code_begin|obj_err|invalid_doc-comment|expected type expression, found 'a document comment'#}
/// doc-comment
//! top-level doc-comment
const std = @import("std");
{#code_end#}
{#code_begin|obj_err|unattached_doc-comment|unattached documentation comment#}
pub fn main() void {}
/// End of file
{#code_end#}
<p>
Doc comments can be interleaved with normal comments. Currently, when producing
the package documentation, normal comments are merged with doc comments.
</p>
{#header_close#}
{#header_open|Top-Level Doc Comments#}
<p>
A top-level doc comment is one that begins with two slashes and an exclamation
point: {#syntax#}//!{#endsyntax#}; it documents the current module.
</p>
<p>
It is a compile error if a top-level doc comment is not placed at the start
of a {#link|container|Containers#}, before any expressions.
</p>
{#code_begin|syntax|tldoc_comments#}
//! This module provides functions for retrieving the current date and
//! time with varying degrees of precision and accuracy. It does not
//! depend on libc, but will use functions from it if available.
const S = struct {
//! Top level comments are allowed inside a container other than a module,
//! but it is not very useful. Currently, when producing the package
//! documentation, these comments are ignored.
};
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Values#}
{#code_begin|exe|values#}
// Top-level declarations are order-independent:
const print = std.debug.print;
const std = @import("std");
const os = std.os;
const assert = std.debug.assert;
pub fn main() void {
// integers
const one_plus_one: i32 = 1 + 1;
print("1 + 1 = {}\n", .{one_plus_one});
// floats
const seven_div_three: f32 = 7.0 / 3.0;
print("7.0 / 3.0 = {}\n", .{seven_div_three});
// boolean
print("{}\n{}\n{}\n", .{
true and false,
true or false,
!true,
});
// optional
var optional_value: ?[]const u8 = null;
assert(optional_value == null);
print("\noptional 1\ntype: {}\nvalue: {?s}\n", .{
@TypeOf(optional_value), optional_value,
});
optional_value = "hi";
assert(optional_value != null);
print("\noptional 2\ntype: {}\nvalue: {?s}\n", .{
@TypeOf(optional_value), optional_value,
});
// error union
var number_or_error: anyerror!i32 = error.ArgNotFound;
print("\nerror union 1\ntype: {}\nvalue: {!}\n", .{
@TypeOf(number_or_error), number_or_error, });
number_or_error = 1234;
print("\nerror union 2\ntype: {}\nvalue: {!}\n", .{
@TypeOf(number_or_error), number_or_error,
});
}
{#code_end#}
{#header_open|Primitive Types#}
<div class="table-wrapper">
<table>
<caption>Primitive Types</caption>
<thead>
<tr>
<th scope="col">Type</th>
<th scope="col">C Equivalent</th>
<th scope="col">Description</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">{#syntax#}i8{#endsyntax#}</th>
<td><code class="c">int8_t</code></td>
<td>signed 8-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u8{#endsyntax#}</th>
<td><code class="c">uint8_t</code></td>
<td>unsigned 8-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i16{#endsyntax#}</th>
<td><code class="c">int16_t</code></td>
<td>signed 16-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u16{#endsyntax#}</th>
<td><code class="c">uint16_t</code></td>
<td>unsigned 16-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i32{#endsyntax#}</th>
<td><code class="c">int32_t</code></td>
<td>signed 32-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u32{#endsyntax#}</th>
<td><code class="c">uint32_t</code></td>
<td>unsigned 32-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i64{#endsyntax#}</th>
<td><code class="c">int64_t</code></td>
<td>signed 64-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u64{#endsyntax#}</th>
<td><code class="c">uint64_t</code></td>
<td>unsigned 64-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i128{#endsyntax#}</th>
<td><code class="c">__int128</code></td>
<td>signed 128-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u128{#endsyntax#}</th>
<td><code class="c">unsigned __int128</code></td>
<td>unsigned 128-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}isize{#endsyntax#}</th>
<td><code class="c">intptr_t</code></td>
<td>signed pointer sized integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}usize{#endsyntax#}</th>
<td><code class="c">uintptr_t</code>, <code class="c">size_t</code></td>
<td>unsigned pointer sized integer. Also see <a href="https://github.com/ziglang/zig/issues/5185">#5185</a></td>
</tr>
<tr>
<th scope="row">{#syntax#}c_char{#endsyntax#}</th>
<td><code class="c">char</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_short{#endsyntax#}</th>
<td><code class="c">short</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_ushort{#endsyntax#}</th>
<td><code class="c">unsigned short</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_int{#endsyntax#}</th>
<td><code class="c">int</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_uint{#endsyntax#}</th>
<td><code class="c">unsigned int</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_long{#endsyntax#}</th>
<td><code class="c">long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_ulong{#endsyntax#}</th>
<td><code class="c">unsigned long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_longlong{#endsyntax#}</th>
<td><code class="c">long long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_ulonglong{#endsyntax#}</th>
<td><code class="c">unsigned long long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_longdouble{#endsyntax#}</th>
<td><code class="c">long double</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}f16{#endsyntax#}</th>
<td><code class="c">_Float16</code></td>
<td>16-bit floating point (10-bit mantissa) IEEE-754-2008 binary16</td>
</tr>
<tr>
<th scope="row">{#syntax#}f32{#endsyntax#}</th>
<td><code class="c">float</code></td>
<td>32-bit floating point (23-bit mantissa) IEEE-754-2008 binary32</td>
</tr>
<tr>
<th scope="row">{#syntax#}f64{#endsyntax#}</th>
<td><code class="c">double</code></td>
<td>64-bit floating point (52-bit mantissa) IEEE-754-2008 binary64</td>
</tr>
<tr>
<th scope="row">{#syntax#}f80{#endsyntax#}</th>
<td><code class="c">double</code></td>
<td>80-bit floating point (64-bit mantissa) IEEE-754-2008 80-bit extended precision</td>
</tr>
<tr>
<th scope="row">{#syntax#}f128{#endsyntax#}</th>
<td><code class="c">_Float128</code></td>
<td>128-bit floating point (112-bit mantissa) IEEE-754-2008 binary128</td>
</tr>
<tr>
<th scope="row">{#syntax#}bool{#endsyntax#}</th>
<td><code class="c">bool</code></td>
<td>{#syntax#}true{#endsyntax#} or {#syntax#}false{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}anyopaque{#endsyntax#}</th>
<td><code class="c">void</code></td>
<td>Used for type-erased pointers.</td>
</tr>
<tr>
<th scope="row">{#syntax#}void{#endsyntax#}</th>
<td>(none)</td>
<td>Always the value {#syntax#}void{}{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}noreturn{#endsyntax#}</th>
<td>(none)</td>
<td>the type of {#syntax#}break{#endsyntax#}, {#syntax#}continue{#endsyntax#}, {#syntax#}return{#endsyntax#}, {#syntax#}unreachable{#endsyntax#}, and {#syntax#}while (true) {}{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}type{#endsyntax#}</th>
<td>(none)</td>
<td>the type of types</td>
</tr>
<tr>
<th scope="row">{#syntax#}anyerror{#endsyntax#}</th>
<td>(none)</td>
<td>an error code</td>
</tr>
<tr>
<th scope="row">{#syntax#}comptime_int{#endsyntax#}</th>
<td>(none)</td>
<td>Only allowed for {#link|comptime#}-known values. The type of integer literals.</td>
</tr>
<tr>
<th scope="row">{#syntax#}comptime_float{#endsyntax#}</th>
<td>(none)</td>
<td>Only allowed for {#link|comptime#}-known values. The type of float literals.</td>
</tr>
</tbody>
</table>
</div>
<p>
In addition to the integer types above, arbitrary bit-width integers can be referenced by using
an identifier of <code>i</code> or <code>u</code> followed by digits. For example, the identifier
{#syntax#}i7{#endsyntax#} refers to a signed 7-bit integer. The maximum allowed bit-width of an
integer type is {#syntax#}65535{#endsyntax#}.
</p>
{#see_also|Integers|Floats|void|Errors|@Type#}
{#header_close#}
{#header_open|Primitive Values#}
<div class="table-wrapper">
<table>
<caption>Primitive Values</caption>
<thead>
<tr>
<th scope="col">Name</th>
<th scope="col">Description</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">{#syntax#}true{#endsyntax#} and {#syntax#}false{#endsyntax#}</th>
<td>{#syntax#}bool{#endsyntax#} values</td>
</tr>
<tr>
<th scope="row">{#syntax#}null{#endsyntax#}</th>
<td>used to set an optional type to {#syntax#}null{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}undefined{#endsyntax#}</th>
<td>used to leave a value unspecified</td>
</tr>
</tbody>
</table>
</div>
{#see_also|Optionals|undefined#}
{#header_close#}
{#header_open|String Literals and Unicode Code Point Literals#}
<p>
String literals are constant single-item {#link|Pointers#} to null-terminated byte arrays.
The type of string literals encodes both the length, and the fact that they are null-terminated,
and thus they can be {#link|coerced|Type Coercion#} to both {#link|Slices#} and
{#link|Null-Terminated Pointers|Sentinel-Terminated Pointers#}.
Dereferencing string literals converts them to {#link|Arrays#}.
</p>
<p>
Because Zig source code is {#link|UTF-8 encoded|Source Encoding#}, any
non-ASCII bytes appearing within a string literal in source code carry
their UTF-8 meaning into the content of the string in the Zig program;
the bytes are not modified by the compiler. It is possible to embed
non-UTF-8 bytes into a string literal using <code>\xNN</code> notation.
</p>
<p>Indexing into a string containing non-ASCII bytes returns individual
bytes, whether valid UTF-8 or not.</p>
<p>
Unicode code point literals have type {#syntax#}comptime_int{#endsyntax#}, the same as
{#link|Integer Literals#}. All {#link|Escape Sequences#} are valid in both string literals
and Unicode code point literals.
</p>
{#code_begin|exe|string_literals#}
const print = @import("std").debug.print;
const mem = @import("std").mem; // will be used to compare bytes
pub fn main() void {
const bytes = "hello";
print("{}\n", .{@TypeOf(bytes)}); // *const [5:0]u8
print("{d}\n", .{bytes.len}); // 5
print("{c}\n", .{bytes[1]}); // 'e'
print("{d}\n", .{bytes[5]}); // 0
print("{}\n", .{'e' == '\x65'}); // true
print("{d}\n", .{'\u{1f4a9}'}); // 128169
print("{d}\n", .{'💯'}); // 128175
print("{u}\n", .{'⚡'});
print("{}\n", .{mem.eql(u8, "hello", "h\x65llo")}); // true
print("{}\n", .{mem.eql(u8, "💯", "\xf0\x9f\x92\xaf")}); // also true
const invalid_utf8 = "\xff\xfe"; // non-UTF-8 strings are possible with \xNN notation.
print("0x{x}\n", .{invalid_utf8[1]}); // indexing them returns individual bytes...
print("0x{x}\n", .{"💯"[1]}); // ...as does indexing part-way through non-ASCII characters
}
{#code_end#}
{#see_also|Arrays|Source Encoding#}
{#header_open|Escape Sequences#}
<div class="table-wrapper">
<table>
<caption>Escape Sequences</caption>
<thead>
<tr>
<th scope="col">Escape Sequence</th>
<th scope="col">Name</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row"><code>\n</code></th>
<td>Newline</td>
</tr>
<tr>
<th scope="row"><code>\r</code></th>
<td>Carriage Return</td>
</tr>
<tr>
<th scope="row"><code>\t</code></th>
<td>Tab</td>
</tr>
<tr>
<th scope="row"><code>\\</code></th>
<td>Backslash</td>
</tr>
<tr>
<th scope="row"><code>\'</code></th>
<td>Single Quote</td>
</tr>
<tr>
<th scope="row"><code>\"</code></th>
<td>Double Quote</td>
</tr>
<tr>
<th scope="row"><code>\xNN</code></th>
<td>hexadecimal 8-bit byte value (2 digits)</td>
</tr>
<tr>
<th scope="row"><code>\u{NNNNNN}</code></th>
<td>hexadecimal Unicode code point UTF-8 encoded (1 or more digits)</td>
</tr>
</tbody>
</table>
</div>
<p>Note that the maximum valid Unicode point is {#syntax#}0x10ffff{#endsyntax#}.</p>
{#header_close#}
{#header_open|Multiline String Literals#}
<p>
Multiline string literals have no escapes and can span across multiple lines.
To start a multiline string literal, use the {#syntax#}\\{#endsyntax#} token. Just like a comment,
the string literal goes until the end of the line. The end of the line is
not included in the string literal.
However, if the next line begins with {#syntax#}\\{#endsyntax#} then a newline is appended and
the string literal continues.
</p>
{#code_begin|syntax|multiline_string_literals#}
const hello_world_in_c =
\\#include <stdio.h>
\\
\\int main(int argc, char **argv) {
\\ printf("hello world\n");
\\ return 0;
\\}
;
{#code_end#}
{#see_also|@embedFile#}
{#header_close#}
{#header_close#}
{#header_open|Assignment#}
<p>Use the {#syntax#}const{#endsyntax#} keyword to assign a value to an identifier:</p>
{#code_begin|exe_build_err|constant_identifier_cannot_change#}
const x = 1234;
fn foo() void {
// It works at file scope as well as inside functions.
const y = 5678;
// Once assigned, an identifier cannot be changed.
y += 1;
}
pub fn main() void {
foo();
}
{#code_end#}
<p>{#syntax#}const{#endsyntax#} applies to all of the bytes that the identifier immediately addresses. {#link|Pointers#} have their own const-ness.</p>
<p>If you need a variable that you can modify, use the {#syntax#}var{#endsyntax#} keyword:</p>
{#code_begin|exe|mutable_var#}
const print = @import("std").debug.print;
pub fn main() void {
var y: i32 = 5678;
y += 1;
print("{d}", .{y});
}
{#code_end#}
<p>Variables must be initialized:</p>
{#code_begin|exe_build_err|var_must_be_initialized#}
pub fn main() void {
var x: i32;
x = 1;
}
{#code_end#}
{#header_open|undefined#}
<p>Use {#syntax#}undefined{#endsyntax#} to leave variables uninitialized:</p>
{#code_begin|exe|assign_undefined#}
const print = @import("std").debug.print;
pub fn main() void {
var x: i32 = undefined;
x = 1;
print("{d}", .{x});
}
{#code_end#}
<p>
{#syntax#}undefined{#endsyntax#} can be {#link|coerced|Type Coercion#} to any type.
Once this happens, it is no longer possible to detect that the value is {#syntax#}undefined{#endsyntax#}.
{#syntax#}undefined{#endsyntax#} means the value could be anything, even something that is nonsense
according to the type. Translated into English, {#syntax#}undefined{#endsyntax#} means "Not a meaningful
value. Using this value would be a bug. The value will be unused, or overwritten before being used."
</p>
<p>
In {#link|Debug#} mode, Zig writes {#syntax#}0xaa{#endsyntax#} bytes to undefined memory. This is to catch
bugs early, and to help detect use of undefined memory in a debugger. However, this behavior is only an
implementation feature, not a language semantic, so it is not guaranteed to be observable to code.
</p>
{#header_close#}
{#header_close#}
{#header_close#}
{#header_open|Zig Test#}
<p>
Code written within one or more {#syntax#}test{#endsyntax#} declarations can be used to ensure behavior meets expectations:
</p>
{#code_begin|test|testing_introduction#}
const std = @import("std");
test "expect addOne adds one to 41" {
// The Standard Library contains useful functions to help create tests.
// `expect` is a function that verifies its argument is true.
// It will return an error if its argument is false to indicate a failure.
// `try` is used to return an error to the test runner to notify it that the test failed.
try std.testing.expect(addOne(41) == 42);
}
test addOne {
// A test name can also be written using an identifier.
// This is a doctest, and serves as documentation for `addOne`.
try std.testing.expect(addOne(41) == 42);
}
/// The function `addOne` adds one to the number given as its argument.
fn addOne(number: i32) i32 {
return number + 1;
}
{#code_end#}
<p>
The <code class="file">testing_introduction.zig</code> code sample tests the {#link|function|Functions#}
{#syntax#}addOne{#endsyntax#} to ensure that it returns {#syntax#}42{#endsyntax#} given the input
{#syntax#}41{#endsyntax#}. From this test's perspective, the {#syntax#}addOne{#endsyntax#} function is
said to be <em>code under test</em>.
</p>
<p>
<kbd>zig test</kbd> is a tool that creates and runs a test build. By default, it builds and runs an
executable program using the <em>default test runner</em> provided by the {#link|Zig Standard Library#}
as its main entry point. During the build, {#syntax#}test{#endsyntax#} declarations found while
{#link|resolving|Root Source File#} the given Zig source file are included for the default test runner
to run and report on.
</p>
<aside>
This documentation discusses the features of the default test runner as provided by the Zig Standard Library.
Its source code is located in <code class="file">lib/test_runner.zig</code>.
</aside>
<p>
The shell output shown above displays two lines after the <kbd>zig test</kbd> command. These lines are
printed to standard error by the default test runner:
</p>
<dl>
<dt><samp>1/2 testing_introduction.test.expect addOne adds one to 41...</samp></dt>
<dd>Lines like this indicate which test, out of the total number of tests, is being run.
In this case, <samp>1/2</samp> indicates that the first test, out of a total of two tests,
is being run. Note that, when the test runner program's standard error is output
to the terminal, these lines are cleared when a test succeeds.
</dd>
<dt><samp>2/2 testing_introduction.decltest.addOne...</samp></dt>
<dd>When the test name is an identifier, the default test runner uses the text
decltest instead of test.
</dd>
<dt><samp>All 2 tests passed.</samp></dt>
<dd>This line indicates the total number of tests that have passed.</dd>
</dl>
{#header_open|Test Declarations#}
<p>
Test declarations contain the {#link|keyword|Keyword Reference#} {#syntax#}test{#endsyntax#}, followed by an
optional name written as a {#link|string literal|String Literals and Unicode Code Point Literals#} or an
{#link|identifier|Identifiers#}, followed by a {#link|block|Blocks#} containing any valid Zig code that
is allowed in a {#link|function|Functions#}.
</p>
<p>Non-named test blocks always run during test builds and are exempt from
{#link|Skip Tests#}.</p>
<p>
Test declarations are similar to {#link|Functions#}: they have a return type and a block of code. The implicit
return type of {#syntax#}test{#endsyntax#} is the {#link|Error Union Type#} {#syntax#}anyerror!void{#endsyntax#},
and it cannot be changed. When a Zig source file is not built using the <kbd>zig test</kbd> tool, the test
declarations are omitted from the build.
</p>
<p>
Test declarations can be written in the same file, where code under test is written, or in a separate Zig source file.
Since test declarations are top-level declarations, they are order-independent and can
be written before or after the code under test.
</p>
{#see_also|The Global Error Set|Grammar#}
{#header_open|Doctests#}
<p>
Test declarations named using an identifier are <em>doctests</em>. The identifier must refer to another declaration in
scope. A doctest, like a {#link|doc comment|Doc Comments#}, serves as documentation for the associated declaration, and
will appear in the generated documentation for the declaration.
</p>
<p>
An effective doctest should be self-contained and focused on the declaration being tested, answering questions a new
user might have about its interface or intended usage, while avoiding unnecessary or confusing details. A doctest is not
a substitute for a doc comment, but rather a supplement and companion providing a testable, code-driven example, verified
by <kbd>zig test</kbd>.
</p>
{#header_close#}
{#header_close#}
{#header_open|Test Failure#}
<p>
The default test runner checks for an {#link|error|Errors#} returned from a test.
When a test returns an error, the test is considered a failure and its {#link|error return trace|Error Return Traces#}
is output to standard error. The total number of failures will be reported after all tests have run.
</p>
{#code_begin|test_err|testing_failure#}
const std = @import("std");
test "expect this to fail" {
try std.testing.expect(false);
}
test "expect this to succeed" {
try std.testing.expect(true);
}
{#code_end#}
{#header_close#}
{#header_open|Skip Tests#}
<p>
One way to skip tests is to filter them out by using the <kbd>zig test</kbd> command line parameter
<kbd>--test-filter [text]</kbd>. This makes the test build only include tests whose name contains the
supplied filter text. Note that non-named tests are run even when using the <kbd>--test-filter [text]</kbd>
command line parameter.
</p>
<p>
To programmatically skip a test, make a {#syntax#}test{#endsyntax#} return the error
{#syntax#}error.SkipZigTest{#endsyntax#} and the default test runner will consider the test as being skipped.
The total number of skipped tests will be reported after all tests have run.
</p>
{#code_begin|test|testing_skip#}
test "this will be skipped" {
return error.SkipZigTest;
}
{#code_end#}
{#header_close#}
{#header_open|Report Memory Leaks#}
<p>
When code allocates {#link|Memory#} using the {#link|Zig Standard Library#}'s testing allocator,
{#syntax#}std.testing.allocator{#endsyntax#}, the default test runner will report any leaks that are
found from using the testing allocator:
</p>
{#code_begin|test_err|testing_detect_leak|1 tests leaked memory#}
const std = @import("std");
test "detect leak" {
var list = std.ArrayList(u21).init(std.testing.allocator);
// missing `defer list.deinit();`
try list.append('☔');
try std.testing.expect(list.items.len == 1);
}
{#code_end#}
{#see_also|defer|Memory#}
{#header_close#}
{#header_open|Detecting Test Build#}
<p>
Use the {#link|compile variable|Compile Variables#} {#syntax#}@import("builtin").is_test{#endsyntax#}
to detect a test build:
</p>
{#code_begin|test|testing_detect_test#}
const std = @import("std");
const builtin = @import("builtin");
const expect = std.testing.expect;
test "builtin.is_test" {
try expect(isATest());
}
fn isATest() bool {
return builtin.is_test;
}
{#code_end#}
{#header_close#}
{#header_open|Test Output and Logging#}
<p>
The default test runner and the Zig Standard Library's testing namespace output messages to standard error.
</p>
{#header_close#}
{#header_open|The Testing Namespace#}
<p>
The Zig Standard Library's <code>testing</code> namespace contains useful functions to help
you create tests. In addition to the <code>expect</code> function, this document uses a couple of more functions
as exemplified here:
</p>
{#code_begin|test|testing_namespace#}
const std = @import("std");
test "expectEqual demo" {
const expected: i32 = 42;
const actual = 42;
// The first argument to `expectEqual` is the known, expected, result.
// The second argument is the result of some expression.
// The actual's type is casted to the type of expected.
try std.testing.expectEqual(expected, actual);
}
test "expectError demo" {
const expected_error = error.DemoError;
const actual_error_union: anyerror!void = error.DemoError;
// `expectError` will fail when the actual error is different than
// the expected error.
try std.testing.expectError(expected_error, actual_error_union);
}
{#code_end#}
<p>The Zig Standard Library also contains functions to compare {#link|Slices#}, strings, and more. See the rest of the
{#syntax#}std.testing{#endsyntax#} namespace in the {#link|Zig Standard Library#} for more available functions.</p>
{#header_close#}
{#header_open|Test Tool Documentation#}
<p>
<kbd>zig test</kbd> has a few command line parameters which affect the compilation.
See <kbd>zig test --help</kbd> for a full list.
</p>
{#header_close#}
{#header_close#}
{#header_open|Variables#}
<p>
A variable is a unit of {#link|Memory#} storage.
</p>
<p>
It is generally preferable to use {#syntax#}const{#endsyntax#} rather than
{#syntax#}var{#endsyntax#} when declaring a variable. This causes less work for both
humans and computers to do when reading code, and creates more optimization opportunities.
</p>
<p>
The {#syntax#}extern{#endsyntax#} keyword or {#link|@extern#} builtin function can be used to link against a variable that is exported
from another object. The {#syntax#}export{#endsyntax#} keyword or {#link|@export#} builtin function
can be used to make a variable available to other objects at link time. In both cases,
the type of the variable must be C ABI compatible.
</p>
{#see_also|Exporting a C Library#}
{#header_open|Identifiers#}
<p>
Variable identifiers are never allowed to shadow identifiers from an outer scope.
</p>
<p>
Identifiers must start with an alphabetic character or underscore and may be followed
by any number of alphanumeric characters or underscores.
They must not overlap with any keywords. See {#link|Keyword Reference#}.
</p>
<p>
If a name that does not fit these requirements is needed, such as for linking with external libraries, the {#syntax#}@""{#endsyntax#} syntax may be used.
</p>
{#code_begin|syntax|identifiers#}
const @"identifier with spaces in it" = 0xff;
const @"1SmallStep4Man" = 112358;
const c = @import("std").c;
pub extern "c" fn @"error"() void;
pub extern "c" fn @"fstat$INODE64"(fd: c.fd_t, buf: *c.Stat) c_int;
const Color = enum {
red,
@"really red",
};
const color: Color = .@"really red";
{#code_end#}
{#header_close#}
{#header_open|Container Level Variables#}
<p>
{#link|Container|Containers#} level variables have static lifetime and are order-independent and lazily analyzed.
The initialization value of container level variables is implicitly
{#link|comptime#}. If a container level variable is {#syntax#}const{#endsyntax#} then its value is
{#syntax#}comptime{#endsyntax#}-known, otherwise it is runtime-known.
</p>
{#code_begin|test|test_container_level_variables#}
var y: i32 = add(10, x);
const x: i32 = add(12, 34);
test "container level variables" {
try expect(x == 46);
try expect(y == 56);
}
fn add(a: i32, b: i32) i32 {
return a + b;
}
const std = @import("std");
const expect = std.testing.expect;
{#code_end#}
<p>
Container level variables may be declared inside a {#link|struct#}, {#link|union#}, {#link|enum#}, or {#link|opaque#}:
</p>
{#code_begin|test|test_namespaced_container_level_variable#}
const std = @import("std");
const expect = std.testing.expect;
test "namespaced container level variable" {
try expect(foo() == 1235);
try expect(foo() == 1236);
}
const S = struct {
var x: i32 = 1234;
};
fn foo() i32 {
S.x += 1;
return S.x;
}
{#code_end#}
{#header_close#}
{#header_open|Static Local Variables#}
<p>
It is also possible to have local variables with static lifetime by using containers inside functions.
</p>
{#code_begin|test|test_static_local_variable#}
const std = @import("std");
const expect = std.testing.expect;
test "static local variable" {
try expect(foo() == 1235);
try expect(foo() == 1236);
}
fn foo() i32 {
const S = struct {
var x: i32 = 1234;
};
S.x += 1;
return S.x;
}
{#code_end#}
{#header_close#}
{#header_open|Thread Local Variables#}
<p>A variable may be specified to be a thread-local variable using the
{#syntax#}threadlocal{#endsyntax#} keyword,
which makes each thread work with a separate instance of the variable:</p>
{#code_begin|test|test_thread_local_variables#}
const std = @import("std");
const assert = std.debug.assert;
threadlocal var x: i32 = 1234;
test "thread local storage" {
const thread1 = try std.Thread.spawn(.{}, testTls, .{});
const thread2 = try std.Thread.spawn(.{}, testTls, .{});
testTls();
thread1.join();
thread2.join();
}
fn testTls() void {
assert(x == 1234);
x += 1;
assert(x == 1235);
}
{#code_end#}
<p>
For {#link|Single Threaded Builds#}, all thread local variables are treated as regular {#link|Container Level Variables#}.
</p>
<p>
Thread local variables may not be {#syntax#}const{#endsyntax#}.
</p>
{#header_close#}
{#header_open|Local Variables#}
<p>
Local variables occur inside {#link|Functions#}, {#link|comptime#} blocks, and {#link|@cImport#} blocks.
</p>
<p>
When a local variable is {#syntax#}const{#endsyntax#}, it means that after initialization, the variable's
value will not change. If the initialization value of a {#syntax#}const{#endsyntax#} variable is
{#link|comptime#}-known, then the variable is also {#syntax#}comptime{#endsyntax#}-known.
</p>
<p>
A local variable may be qualified with the {#syntax#}comptime{#endsyntax#} keyword. This causes
the variable's value to be {#syntax#}comptime{#endsyntax#}-known, and all loads and stores of the
variable to happen during semantic analysis of the program, rather than at runtime.
All variables declared in a {#syntax#}comptime{#endsyntax#} expression are implicitly
{#syntax#}comptime{#endsyntax#} variables.
</p>
{#code_begin|test|test_comptime_variables#}
const std = @import("std");
const expect = std.testing.expect;
test "comptime vars" {
var x: i32 = 1;
comptime var y: i32 = 1;
x += 1;
y += 1;
try expect(x == 2);
try expect(y == 2);
if (y != 2) {
// This compile error never triggers because y is a comptime variable,
// and so `y != 2` is a comptime value, and this if is statically evaluated.
@compileError("wrong y value");
}
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Integers#}
{#header_open|Integer Literals#}
{#code_begin|syntax|integer_literals#}
const decimal_int = 98222;
const hex_int = 0xff;
const another_hex_int = 0xFF;
const octal_int = 0o755;
const binary_int = 0b11110000;
// underscores may be placed between two digits as a visual separator
const one_billion = 1_000_000_000;
const binary_mask = 0b1_1111_1111;
const permissions = 0o7_5_5;
const big_address = 0xFF80_0000_0000_0000;
{#code_end#}
{#header_close#}
{#header_open|Runtime Integer Values#}
<p>
Integer literals have no size limitation, and if any undefined behavior occurs,
the compiler catches it.
</p>
<p>
However, once an integer value is no longer known at compile-time, it must have a
known size, and is vulnerable to undefined behavior.
</p>
{#code_begin|syntax|runtime_vs_comptime#}
fn divide(a: i32, b: i32) i32 {
return a / b;
}
{#code_end#}
<p>
In this function, values {#syntax#}a{#endsyntax#} and {#syntax#}b{#endsyntax#} are known only at runtime,
and thus this division operation is vulnerable to both {#link|Integer Overflow#} and
{#link|Division by Zero#}.
</p>
<p>
Operators such as {#syntax#}+{#endsyntax#} and {#syntax#}-{#endsyntax#} cause undefined behavior on
integer overflow. Alternative operators are provided for wrapping and saturating arithmetic on all targets.
{#syntax#}+%{#endsyntax#} and {#syntax#}-%{#endsyntax#} perform wrapping arithmetic
while {#syntax#}+|{#endsyntax#} and {#syntax#}-|{#endsyntax#} perform saturating arithmetic.
</p>
<p>
Zig supports arbitrary bit-width integers, referenced by using
an identifier of <code>i</code> or <code>u</code> followed by digits. For example, the identifier
{#syntax#}i7{#endsyntax#} refers to a signed 7-bit integer. The maximum allowed bit-width of an
integer type is {#syntax#}65535{#endsyntax#}. For signed integer types, Zig uses a
<a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement</a> representation.
</p>
{#see_also|Wrapping Operations#}
{#header_close#}
{#header_close#}
{#header_open|Floats#}
<p>Zig has the following floating point types:</p>
<ul>
<li>{#syntax#}f16{#endsyntax#} - IEEE-754-2008 binary16</li>
<li>{#syntax#}f32{#endsyntax#} - IEEE-754-2008 binary32</li>
<li>{#syntax#}f64{#endsyntax#} - IEEE-754-2008 binary64</li>
<li>{#syntax#}f80{#endsyntax#} - IEEE-754-2008 80-bit extended precision</li>
<li>{#syntax#}f128{#endsyntax#} - IEEE-754-2008 binary128</li>
<li>{#syntax#}c_longdouble{#endsyntax#} - matches <code class="c">long double</code> for the target C ABI</li>
</ul>
{#header_open|Float Literals#}
<p>
Float literals have type {#syntax#}comptime_float{#endsyntax#} which is guaranteed to have
the same precision and operations of the largest other floating point type, which is
{#syntax#}f128{#endsyntax#}.
</p>
<p>
Float literals {#link|coerce|Type Coercion#} to any floating point type,
and to any {#link|integer|Integers#} type when there is no fractional component.
</p>
{#code_begin|syntax|float_literals#}
const floating_point = 123.0E+77;
const another_float = 123.0;
const yet_another = 123.0e+77;
const hex_floating_point = 0x103.70p-5;
const another_hex_float = 0x103.70;
const yet_another_hex_float = 0x103.70P-5;
// underscores may be placed between two digits as a visual separator
const lightspeed = 299_792_458.000_000;
const nanosecond = 0.000_000_001;
const more_hex = 0x1234_5678.9ABC_CDEFp-10;
{#code_end#}
<p>
There is no syntax for NaN, infinity, or negative infinity. For these special values,
one must use the standard library:
</p>
{#code_begin|syntax|float_special_values#}
const std = @import("std");
const inf = std.math.inf(f32);
const negative_inf = -std.math.inf(f64);
const nan = std.math.nan(f128);
{#code_end#}
{#header_close#}
{#header_open|Floating Point Operations#}
<p>By default floating point operations use {#syntax#}Strict{#endsyntax#} mode,
but you can switch to {#syntax#}Optimized{#endsyntax#} mode on a per-block basis:</p>
{#code_begin|obj|float_mode_obj#}
{#code_release_fast#}
{#code_disable_cache#}
const std = @import("std");
const big = @as(f64, 1 << 40);
export fn foo_strict(x: f64) f64 {
return x + big - big;
}
export fn foo_optimized(x: f64) f64 {
@setFloatMode(.optimized);
return x + big - big;
}
{#code_end#}
<p>For this test we have to separate code into two object files -
otherwise the optimizer figures out all the values at compile-time,
which operates in strict mode.</p>
{#code_begin|exe|float_mode_exe#}
{#code_link_object|float_mode_obj#}
const print = @import("std").debug.print;
extern fn foo_strict(x: f64) f64;
extern fn foo_optimized(x: f64) f64;
pub fn main() void {
const x = 0.001;
print("optimized = {}\n", .{foo_optimized(x)});
print("strict = {}\n", .{foo_strict(x)});
}
{#code_end#}
{#see_also|@setFloatMode|Division by Zero#}
{#header_close#}
{#header_close#}
{#header_open|Operators#}
<p>
There is no operator overloading. When you see an operator in Zig, you know that
it is doing something from this table, and nothing else.
</p>
{#header_open|Table of Operators#}
<div class="table-wrapper">
<table>
<thead>
<tr>
<th scope="col">Name</th>
<th scope="col">Syntax</th>
<th scope="col">Types</th>
<th scope="col">Remarks</th>
<th scope="col">Example</th>
</tr>
</thead>
<tbody>
<tr>
<td>Addition</td>
<td><pre>{#syntax#}a + b
a += b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
<ul>
<li>Can cause {#link|overflow|Default Operations#} for integers.</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
<li>See also {#link|@addWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}2 + 5 == 7{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Wrapping Addition</td>
<td><pre>{#syntax#}a +% b
a +%= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Twos-complement wrapping behavior.</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
<li>See also {#link|@addWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u32, 0xffffffff) +% 1 == 0{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Saturating Addition</td>
<td><pre>{#syntax#}a +| b
a +|= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u8, 255) +| 1 == @as(u8, 255){#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Subtraction</td>
<td><pre>{#syntax#}a - b
a -= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
<ul>
<li>Can cause {#link|overflow|Default Operations#} for integers.</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
<li>See also {#link|@subWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}2 - 5 == -3{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Wrapping Subtraction</td>
<td><pre>{#syntax#}a -% b
a -%= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Twos-complement wrapping behavior.</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
<li>See also {#link|@subWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u8, 0) -% 1 == 255{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Saturating Subtraction</td>
<td><pre>{#syntax#}a -| b
a -|= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u32, 0) -| 1 == 0{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Negation</td>
<td><pre>{#syntax#}-a{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
<ul>
<li>Can cause {#link|overflow|Default Operations#} for integers.</li>
</ul>
</td>
<td>
<pre>{#syntax#}-1 == 0 - 1{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Wrapping Negation</td>
<td><pre>{#syntax#}-%a{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Twos-complement wrapping behavior.</li>
</ul>
</td>
<td>
<pre>{#syntax#}-%@as(i8, -128) == -128{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Multiplication</td>
<td><pre>{#syntax#}a * b
a *= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
<ul>
<li>Can cause {#link|overflow|Default Operations#} for integers.</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
<li>See also {#link|@mulWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}2 * 5 == 10{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Wrapping Multiplication</td>
<td><pre>{#syntax#}a *% b
a *%= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Twos-complement wrapping behavior.</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
<li>See also {#link|@mulWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u8, 200) *% 2 == 144{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Saturating Multiplication</td>
<td><pre>{#syntax#}a *| b
a *|= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u8, 200) *| 2 == 255{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Division</td>
<td><pre>{#syntax#}a / b
a /= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
<ul>
<li>Can cause {#link|overflow|Default Operations#} for integers.</li>
<li>Can cause {#link|Division by Zero#} for integers.</li>
<li>Can cause {#link|Division by Zero#} for floats in {#link|FloatMode.Optimized Mode|Floating Point Operations#}.</li>
<li>Signed integer operands must be comptime-known and positive. In other cases, use
{#link|@divTrunc#},
{#link|@divFloor#}, or
{#link|@divExact#} instead.
</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}10 / 5 == 2{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Remainder Division</td>
<td><pre>{#syntax#}a % b
a %= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
<ul>
<li>Can cause {#link|Division by Zero#} for integers.</li>
<li>Can cause {#link|Division by Zero#} for floats in {#link|FloatMode.Optimized Mode|Floating Point Operations#}.</li>
<li>Signed or floating-point operands must be comptime-known and positive. In other cases, use
{#link|@rem#} or
{#link|@mod#} instead.
</li>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}10 % 3 == 1{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Bit Shift Left</td>
<td><pre>{#syntax#}a << b
a <<= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Moves all bits to the left, inserting new zeroes at the
least-significant bit.</li>
<li>{#syntax#}b{#endsyntax#} must be
{#link|comptime-known|comptime#} or have a type with log2 number
of bits as {#syntax#}a{#endsyntax#}.</li>
<li>See also {#link|@shlExact#}.</li>
<li>See also {#link|@shlWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}0b1 << 8 == 0b100000000{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Saturating Bit Shift Left</td>
<td><pre>{#syntax#}a <<| b
a <<|= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>See also {#link|@shlExact#}.</li>
<li>See also {#link|@shlWithOverflow#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}@as(u8, 1) <<| 8 == 255{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Bit Shift Right</td>
<td><pre>{#syntax#}a >> b
a >>= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Moves all bits to the right, inserting zeroes at the most-significant bit.</li>
<li>{#syntax#}b{#endsyntax#} must be
{#link|comptime-known|comptime#} or have a type with log2 number
of bits as {#syntax#}a{#endsyntax#}.</li>
<li>See also {#link|@shrExact#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}0b1010 >> 1 == 0b101{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Bitwise And</td>
<td><pre>{#syntax#}a & b
a &= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}0b011 & 0b101 == 0b001{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Bitwise Or</td>
<td><pre>{#syntax#}a | b
a |= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}0b010 | 0b100 == 0b110{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Bitwise Xor</td>
<td><pre>{#syntax#}a ^ b
a ^= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td>
<ul>
<li>Invokes {#link|Peer Type Resolution#} for the operands.</li>
</ul>
</td>
<td>
<pre>{#syntax#}0b011 ^ 0b101 == 0b110{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Bitwise Not</td>
<td><pre>{#syntax#}~a{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
</ul>
</td>
<td></td>
<td>
<pre>{#syntax#}~@as(u8, 0b10101111) == 0b01010000{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Defaulting Optional Unwrap</td>
<td><pre>{#syntax#}a orelse b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Optionals#}</li>
</ul>
</td>
<td>If {#syntax#}a{#endsyntax#} is {#syntax#}null{#endsyntax#},
returns {#syntax#}b{#endsyntax#} ("default value"),
otherwise returns the unwrapped value of {#syntax#}a{#endsyntax#}.
Note that {#syntax#}b{#endsyntax#} may be a value of type {#link|noreturn#}.
</td>
<td>
<pre>{#syntax#}const value: ?u32 = null;
const unwrapped = value orelse 1234;
unwrapped == 1234{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Optional Unwrap</td>
<td><pre>{#syntax#}a.?{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Optionals#}</li>
</ul>
</td>
<td>
Equivalent to:
<pre>{#syntax#}a orelse unreachable{#endsyntax#}</pre>
</td>
<td>
<pre>{#syntax#}const value: ?u32 = 5678;
value.? == 5678{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Defaulting Error Unwrap</td>
<td><pre>{#syntax#}a catch b
a catch |err| b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Error Unions|Errors#}</li>
</ul>
</td>
<td>If {#syntax#}a{#endsyntax#} is an {#syntax#}error{#endsyntax#},
returns {#syntax#}b{#endsyntax#} ("default value"),
otherwise returns the unwrapped value of {#syntax#}a{#endsyntax#}.
Note that {#syntax#}b{#endsyntax#} may be a value of type {#link|noreturn#}.
{#syntax#}err{#endsyntax#} is the {#syntax#}error{#endsyntax#} and is in scope of the expression {#syntax#}b{#endsyntax#}.
</td>
<td>
<pre>{#syntax#}const value: anyerror!u32 = error.Broken;
const unwrapped = value catch 1234;
unwrapped == 1234{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Logical And</td>
<td><pre>{#syntax#}a and b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|bool|Primitive Types#}</li>
</ul>
</td>
<td>
If {#syntax#}a{#endsyntax#} is {#syntax#}false{#endsyntax#}, returns {#syntax#}false{#endsyntax#}
without evaluating {#syntax#}b{#endsyntax#}. Otherwise, returns {#syntax#}b{#endsyntax#}.
</td>
<td>
<pre>{#syntax#}(false and true) == false{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Logical Or</td>
<td><pre>{#syntax#}a or b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|bool|Primitive Types#}</li>
</ul>
</td>
<td>
If {#syntax#}a{#endsyntax#} is {#syntax#}true{#endsyntax#},
returns {#syntax#}true{#endsyntax#} without evaluating
{#syntax#}b{#endsyntax#}. Otherwise, returns
{#syntax#}b{#endsyntax#}.
</td>
<td>
<pre>{#syntax#}(false or true) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Boolean Not</td>
<td><pre>{#syntax#}!a{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|bool|Primitive Types#}</li>
</ul>
</td>
<td></td>
<td>
<pre>{#syntax#}!false == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Equality</td>
<td><pre>{#syntax#}a == b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
<li>{#link|bool|Primitive Types#}</li>
<li>{#link|type|Primitive Types#}</li>
</ul>
</td>
<td>
Returns {#syntax#}true{#endsyntax#} if a and b are equal, otherwise returns {#syntax#}false{#endsyntax#}.
Invokes {#link|Peer Type Resolution#} for the operands.
</td>
<td>
<pre>{#syntax#}(1 == 1) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Null Check</td>
<td><pre>{#syntax#}a == null{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Optionals#}</li>
</ul>
</td>
<td>
Returns {#syntax#}true{#endsyntax#} if a is {#syntax#}null{#endsyntax#}, otherwise returns {#syntax#}false{#endsyntax#}.
</td>
<td>
<pre>{#syntax#}const value: ?u32 = null;
(value == null) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Inequality</td>
<td><pre>{#syntax#}a != b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
<li>{#link|bool|Primitive Types#}</li>
<li>{#link|type|Primitive Types#}</li>
</ul>
</td>
<td>
Returns {#syntax#}false{#endsyntax#} if a and b are equal, otherwise returns {#syntax#}true{#endsyntax#}.
Invokes {#link|Peer Type Resolution#} for the operands.
</td>
<td>
<pre>{#syntax#}(1 != 1) == false{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Non-Null Check</td>
<td><pre>{#syntax#}a != null{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Optionals#}</li>
</ul>
</td>
<td>
Returns {#syntax#}false{#endsyntax#} if a is {#syntax#}null{#endsyntax#}, otherwise returns {#syntax#}true{#endsyntax#}.
</td>
<td>
<pre>{#syntax#}const value: ?u32 = null;
(value != null) == false{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Greater Than</td>
<td><pre>{#syntax#}a > b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
Returns {#syntax#}true{#endsyntax#} if a is greater than b, otherwise returns {#syntax#}false{#endsyntax#}.
Invokes {#link|Peer Type Resolution#} for the operands.
</td>
<td>
<pre>{#syntax#}(2 > 1) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Greater or Equal</td>
<td><pre>{#syntax#}a >= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
Returns {#syntax#}true{#endsyntax#} if a is greater than or equal to b, otherwise returns {#syntax#}false{#endsyntax#}.
Invokes {#link|Peer Type Resolution#} for the operands.
</td>
<td>
<pre>{#syntax#}(2 >= 1) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Less Than</td>
<td><pre>{#syntax#}a < b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
Returns {#syntax#}true{#endsyntax#} if a is less than b, otherwise returns {#syntax#}false{#endsyntax#}.
Invokes {#link|Peer Type Resolution#} for the operands.
</td>
<td>
<pre>{#syntax#}(1 < 2) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Lesser or Equal</td>
<td><pre>{#syntax#}a <= b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Integers#}</li>
<li>{#link|Floats#}</li>
</ul>
</td>
<td>
Returns {#syntax#}true{#endsyntax#} if a is less than or equal to b, otherwise returns {#syntax#}false{#endsyntax#}.
Invokes {#link|Peer Type Resolution#} for the operands.
</td>
<td>
<pre>{#syntax#}(1 <= 2) == true{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Array Concatenation</td>
<td><pre>{#syntax#}a ++ b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Arrays#}</li>
</ul>
</td>
<td>
<ul>
<li>Only available when the lengths of both {#syntax#}a{#endsyntax#} and {#syntax#}b{#endsyntax#} are {#link|compile-time known|comptime#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}const mem = @import("std").mem;
const array1 = [_]u32{1,2};
const array2 = [_]u32{3,4};
const together = array1 ++ array2;
mem.eql(u32, &together, &[_]u32{1,2,3,4}){#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Array Multiplication</td>
<td><pre>{#syntax#}a ** b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Arrays#}</li>
</ul>
</td>
<td>
<ul>
<li>Only available when the length of {#syntax#}a{#endsyntax#} and {#syntax#}b{#endsyntax#} are {#link|compile-time known|comptime#}.</li>
</ul>
</td>
<td>
<pre>{#syntax#}const mem = @import("std").mem;
const pattern = "ab" ** 3;
mem.eql(u8, pattern, "ababab"){#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Pointer Dereference</td>
<td><pre>{#syntax#}a.*{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Pointers#}</li>
</ul>
</td>
<td>
Pointer dereference.
</td>
<td>
<pre>{#syntax#}const x: u32 = 1234;
const ptr = &x;
ptr.* == 1234{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Address Of</td>
<td><pre>{#syntax#}&a{#endsyntax#}</pre></td>
<td>
All types
</td>
<td>
</td>
<td>
<pre>{#syntax#}const x: u32 = 1234;
const ptr = &x;
ptr.* == 1234{#endsyntax#}</pre>
</td>
</tr>
<tr>
<td>Error Set Merge</td>
<td><pre>{#syntax#}a || b{#endsyntax#}</pre></td>
<td>
<ul>
<li>{#link|Error Set Type#}</li>
</ul>
</td>
<td>
{#link|Merging Error Sets#}
</td>
<td>
<pre>{#syntax#}const A = error{One};
const B = error{Two};
(A || B) == error{One, Two}{#endsyntax#}</pre>
</td>
</tr>
</tbody>
</table>
</div>
{#header_close#}
{#header_open|Precedence#}
<pre>{#syntax#}x() x[] x.y x.* x.?
a!b
x{}
!x -x -%x ~x &x ?x
* / % ** *% *| ||
+ - ++ +% -% +| -|
<< >> <<|
& ^ | orelse catch
== != < > <= >=
and
or
= *= *%= *|= /= %= += +%= +|= -= -%= -|= <<= <<|= >>= &= ^= |={#endsyntax#}</pre>
{#header_close#}
{#header_close#}
{#header_open|Arrays#}
{#code_begin|test|test_arrays#}
const expect = @import("std").testing.expect;
const assert = @import("std").debug.assert;
const mem = @import("std").mem;
// array literal
const message = [_]u8{ 'h', 'e', 'l', 'l', 'o' };
// get the size of an array
comptime {
assert(message.len == 5);
}
// A string literal is a single-item pointer to an array.
const same_message = "hello";
comptime {
assert(mem.eql(u8, &message, same_message));
}
test "iterate over an array" {
var sum: usize = 0;
for (message) |byte| {
sum += byte;
}
try expect(sum == 'h' + 'e' + 'l' * 2 + 'o');
}
// modifiable array
var some_integers: [100]i32 = undefined;
test "modify an array" {
for (&some_integers, 0..) |*item, i| {
item.* = @intCast(i);
}
try expect(some_integers[10] == 10);
try expect(some_integers[99] == 99);
}
// array concatenation works if the values are known
// at compile time
const part_one = [_]i32{ 1, 2, 3, 4 };
const part_two = [_]i32{ 5, 6, 7, 8 };
const all_of_it = part_one ++ part_two;
comptime {
assert(mem.eql(i32, &all_of_it, &[_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }));
}
// remember that string literals are arrays
const hello = "hello";
const world = "world";
const hello_world = hello ++ " " ++ world;
comptime {
assert(mem.eql(u8, hello_world, "hello world"));
}
// ** does repeating patterns
const pattern = "ab" ** 3;
comptime {
assert(mem.eql(u8, pattern, "ababab"));
}
// initialize an array to zero
const all_zero = [_]u16{0} ** 10;
comptime {
assert(all_zero.len == 10);
assert(all_zero[5] == 0);
}
// use compile-time code to initialize an array
var fancy_array = init: {
var initial_value: [10]Point = undefined;
for (&initial_value, 0..) |*pt, i| {
pt.* = Point{
.x = @intCast(i),
.y = @intCast(i * 2),
};
}
break :init initial_value;
};
const Point = struct {
x: i32,
y: i32,
};
test "compile-time array initialization" {
try expect(fancy_array[4].x == 4);
try expect(fancy_array[4].y == 8);
}
// call a function to initialize an array
var more_points = [_]Point{makePoint(3)} ** 10;
fn makePoint(x: i32) Point {
return Point{
.x = x,
.y = x * 2,
};
}
test "array initialization with function calls" {
try expect(more_points[4].x == 3);
try expect(more_points[4].y == 6);
try expect(more_points.len == 10);
}
{#code_end#}
{#see_also|for|Slices#}
{#header_open|Multidimensional Arrays#}
<p>
Multidimensional arrays can be created by nesting arrays:
</p>
{#code_begin|test|test_multidimensional_arrays#}
const std = @import("std");
const expect = std.testing.expect;
const mat4x4 = [4][4]f32{
[_]f32{ 1.0, 0.0, 0.0, 0.0 },
[_]f32{ 0.0, 1.0, 0.0, 1.0 },
[_]f32{ 0.0, 0.0, 1.0, 0.0 },
[_]f32{ 0.0, 0.0, 0.0, 1.0 },
};
test "multidimensional arrays" {
// Access the 2D array by indexing the outer array, and then the inner array.
try expect(mat4x4[1][1] == 1.0);
// Here we iterate with for loops.
for (mat4x4, 0..) |row, row_index| {
for (row, 0..) |cell, column_index| {
if (row_index == column_index) {
try expect(cell == 1.0);
}
}
}
}
{#code_end#}
{#header_close#}
{#header_open|Sentinel-Terminated Arrays#}
<p>
The syntax {#syntax#}[N:x]T{#endsyntax#} describes an array which has a sentinel element of value {#syntax#}x{#endsyntax#} at the
index corresponding to the length {#syntax#}N{#endsyntax#}.
</p>
{#code_begin|test|test_null_terminated_array#}
const std = @import("std");
const expect = std.testing.expect;
test "0-terminated sentinel array" {
const array = [_:0]u8 {1, 2, 3, 4};
try expect(@TypeOf(array) == [4:0]u8);
try expect(array.len == 4);
try expect(array[4] == 0);
}
test "extra 0s in 0-terminated sentinel array" {
// The sentinel value may appear earlier, but does not influence the compile-time 'len'.
const array = [_:0]u8 {1, 0, 0, 4};
try expect(@TypeOf(array) == [4:0]u8);
try expect(array.len == 4);
try expect(array[4] == 0);
}
{#code_end#}
{#see_also|Sentinel-Terminated Pointers|Sentinel-Terminated Slices#}
{#header_close#}
{#header_close#}
{#header_open|Vectors#}
<p>
A vector is a group of booleans, {#link|Integers#}, {#link|Floats#}, or
{#link|Pointers#} which are operated on in parallel, using SIMD instructions if possible.
Vector types are created with the builtin function {#link|@Vector#}.
</p>
<p>
Vectors support the same builtin operators as their underlying base types.
These operations are performed element-wise, and return a vector of the same length
as the input vectors. This includes:
</p>
<ul>
<li>Arithmetic ({#syntax#}+{#endsyntax#}, {#syntax#}-{#endsyntax#}, {#syntax#}/{#endsyntax#}, {#syntax#}*{#endsyntax#},
{#syntax#}@divFloor{#endsyntax#}, {#syntax#}@sqrt{#endsyntax#}, {#syntax#}@ceil{#endsyntax#},
{#syntax#}@log{#endsyntax#}, etc.)</li>
<li>Bitwise operators ({#syntax#}>>{#endsyntax#}, {#syntax#}<<{#endsyntax#}, {#syntax#}&{#endsyntax#},
{#syntax#}|{#endsyntax#}, {#syntax#}~{#endsyntax#}, etc.)</li>
<li>Comparison operators ({#syntax#}<{#endsyntax#}, {#syntax#}>{#endsyntax#}, {#syntax#}=={#endsyntax#}, etc.)</li>
</ul>
<p>
It is prohibited to use a math operator on a mixture of scalars (individual numbers)
and vectors. Zig provides the {#link|@splat#} builtin to easily convert from scalars
to vectors, and it supports {#link|@reduce#} and array indexing syntax to convert
from vectors to scalars. Vectors also support assignment to and from fixed-length
arrays with comptime-known length.
</p>
<p>
For rearranging elements within and between vectors, Zig provides the {#link|@shuffle#} and {#link|@select#} functions.
</p>
<p>
Operations on vectors shorter than the target machine's native SIMD size will typically compile to single SIMD
instructions, while vectors longer than the target machine's native SIMD size will compile to multiple SIMD
instructions. If a given operation doesn't have SIMD support on the target architecture, the compiler will default
to operating on each vector element one at a time. Zig supports any comptime-known vector length up to 2^32-1,
although small powers of two (2-64) are most typical. Note that excessively long vector lengths (e.g. 2^20) may
result in compiler crashes on current versions of Zig.
</p>
{#code_begin|test|test_vector#}
const std = @import("std");
const expectEqual = std.testing.expectEqual;
test "Basic vector usage" {
// Vectors have a compile-time known length and base type.
const a = @Vector(4, i32){ 1, 2, 3, 4 };
const b = @Vector(4, i32){ 5, 6, 7, 8 };
// Math operations take place element-wise.
const c = a + b;
// Individual vector elements can be accessed using array indexing syntax.
try expectEqual(6, c[0]);
try expectEqual(8, c[1]);
try expectEqual(10, c[2]);
try expectEqual(12, c[3]);
}
test "Conversion between vectors, arrays, and slices" {
// Vectors and fixed-length arrays can be automatically assigned back and forth
const arr1: [4]f32 = [_]f32{ 1.1, 3.2, 4.5, 5.6 };
const vec: @Vector(4, f32) = arr1;
const arr2: [4]f32 = vec;
try expectEqual(arr1, arr2);
// You can also assign from a slice with comptime-known length to a vector using .*
const vec2: @Vector(2, f32) = arr1[1..3].*;
const slice: []const f32 = &arr1;
var offset: u32 = 1; // var to make it runtime-known
_ = &offset; // suppress 'var is never mutated' error
// To extract a comptime-known length from a runtime-known offset,
// first extract a new slice from the starting offset, then an array of
// comptime-known length
const vec3: @Vector(2, f32) = slice[offset..][0..2].*;
try expectEqual(slice[offset], vec2[0]);
try expectEqual(slice[offset + 1], vec2[1]);
try expectEqual(vec2, vec3);
}
{#code_end#}
<p>
TODO talk about C ABI interop<br>
TODO consider suggesting std.MultiArrayList
</p>
{#see_also|@splat|@shuffle|@select|@reduce#}
{#header_close#}
{#header_open|Pointers#}
<p>
Zig has two kinds of pointers: single-item and many-item.
</p>
<ul>
<li>{#syntax#}*T{#endsyntax#} - single-item pointer to exactly one item.
<ul>
<li>Supports deref syntax: {#syntax#}ptr.*{#endsyntax#}</li>
</ul>
</li>
<li>{#syntax#}[*]T{#endsyntax#} - many-item pointer to unknown number of items.
<ul>
<li>Supports index syntax: {#syntax#}ptr[i]{#endsyntax#}</li>
<li>Supports slice syntax: {#syntax#}ptr[start..end]{#endsyntax#} and {#syntax#}ptr[start..]{#endsyntax#}</li>
<li>Supports pointer arithmetic: {#syntax#}ptr + x{#endsyntax#}, {#syntax#}ptr - x{#endsyntax#}</li>
<li>{#syntax#}T{#endsyntax#} must have a known size, which means that it cannot be
{#syntax#}anyopaque{#endsyntax#} or any other {#link|opaque type|opaque#}.</li>
</ul>
</li>
</ul>
<p>These types are closely related to {#link|Arrays#} and {#link|Slices#}:</p>
<ul>
<li>{#syntax#}*[N]T{#endsyntax#} - pointer to N items, same as single-item pointer to an array.
<ul>
<li>Supports index syntax: {#syntax#}array_ptr[i]{#endsyntax#}</li>
<li>Supports slice syntax: {#syntax#}array_ptr[start..end]{#endsyntax#}</li>
<li>Supports len property: {#syntax#}array_ptr.len{#endsyntax#}</li>
</ul>
</li>
</ul>
<ul>
<li>{#syntax#}[]T{#endsyntax#} - is a slice (a fat pointer, which contains a pointer of type {#syntax#}[*]T{#endsyntax#} and a length).
<ul>
<li>Supports index syntax: {#syntax#}slice[i]{#endsyntax#}</li>
<li>Supports slice syntax: {#syntax#}slice[start..end]{#endsyntax#}</li>
<li>Supports len property: {#syntax#}slice.len{#endsyntax#}</li>
</ul>
</li>
</ul>
<p>Use {#syntax#}&x{#endsyntax#} to obtain a single-item pointer:</p>
{#code_begin|test|test_single_item_pointer#}
const expect = @import("std").testing.expect;
test "address of syntax" {
// Get the address of a variable:
const x: i32 = 1234;
const x_ptr = &x;
// Dereference a pointer:
try expect(x_ptr.* == 1234);
// When you get the address of a const variable, you get a const single-item pointer.
try expect(@TypeOf(x_ptr) == *const i32);
// If you want to mutate the value, you'd need an address of a mutable variable:
var y: i32 = 5678;
const y_ptr = &y;
try expect(@TypeOf(y_ptr) == *i32);
y_ptr.* += 1;
try expect(y_ptr.* == 5679);
}
test "pointer array access" {
// Taking an address of an individual element gives a
// single-item pointer. This kind of pointer
// does not support pointer arithmetic.
var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
const ptr = &array[2];
try expect(@TypeOf(ptr) == *u8);
try expect(array[2] == 3);
ptr.* += 1;
try expect(array[2] == 4);
}
{#code_end#}
<p>
Zig supports pointer arithmetic. It's better to assign the pointer to {#syntax#}[*]T{#endsyntax#} and increment that variable. For example, directly incrementing the pointer from a slice will corrupt it.
</p>
{#code_begin|test|test_pointer_arithmetic#}
const expect = @import("std").testing.expect;
test "pointer arithmetic with many-item pointer" {
const array = [_]i32{ 1, 2, 3, 4 };
var ptr: [*]const i32 = &array;
try expect(ptr[0] == 1);
ptr += 1;
try expect(ptr[0] == 2);
// slicing a many-item pointer without an end is equivalent to
// pointer arithmetic: `ptr[start..] == ptr + start`
try expect(ptr[1..] == ptr + 1);
}
test "pointer arithmetic with slices" {
var array = [_]i32{ 1, 2, 3, 4 };
var length: usize = 0; // var to make it runtime-known
_ = &length; // suppress 'var is never mutated' error
var slice = array[length..array.len];
try expect(slice[0] == 1);
try expect(slice.len == 4);
slice.ptr += 1;
// now the slice is in an bad state since len has not been updated
try expect(slice[0] == 2);
try expect(slice.len == 4);
}
{#code_end#}
<p>
In Zig, we generally prefer {#link|Slices#} rather than {#link|Sentinel-Terminated Pointers#}.
You can turn an array or pointer into a slice using slice syntax.
</p>
<p>
Slices have bounds checking and are therefore protected
against this kind of undefined behavior. This is one reason
we prefer slices to pointers.
</p>
{#code_begin|test|test_slice_bounds#}
const expect = @import("std").testing.expect;
test "pointer slicing" {
var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
var start: usize = 2; // var to make it runtime-known
_ = &start; // suppress 'var is never mutated' error
const slice = array[start..4];
try expect(slice.len == 2);
try expect(array[3] == 4);
slice[1] += 1;
try expect(array[3] == 5);
}
{#code_end#}
<p>Pointers work at compile-time too, as long as the code does not depend on
an undefined memory layout:</p>
{#code_begin|test|test_comptime_pointers#}
const expect = @import("std").testing.expect;
test "comptime pointers" {
comptime {
var x: i32 = 1;
const ptr = &x;
ptr.* += 1;
x += 1;
try expect(ptr.* == 3);
}
}
{#code_end#}
<p>To convert an integer address into a pointer, use {#syntax#}@ptrFromInt{#endsyntax#}.
To convert a pointer to an integer, use {#syntax#}@intFromPtr{#endsyntax#}:</p>
{#code_begin|test|test_integer_pointer_conversion#}
const expect = @import("std").testing.expect;
test "@intFromPtr and @ptrFromInt" {
const ptr: *i32 = @ptrFromInt(0xdeadbee0);
const addr = @intFromPtr(ptr);
try expect(@TypeOf(addr) == usize);
try expect(addr == 0xdeadbee0);
}
{#code_end#}
<p>Zig is able to preserve memory addresses in comptime code, as long as
the pointer is never dereferenced:</p>
{#code_begin|test|test_comptime_pointer_conversion#}
const expect = @import("std").testing.expect;
test "comptime @ptrFromInt" {
comptime {
// Zig is able to do this at compile-time, as long as
// ptr is never dereferenced.
const ptr: *i32 = @ptrFromInt(0xdeadbee0);
const addr = @intFromPtr(ptr);
try expect(@TypeOf(addr) == usize);
try expect(addr == 0xdeadbee0);
}
}
{#code_end#}
{#see_also|Optional Pointers|@ptrFromInt|@intFromPtr|C Pointers#}
{#header_open|volatile#}
<p>Loads and stores are assumed to not have side effects. If a given load or store
should have side effects, such as Memory Mapped Input/Output (MMIO), use {#syntax#}volatile{#endsyntax#}.
In the following code, loads and stores with {#syntax#}mmio_ptr{#endsyntax#} are guaranteed to all happen
and in the same order as in source code:</p>
{#code_begin|test|test_volatile#}
const expect = @import("std").testing.expect;
test "volatile" {
const mmio_ptr: *volatile u8 = @ptrFromInt(0x12345678);
try expect(@TypeOf(mmio_ptr) == *volatile u8);
}
{#code_end#}
<p>
Note that {#syntax#}volatile{#endsyntax#} is unrelated to concurrency and {#link|Atomics#}.
If you see code that is using {#syntax#}volatile{#endsyntax#} for something other than Memory Mapped
Input/Output, it is probably a bug.
</p>
{#header_close#}
<p>
{#link|@ptrCast#} converts a pointer's element type to another. This
creates a new pointer that can cause undetectable illegal behavior
depending on the loads and stores that pass through it. Generally, other
kinds of type conversions are preferable to
{#syntax#}@ptrCast{#endsyntax#} if possible.
</p>
{#code_begin|test|test_pointer_casting#}
const std = @import("std");
const expect = std.testing.expect;
test "pointer casting" {
const bytes align(@alignOf(u32)) = [_]u8{ 0x12, 0x12, 0x12, 0x12 };
const u32_ptr: *const u32 = @ptrCast(&bytes);
try expect(u32_ptr.* == 0x12121212);
// Even this example is contrived - there are better ways to do the above than
// pointer casting. For example, using a slice narrowing cast:
const u32_value = std.mem.bytesAsSlice(u32, bytes[0..])[0];
try expect(u32_value == 0x12121212);
// And even another way, the most straightforward way to do it:
try expect(@as(u32, @bitCast(bytes)) == 0x12121212);
}
test "pointer child type" {
// pointer types have a `child` field which tells you the type they point to.
try expect(@typeInfo(*u32).Pointer.child == u32);
}
{#code_end#}
{#header_open|Alignment#}
<p>
Each type has an <strong>alignment</strong> - a number of bytes such that,
when a value of the type is loaded from or stored to memory,
the memory address must be evenly divisible by this number. You can use
{#link|@alignOf#} to find out this value for any type.
</p>
<p>
Alignment depends on the CPU architecture, but is always a power of two, and
less than {#syntax#}1 << 29{#endsyntax#}.
</p>
<p>
In Zig, a pointer type has an alignment value. If the value is equal to the
alignment of the underlying type, it can be omitted from the type:
</p>
{#code_begin|test|test_variable_alignment#}
const std = @import("std");
const builtin = @import("builtin");
const expect = std.testing.expect;
test "variable alignment" {
var x: i32 = 1234;
const align_of_i32 = @alignOf(@TypeOf(x));
try expect(@TypeOf(&x) == *i32);
try expect(*i32 == *align(align_of_i32) i32);
if (builtin.target.cpu.arch == .x86_64) {
try expect(@typeInfo(*i32).Pointer.alignment == 4);
}
}
{#code_end#}
<p>In the same way that a {#syntax#}*i32{#endsyntax#} can be {#link|coerced|Type Coercion#} to a
{#syntax#}*const i32{#endsyntax#}, a pointer with a larger alignment can be implicitly
cast to a pointer with a smaller alignment, but not vice versa.
</p>
<p>
You can specify alignment on variables and functions. If you do this, then
pointers to them get the specified alignment:
</p>
{#code_begin|test|test_variable_func_alignment#}
const expect = @import("std").testing.expect;
var foo: u8 align(4) = 100;
test "global variable alignment" {
try expect(@typeInfo(@TypeOf(&foo)).Pointer.alignment == 4);
try expect(@TypeOf(&foo) == *align(4) u8);
const as_pointer_to_array: *align(4) [1]u8 = &foo;
const as_slice: []align(4) u8 = as_pointer_to_array;
const as_unaligned_slice: []u8 = as_slice;
try expect(as_unaligned_slice[0] == 100);
}
fn derp() align(@sizeOf(usize) * 2) i32 {
return 1234;
}
fn noop1() align(1) void {}
fn noop4() align(4) void {}
test "function alignment" {
try expect(derp() == 1234);
try expect(@TypeOf(derp) == fn () i32);
try expect(@TypeOf(&derp) == *align(@sizeOf(usize) * 2) const fn () i32);
noop1();
try expect(@TypeOf(noop1) == fn () void);
try expect(@TypeOf(&noop1) == *align(1) const fn () void);
noop4();
try expect(@TypeOf(noop4) == fn () void);
try expect(@TypeOf(&noop4) == *align(4) const fn () void);
}
{#code_end#}
<p>
If you have a pointer or a slice that has a small alignment, but you know that it actually
has a bigger alignment, use {#link|@alignCast#} to change the
pointer into a more aligned pointer. This is a no-op at runtime, but inserts a
{#link|safety check|Incorrect Pointer Alignment#}:
</p>
{#code_begin|test_safety|test_incorrect_pointer_alignment|incorrect alignment#}
const std = @import("std");
test "pointer alignment safety" {
var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
const bytes = std.mem.sliceAsBytes(array[0..]);
try std.testing.expect(foo(bytes) == 0x11111111);
}
fn foo(bytes: []u8) u32 {
const slice4 = bytes[1..5];
const int_slice = std.mem.bytesAsSlice(u32, @as([]align(4) u8, @alignCast(slice4)));
return int_slice[0];
}
{#code_end#}
{#header_close#}
{#header_open|allowzero#}
<p>
This pointer attribute allows a pointer to have address zero. This is only ever needed on the
freestanding OS target, where the address zero is mappable. If you want to represent null pointers, use
{#link|Optional Pointers#} instead. {#link|Optional Pointers#} with {#syntax#}allowzero{#endsyntax#}
are not the same size as pointers. In this code example, if the pointer
did not have the {#syntax#}allowzero{#endsyntax#} attribute, this would be a
{#link|Pointer Cast Invalid Null#} panic:
</p>
{#code_begin|test|test_allowzero#}
const std = @import("std");
const expect = std.testing.expect;
test "allowzero" {
var zero: usize = 0; // var to make to runtime-known
_ = &zero; // suppress 'var is never mutated' error
const ptr: *allowzero i32 = @ptrFromInt(zero);
try expect(@intFromPtr(ptr) == 0);
}
{#code_end#}
{#header_close#}
{#header_open|Sentinel-Terminated Pointers#}
<p>
The syntax {#syntax#}[*:x]T{#endsyntax#} describes a pointer that
has a length determined by a sentinel value. This provides protection
against buffer overflow and overreads.
</p>
{#code_begin|exe_build_err|sentinel-terminated_pointer#}
{#link_libc#}
const std = @import("std");
// This is also available as `std.c.printf`.
pub extern "c" fn printf(format: [*:0]const u8, ...) c_int;
pub fn main() anyerror!void {
_ = printf("Hello, world!\n"); // OK
const msg = "Hello, world!\n";
const non_null_terminated_msg: [msg.len]u8 = msg.*;
_ = printf(&non_null_terminated_msg);
}
{#code_end#}
{#see_also|Sentinel-Terminated Slices|Sentinel-Terminated Arrays#}
{#header_close#}
{#header_close#}
{#header_open|Slices#}
<p>
A slice is a pointer and a length. The difference between an array and
a slice is that the array's length is part of the type and known at
compile-time, whereas the slice's length is known at runtime.
Both can be accessed with the `len` field.
</p>
{#code_begin|test_safety|test_basic_slices|index out of bounds#}
const expect = @import("std").testing.expect;
test "basic slices" {
var array = [_]i32{ 1, 2, 3, 4 };
var known_at_runtime_zero: usize = 0;
_ = &known_at_runtime_zero;
const slice = array[known_at_runtime_zero..array.len];
try expect(@TypeOf(slice) == []i32);
try expect(&slice[0] == &array[0]);
try expect(slice.len == array.len);
// If you slice with comptime-known start and end positions, the result is
// a pointer to an array, rather than a slice.
const array_ptr = array[0..array.len];
try expect(@TypeOf(array_ptr) == *[array.len]i32);
// You can perform a slice-by-length by slicing twice. This allows the compiler
// to perform some optimisations like recognising a comptime-known length when
// the start position is only known at runtime.
var runtime_start: usize = 1;
_ = &runtime_start;
const length = 2;
const array_ptr_len = array[runtime_start..][0..length];
try expect(@TypeOf(array_ptr_len) == *[length]i32);
// Using the address-of operator on a slice gives a single-item pointer.
try expect(@TypeOf(&slice[0]) == *i32);
// Using the `ptr` field gives a many-item pointer.
try expect(@TypeOf(slice.ptr) == [*]i32);
try expect(@intFromPtr(slice.ptr) == @intFromPtr(&slice[0]));
// Slices have array bounds checking. If you try to access something out
// of bounds, you'll get a safety check failure:
slice[10] += 1;
// Note that `slice.ptr` does not invoke safety checking, while `&slice[0]`
// asserts that the slice has len > 0.
}
{#code_end#}
<p>This is one reason we prefer slices to pointers.</p>
{#code_begin|test|test_slices#}
const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;
const fmt = std.fmt;
test "using slices for strings" {
// Zig has no concept of strings. String literals are const pointers
// to null-terminated arrays of u8, and by convention parameters
// that are "strings" are expected to be UTF-8 encoded slices of u8.
// Here we coerce *const [5:0]u8 and *const [6:0]u8 to []const u8
const hello: []const u8 = "hello";
const world: []const u8 = "世界";
var all_together: [100]u8 = undefined;
// You can use slice syntax with at least one runtime-known index on an
// array to convert an array into a slice.
var start: usize = 0;
_ = &start;
const all_together_slice = all_together[start..];
// String concatenation example.
const hello_world = try fmt.bufPrint(all_together_slice, "{s} {s}", .{ hello, world });
// Generally, you can use UTF-8 and not worry about whether something is a
// string. If you don't need to deal with individual characters, no need
// to decode.
try expect(mem.eql(u8, hello_world, "hello 世界"));
}
test "slice pointer" {
var array: [10]u8 = undefined;
const ptr = &array;
try expect(@TypeOf(ptr) == *[10]u8);
// A pointer to an array can be sliced just like an array:
var start: usize = 0;
var end: usize = 5;
_ = .{ &start, &end };
const slice = ptr[start..end];
// The slice is mutable because we sliced a mutable pointer.
try expect(@TypeOf(slice) == []u8);
slice[2] = 3;
try expect(array[2] == 3);
// Again, slicing with comptime-known indexes will produce another pointer
// to an array:
const ptr2 = slice[2..3];
try expect(ptr2.len == 1);
try expect(ptr2[0] == 3);
try expect(@TypeOf(ptr2) == *[1]u8);
}
{#code_end#}
{#see_also|Pointers|for|Arrays#}
{#header_open|Sentinel-Terminated Slices#}
<p>
The syntax {#syntax#}[:x]T{#endsyntax#} is a slice which has a runtime-known length
and also guarantees a sentinel value at the element indexed by the length. The type does not
guarantee that there are no sentinel elements before that. Sentinel-terminated slices allow element
access to the {#syntax#}len{#endsyntax#} index.
</p>
{#code_begin|test|test_null_terminated_slice#}
const std = @import("std");
const expect = std.testing.expect;
test "0-terminated slice" {
const slice: [:0]const u8 = "hello";
try expect(slice.len == 5);
try expect(slice[5] == 0);
}
{#code_end#}
<p>
Sentinel-terminated slices can also be created using a variation of the slice syntax
{#syntax#}data[start..end :x]{#endsyntax#}, where {#syntax#}data{#endsyntax#} is a many-item pointer,
array or slice and {#syntax#}x{#endsyntax#} is the sentinel value.
</p>
{#code_begin|test|test_null_terminated_slicing#}
const std = @import("std");
const expect = std.testing.expect;
test "0-terminated slicing" {
var array = [_]u8{ 3, 2, 1, 0, 3, 2, 1, 0 };
var runtime_length: usize = 3;
_ = &runtime_length;
const slice = array[0..runtime_length :0];
try expect(@TypeOf(slice) == [:0]u8);
try expect(slice.len == 3);
}
{#code_end#}
<p>
Sentinel-terminated slicing asserts that the element in the sentinel position of the backing data is
actually the sentinel value. If this is not the case, safety-protected {#link|Undefined Behavior#} results.
</p>
{#code_begin|test_safety|test_sentinel_mismatch|sentinel mismatch#}
const std = @import("std");
const expect = std.testing.expect;
test "sentinel mismatch" {
var array = [_]u8{ 3, 2, 1, 0 };
// Creating a sentinel-terminated slice from the array with a length of 2
// will result in the value `1` occupying the sentinel element position.
// This does not match the indicated sentinel value of `0` and will lead
// to a runtime panic.
var runtime_length: usize = 2;
_ = &runtime_length;
const slice = array[0..runtime_length :0];
_ = slice;
}
{#code_end#}
{#see_also|Sentinel-Terminated Pointers|Sentinel-Terminated Arrays#}
{#header_close#}
{#header_close#}
{#header_open|struct#}
{#code_begin|test|test_structs#}
// Declare a struct.
// Zig gives no guarantees about the order of fields and the size of
// the struct but the fields are guaranteed to be ABI-aligned.
const Point = struct {
x: f32,
y: f32,
};
// Maybe we want to pass it to OpenGL so we want to be particular about
// how the bytes are arranged.
const Point2 = packed struct {
x: f32,
y: f32,
};
// Declare an instance of a struct.
const p = Point {
.x = 0.12,
.y = 0.34,
};
// Maybe we're not ready to fill out some of the fields.
var p2 = Point {
.x = 0.12,
.y = undefined,
};
// Structs can have methods
// Struct methods are not special, they are only namespaced
// functions that you can call with dot syntax.
const Vec3 = struct {
x: f32,
y: f32,
z: f32,
pub fn init(x: f32, y: f32, z: f32) Vec3 {
return Vec3 {
.x = x,
.y = y,
.z = z,
};
}
pub fn dot(self: Vec3, other: Vec3) f32 {
return self.x * other.x + self.y * other.y + self.z * other.z;
}
};
const expect = @import("std").testing.expect;
test "dot product" {
const v1 = Vec3.init(1.0, 0.0, 0.0);
const v2 = Vec3.init(0.0, 1.0, 0.0);
try expect(v1.dot(v2) == 0.0);
// Other than being available to call with dot syntax, struct methods are
// not special. You can reference them as any other declaration inside
// the struct:
try expect(Vec3.dot(v1, v2) == 0.0);
}
// Structs can have declarations.
// Structs can have 0 fields.
const Empty = struct {
pub const PI = 3.14;
};
test "struct namespaced variable" {
try expect(Empty.PI == 3.14);
try expect(@sizeOf(Empty) == 0);
// you can still instantiate an empty struct
const does_nothing = Empty {};
_ = does_nothing;
}
// struct field order is determined by the compiler for optimal performance.
// however, you can still calculate a struct base pointer given a field pointer:
fn setYBasedOnX(x: *f32, y: f32) void {
const point: *Point = @fieldParentPtr("x", x);
point.y = y;
}
test "field parent pointer" {
var point = Point {
.x = 0.1234,
.y = 0.5678,
};
setYBasedOnX(&point.x, 0.9);
try expect(point.y == 0.9);
}
// You can return a struct from a function. This is how we do generics
// in Zig:
fn LinkedList(comptime T: type) type {
return struct {
pub const Node = struct {
prev: ?*Node,
next: ?*Node,
data: T,
};
first: ?*Node,
last: ?*Node,
len: usize,
};
}
test "linked list" {
// Functions called at compile-time are memoized. This means you can
// do this:
try expect(LinkedList(i32) == LinkedList(i32));
const list = LinkedList(i32){
.first = null,
.last = null,
.len = 0,
};
try expect(list.len == 0);
// Since types are first class values you can instantiate the type
// by assigning it to a variable:
const ListOfInts = LinkedList(i32);
try expect(ListOfInts == LinkedList(i32));
var node = ListOfInts.Node{
.prev = null,
.next = null,
.data = 1234,
};
const list2 = LinkedList(i32){
.first = &node,
.last = &node,
.len = 1,
};
// When using a pointer to a struct, fields can be accessed directly,
// without explicitly dereferencing the pointer.
// So you can do
try expect(list2.first.?.data == 1234);
// instead of try expect(list2.first.?.*.data == 1234);
}
{#code_end#}
{#header_open|Default Field Values#}
<p>
Each struct field may have an expression indicating the default field
value. Such expressions are executed at {#link|comptime#}, and allow the
field to be omitted in a struct literal expression:
</p>
{#code_begin|test|struct_default_field_values#}
const Foo = struct {
a: i32 = 1234,
b: i32,
};
test "default struct initialization fields" {
const x: Foo = .{
.b = 5,
};
if (x.a + x.b != 1239) {
comptime unreachable;
}
}
{#code_end#}
<p>
Default field values are only appropriate when the data invariants of a struct
cannot be violated by omitting that field from an initialization.
</p>
<p>
For example, here is an inappropriate use of default struct field initialization:
</p>
{#code_begin|exe_err|bad_default_value#}
const Threshold = struct {
minimum: f32 = 0.25,
maximum: f32 = 0.75,
const Category = enum { low, medium, high };
fn categorize(t: Threshold, value: f32) Category {
assert(t.maximum >= t.minimum);
if (value < t.minimum) return .low;
if (value > t.maximum) return .high;
return .medium;
}
};
pub fn main() !void {
var threshold: Threshold = .{
.maximum = 0.20,
};
const category = threshold.categorize(0.90);
try std.io.getStdOut().writeAll(@tagName(category));
}
const std = @import("std");
const assert = std.debug.assert;
{#code_end#}
<p>
Above you can see the danger of ignoring this principle. The default
field values caused the data invariant to be violated, causing illegal
behavior.
</p>
<p>
To fix this, remove the default values from all the struct fields, and provide
a named default value:
</p>
{#code_begin|syntax|struct_default_value#}
const Threshold = struct {
minimum: f32,
maximum: f32,
const default: Threshold = .{
.minimum = 0.25,
.maximum = 0.75,
};
};
{#code_end#}
<p>If a struct value requires a runtime-known value in order to be initialized
without violating data invariants, then use an initialization method that accepts
those runtime values, and populates the remaining fields.</p>
{#header_close#}
{#header_open|extern struct#}
<p>An {#syntax#}extern struct{#endsyntax#} has in-memory layout matching
the C ABI for the target.</p>
<p>If well-defined in-memory layout is not required, {#link|struct#} is a better choice
because it places fewer restrictions on the compiler.</p>
<p>See {#link|packed struct#} for a struct that has the ABI of its backing integer,
which can be useful for modeling flags.</p>
{#see_also|extern union|extern enum#}
{#header_close#}
{#header_open|packed struct#}
<p>
Unlike normal structs, {#syntax#}packed{#endsyntax#} structs have guaranteed in-memory layout:
</p>
<ul>
<li>Fields remain in the order declared, least to most significant.</li>
<li>There is no padding between fields.</li>
<li>Zig supports arbitrary width {#link|Integers#} and although normally, integers with fewer
than 8 bits will still use 1 byte of memory, in packed structs, they use
exactly their bit width.
</li>
<li>{#syntax#}bool{#endsyntax#} fields use exactly 1 bit.</li>
<li>An {#link|enum#} field uses exactly the bit width of its integer tag type.</li>
<li>A {#link|packed union#} field uses exactly the bit width of the union field with
the largest bit width.</li>
</ul>
<p>
This means that a {#syntax#}packed struct{#endsyntax#} can participate
in a {#link|@bitCast#} or a {#link|@ptrCast#} to reinterpret memory.
This even works at {#link|comptime#}:
</p>
{#code_begin|test|test_packed_structs#}
const std = @import("std");
const native_endian = @import("builtin").target.cpu.arch.endian();
const expect = std.testing.expect;
const Full = packed struct {
number: u16,
};
const Divided = packed struct {
half1: u8,
quarter3: u4,
quarter4: u4,
};
test "@bitCast between packed structs" {
try doTheTest();
try comptime doTheTest();
}
fn doTheTest() !void {
try expect(@sizeOf(Full) == 2);
try expect(@sizeOf(Divided) == 2);
const full = Full{ .number = 0x1234 };
const divided: Divided = @bitCast(full);
try expect(divided.half1 == 0x34);
try expect(divided.quarter3 == 0x2);
try expect(divided.quarter4 == 0x1);
const ordered: [2]u8 = @bitCast(full);
switch (native_endian) {
.big => {
try expect(ordered[0] == 0x12);
try expect(ordered[1] == 0x34);
},
.little => {
try expect(ordered[0] == 0x34);
try expect(ordered[1] == 0x12);
},
}
}
{#code_end#}
<p>
Zig allows the address to be taken of a non-byte-aligned field:
</p>
{#code_begin|test|test_pointer_to_non-byte_aligned_field#}
const std = @import("std");
const expect = std.testing.expect;
const BitField = packed struct {
a: u3,
b: u3,
c: u2,
};
var foo = BitField{
.a = 1,
.b = 2,
.c = 3,
};
test "pointer to non-byte-aligned field" {
const ptr = &foo.b;
try expect(ptr.* == 2);
}
{#code_end#}
<p>
However, the pointer to a non-byte-aligned field has special properties and cannot
be passed when a normal pointer is expected:
</p>
{#code_begin|test_err|test_misaligned_pointer|expected type#}
const std = @import("std");
const expect = std.testing.expect;
const BitField = packed struct {
a: u3,
b: u3,
c: u2,
};
var bit_field = BitField{
.a = 1,
.b = 2,
.c = 3,
};
test "pointer to non-byte-aligned field" {
try expect(bar(&bit_field.b) == 2);
}
fn bar(x: *const u3) u3 {
return x.*;
}
{#code_end#}
<p>
In this case, the function {#syntax#}bar{#endsyntax#} cannot be called because the pointer
to the non-ABI-aligned field mentions the bit offset, but the function expects an ABI-aligned pointer.
</p>
<p>
Pointers to non-ABI-aligned fields share the same address as the other fields within their host integer:
</p>
{#code_begin|test|test_packed_struct_field_address#}
const std = @import("std");
const expect = std.testing.expect;
const BitField = packed struct {
a: u3,
b: u3,
c: u2,
};
var bit_field = BitField{
.a = 1,
.b = 2,
.c = 3,
};
test "pointers of sub-byte-aligned fields share addresses" {
try expect(@intFromPtr(&bit_field.a) == @intFromPtr(&bit_field.b));
try expect(@intFromPtr(&bit_field.a) == @intFromPtr(&bit_field.c));
}
{#code_end#}
<p>
This can be observed with {#link|@bitOffsetOf#} and {#link|offsetOf#}:
</p>
{#code_begin|test|test_bitOffsetOf_offsetOf#}
const std = @import("std");
const expect = std.testing.expect;
const BitField = packed struct {
a: u3,
b: u3,
c: u2,
};
test "offsets of non-byte-aligned fields" {
comptime {
try expect(@bitOffsetOf(BitField, "a") == 0);
try expect(@bitOffsetOf(BitField, "b") == 3);
try expect(@bitOffsetOf(BitField, "c") == 6);
try expect(@offsetOf(BitField, "a") == 0);
try expect(@offsetOf(BitField, "b") == 0);
try expect(@offsetOf(BitField, "c") == 0);
}
}
{#code_end#}
<p>
Packed structs have the same alignment as their backing integer, however, overaligned
pointers to packed structs can override this:
</p>
{#code_begin|test|test_overaligned_packed_struct#}
const std = @import("std");
const expect = std.testing.expect;
const S = packed struct {
a: u32,
b: u32,
};
test "overaligned pointer to packed struct" {
var foo: S align(4) = .{ .a = 1, .b = 2 };
const ptr: *align(4) S = &foo;
const ptr_to_b: *u32 = &ptr.b;
try expect(ptr_to_b.* == 2);
}
{#code_end#}
<p>
It's also possible to set alignment of struct fields:
</p>
{#code_begin|test|test_aligned_struct_fields#}
const std = @import("std");
const expectEqual = std.testing.expectEqual;
test "aligned struct fields" {
const S = struct {
a: u32 align(2),
b: u32 align(64),
};
var foo = S{ .a = 1, .b = 2 };
try expectEqual(64, @alignOf(S));
try expectEqual(*align(2) u32, @TypeOf(&foo.a));
try expectEqual(*align(64) u32, @TypeOf(&foo.b));
}
{#code_end#}
<p>
Using packed structs with {#link|volatile#} is problematic, and may be a compile error in the future.
For details on this subscribe to
<a href="https://github.com/ziglang/zig/issues/1761">this issue</a>.
TODO update these docs with a recommendation on how to use packed structs with MMIO
(the use case for volatile packed structs) once this issue is resolved.
Don't worry, there will be a good solution for this use case in zig.
</p>
{#header_close#}
{#header_open|Struct Naming#}
<p>Since all structs are anonymous, Zig infers the type name based on a few rules.</p>
<ul>
<li>If the struct is in the initialization expression of a variable, it gets named after
that variable.</li>
<li>If the struct is in the {#syntax#}return{#endsyntax#} expression, it gets named after
the function it is returning from, with the parameter values serialized.</li>
<li>Otherwise, the struct gets a name such as <code>(filename.funcname.__struct_ID)</code>.</li>
<li>If the struct is declared inside another struct, it gets named after both the parent
struct and the name inferred by the previous rules, separated by a dot.</li>
</ul>
{#code_begin|exe|struct_name#}
const std = @import("std");
pub fn main() void {
const Foo = struct {};
std.debug.print("variable: {s}\n", .{@typeName(Foo)});
std.debug.print("anonymous: {s}\n", .{@typeName(struct {})});
std.debug.print("function: {s}\n", .{@typeName(List(i32))});
}
fn List(comptime T: type) type {
return struct {
x: T,
};
}
{#code_end#}
{#header_close#}
{#header_open|Anonymous Struct Literals#}
<p>
Zig allows omitting the struct type of a literal. When the result is {#link|coerced|Type Coercion#},
the struct literal will directly instantiate the {#link|result location|Result Location Semantics#},
with no copy:
</p>
{#code_begin|test|test_struct_result#}
const std = @import("std");
const expect = std.testing.expect;
const Point = struct {x: i32, y: i32};
test "anonymous struct literal" {
const pt: Point = .{
.x = 13,
.y = 67,
};
try expect(pt.x == 13);
try expect(pt.y == 67);
}
{#code_end#}
<p>
The struct type can be inferred. Here the {#link|result location|Result Location Semantics#}
does not include a type, and so Zig infers the type:
</p>
{#code_begin|test|test_anonymous_struct#}
const std = @import("std");
const expect = std.testing.expect;
test "fully anonymous struct" {
try check(.{
.int = @as(u32, 1234),
.float = @as(f64, 12.34),
.b = true,
.s = "hi",
});
}
fn check(args: anytype) !void {
try expect(args.int == 1234);
try expect(args.float == 12.34);
try expect(args.b);
try expect(args.s[0] == 'h');
try expect(args.s[1] == 'i');
}
{#code_end#}
{#header_close#}
{#header_open|Tuples#}
<p>
Anonymous structs can be created without specifying field names, and are referred to as "tuples".
</p>
<p>
The fields are implicitly named using numbers starting from 0. Because their names are integers,
they cannot be accessed with {#syntax#}.{#endsyntax#} syntax without also wrapping them in
{#syntax#}@""{#endsyntax#}. Names inside {#syntax#}@""{#endsyntax#} are always recognised as
{#link|identifiers|Identifiers#}.
</p>
<p>
Like arrays, tuples have a .len field, can be indexed (provided the index is comptime-known)
and work with the ++ and ** operators. They can also be iterated over with {#link|inline for#}.
</p>
{#code_begin|test|test_tuples#}
const std = @import("std");
const expect = std.testing.expect;
test "tuple" {
const values = .{
@as(u32, 1234),
@as(f64, 12.34),
true,
"hi",
} ++ .{false} ** 2;
try expect(values[0] == 1234);
try expect(values[4] == false);
inline for (values, 0..) |v, i| {
if (i != 2) continue;
try expect(v);
}
try expect(values.len == 6);
try expect(values.@"3"[0] == 'h');
}
{#code_end#}
{#header_close#}
{#see_also|comptime|@fieldParentPtr#}
{#header_close#}
{#header_open|enum#}
{#code_begin|test|test_enums#}
const expect = @import("std").testing.expect;
const mem = @import("std").mem;
// Declare an enum.
const Type = enum {
ok,
not_ok,
};
// Declare a specific enum field.
const c = Type.ok;
// If you want access to the ordinal value of an enum, you
// can specify the tag type.
const Value = enum(u2) {
zero,
one,
two,
};
// Now you can cast between u2 and Value.
// The ordinal value starts from 0, counting up by 1 from the previous member.
test "enum ordinal value" {
try expect(@intFromEnum(Value.zero) == 0);
try expect(@intFromEnum(Value.one) == 1);
try expect(@intFromEnum(Value.two) == 2);
}
// You can override the ordinal value for an enum.
const Value2 = enum(u32) {
hundred = 100,
thousand = 1000,
million = 1000000,
};
test "set enum ordinal value" {
try expect(@intFromEnum(Value2.hundred) == 100);
try expect(@intFromEnum(Value2.thousand) == 1000);
try expect(@intFromEnum(Value2.million) == 1000000);
}
// You can also override only some values.
const Value3 = enum(u4) {
a,
b = 8,
c,
d = 4,
e,
};
test "enum implicit ordinal values and overridden values" {
try expect(@intFromEnum(Value3.a) == 0);
try expect(@intFromEnum(Value3.b) == 8);
try expect(@intFromEnum(Value3.c) == 9);
try expect(@intFromEnum(Value3.d) == 4);
try expect(@intFromEnum(Value3.e) == 5);
}
// Enums can have methods, the same as structs and unions.
// Enum methods are not special, they are only namespaced
// functions that you can call with dot syntax.
const Suit = enum {
clubs,
spades,
diamonds,
hearts,
pub fn isClubs(self: Suit) bool {
return self == Suit.clubs;
}
};
test "enum method" {
const p = Suit.spades;
try expect(!p.isClubs());
}
// An enum can be switched upon.
const Foo = enum {
string,
number,
none,
};
test "enum switch" {
const p = Foo.number;
const what_is_it = switch (p) {
Foo.string => "this is a string",
Foo.number => "this is a number",
Foo.none => "this is a none",
};
try expect(mem.eql(u8, what_is_it, "this is a number"));
}
// @typeInfo can be used to access the integer tag type of an enum.
const Small = enum {
one,
two,
three,
four,
};
test "std.meta.Tag" {
try expect(@typeInfo(Small).Enum.tag_type == u2);
}
// @typeInfo tells us the field count and the fields names:
test "@typeInfo" {
try expect(@typeInfo(Small).Enum.fields.len == 4);
try expect(mem.eql(u8, @typeInfo(Small).Enum.fields[1].name, "two"));
}
// @tagName gives a [:0]const u8 representation of an enum value:
test "@tagName" {
try expect(mem.eql(u8, @tagName(Small.three), "three"));
}
{#code_end#}
{#see_also|@typeInfo|@tagName|@sizeOf#}
{#header_open|extern enum#}
<p>
By default, enums are not guaranteed to be compatible with the C ABI:
</p>
{#code_begin|obj_err|enum_export_error|parameter of type 'enum_export_error.Foo' not allowed in function with calling convention 'C'#}
const Foo = enum { a, b, c };
export fn entry(foo: Foo) void { _ = foo; }
{#code_end#}
<p>
For a C-ABI-compatible enum, provide an explicit tag type to
the enum:
</p>
{#code_begin|obj|enum_export#}
const Foo = enum(c_int) { a, b, c };
export fn entry(foo: Foo) void { _ = foo; }
{#code_end#}
{#header_close#}
{#header_open|Enum Literals#}
<p>
Enum literals allow specifying the name of an enum field without specifying the enum type:
</p>
{#code_begin|test|test_enum_literals#}
const std = @import("std");
const expect = std.testing.expect;
const Color = enum {
auto,
off,
on,
};
test "enum literals" {
const color1: Color = .auto;
const color2 = Color.auto;
try expect(color1 == color2);
}
test "switch using enum literals" {
const color = Color.on;
const result = switch (color) {
.auto => false,
.on => true,
.off => false,
};
try expect(result);
}
{#code_end#}
{#header_close#}
{#header_open|Non-exhaustive enum#}
<p>
A non-exhaustive enum can be created by adding a trailing {#syntax#}_{#endsyntax#} field.
The enum must specify a tag type and cannot consume every enumeration value.
</p>
<p>
{#link|@enumFromInt#} on a non-exhaustive enum involves the safety semantics
of {#link|@intCast#} to the integer tag type, but beyond that always results in
a well-defined enum value.
</p>
<p>
A switch on a non-exhaustive enum can include a {#syntax#}_{#endsyntax#} prong as an alternative to an {#syntax#}else{#endsyntax#} prong.
With a {#syntax#}_{#endsyntax#} prong the compiler errors if all the known tag names are not handled by the switch.
</p>
{#code_begin|test|test_switch_non-exhaustive#}
const std = @import("std");
const expect = std.testing.expect;
const Number = enum(u8) {
one,
two,
three,
_,
};
test "switch on non-exhaustive enum" {
const number = Number.one;
const result = switch (number) {
.one => true,
.two,
.three => false,
_ => false,
};
try expect(result);
const is_one = switch (number) {
.one => true,
else => false,
};
try expect(is_one);
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|union#}
<p>
A bare {#syntax#}union{#endsyntax#} defines a set of possible types that a value
can be as a list of fields. Only one field can be active at a time.
The in-memory representation of bare unions is not guaranteed.
Bare unions cannot be used to reinterpret memory. For that, use {#link|@ptrCast#},
or use an {#link|extern union#} or a {#link|packed union#} which have
guaranteed in-memory layout.
{#link|Accessing the non-active field|Wrong Union Field Access#} is
safety-checked {#link|Undefined Behavior#}:
</p>
{#code_begin|test_err|test_wrong_union_access|access of union field 'float' while field 'int' is active#}
const Payload = union {
int: i64,
float: f64,
boolean: bool,
};
test "simple union" {
var payload = Payload{ .int = 1234 };
payload.float = 12.34;
}
{#code_end#}
<p>You can activate another field by assigning the entire union:</p>
{#code_begin|test|test_simple_union#}
const std = @import("std");
const expect = std.testing.expect;
const Payload = union {
int: i64,
float: f64,
boolean: bool,
};
test "simple union" {
var payload = Payload{ .int = 1234 };
try expect(payload.int == 1234);
payload = Payload{ .float = 12.34 };
try expect(payload.float == 12.34);
}
{#code_end#}
<p>
In order to use {#link|switch#} with a union, it must be a {#link|Tagged union#}.
</p>
<p>
To initialize a union when the tag is a {#link|comptime#}-known name, see {#link|@unionInit#}.
</p>
{#header_open|Tagged union#}
<p>Unions can be declared with an enum tag type.
This turns the union into a <em>tagged</em> union, which makes it eligible
to use with {#link|switch#} expressions.
Tagged unions coerce to their tag type: {#link|Type Coercion: Unions and Enums#}.
</p>
{#code_begin|test|test_tagged_union#}
const std = @import("std");
const expect = std.testing.expect;
const ComplexTypeTag = enum {
ok,
not_ok,
};
const ComplexType = union(ComplexTypeTag) {
ok: u8,
not_ok: void,
};
test "switch on tagged union" {
const c = ComplexType{ .ok = 42 };
try expect(@as(ComplexTypeTag, c) == ComplexTypeTag.ok);
switch (c) {
ComplexTypeTag.ok => |value| try expect(value == 42),
ComplexTypeTag.not_ok => unreachable,
}
}
test "get tag type" {
try expect(std.meta.Tag(ComplexType) == ComplexTypeTag);
}
{#code_end#}
<p>In order to modify the payload of a tagged union in a switch expression,
place a {#syntax#}*{#endsyntax#} before the variable name to make it a pointer:
</p>
{#code_begin|test|test_switch_modify_tagged_union#}
const std = @import("std");
const expect = std.testing.expect;
const ComplexTypeTag = enum {
ok,
not_ok,
};
const ComplexType = union(ComplexTypeTag) {
ok: u8,
not_ok: void,
};
test "modify tagged union in switch" {
var c = ComplexType{ .ok = 42 };
switch (c) {
ComplexTypeTag.ok => |*value| value.* += 1,
ComplexTypeTag.not_ok => unreachable,
}
try expect(c.ok == 43);
}
{#code_end#}
<p>
Unions can be made to infer the enum tag type.
Further, unions can have methods just like structs and enums.
</p>
{#code_begin|test|test_union_method#}
const std = @import("std");
const expect = std.testing.expect;
const Variant = union(enum) {
int: i32,
boolean: bool,
// void can be omitted when inferring enum tag type.
none,
fn truthy(self: Variant) bool {
return switch (self) {
Variant.int => |x_int| x_int != 0,
Variant.boolean => |x_bool| x_bool,
Variant.none => false,
};
}
};
test "union method" {
var v1 = Variant{ .int = 1 };
var v2 = Variant{ .boolean = false };
try expect(v1.truthy());
try expect(!v2.truthy());
}
{#code_end#}
<p>
{#link|@tagName#} can be used to return a {#link|comptime#}
{#syntax#}[:0]const u8{#endsyntax#} value representing the field name:
</p>
{#code_begin|test|test_tagName#}
const std = @import("std");
const expect = std.testing.expect;
const Small2 = union(enum) {
a: i32,
b: bool,
c: u8,
};
test "@tagName" {
try expect(std.mem.eql(u8, @tagName(Small2.a), "a"));
}
{#code_end#}
{#header_close#}
{#header_open|extern union#}
<p>
An {#syntax#}extern union{#endsyntax#} has memory layout guaranteed to be compatible with
the target C ABI.
</p>
{#see_also|extern struct#}
{#header_close#}
{#header_open|packed union#}
<p>A {#syntax#}packed union{#endsyntax#} has well-defined in-memory layout and is eligible
to be in a {#link|packed struct#}.</p>
{#header_close#}
{#header_open|Anonymous Union Literals#}
<p>{#link|Anonymous Struct Literals#} syntax can be used to initialize unions without specifying
the type:</p>
{#code_begin|test|test_anonymous_union#}
const std = @import("std");
const expect = std.testing.expect;
const Number = union {
int: i32,
float: f64,
};
test "anonymous union literal syntax" {
const i: Number = .{ .int = 42 };
const f = makeNumber();
try expect(i.int == 42);
try expect(f.float == 12.34);
}
fn makeNumber() Number {
return .{ .float = 12.34 };
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|opaque#}
<p>
{#syntax#}opaque {}{#endsyntax#} declares a new type with an unknown (but non-zero) size and alignment.
It can contain declarations the same as {#link|structs|struct#}, {#link|unions|union#},
and {#link|enums|enum#}.
</p>
<p>
This is typically used for type safety when interacting with C code that does not expose struct details.
Example:
</p>
{#code_begin|test_err|test_opaque|expected type '*test_opaque.Derp', found '*test_opaque.Wat'#}
const Derp = opaque {};
const Wat = opaque {};
extern fn bar(d: *Derp) void;
fn foo(w: *Wat) callconv(.C) void {
bar(w);
}
test "call foo" {
foo(undefined);
}
{#code_end#}
{#header_close#}
{#header_open|Blocks#}
<p>
Blocks are used to limit the scope of variable declarations:
</p>
{#code_begin|test_err|test_blocks|use of undeclared identifier 'x'#}
test "access variable after block scope" {
{
var x: i32 = 1;
_ = &x;
}
x += 1;
}
{#code_end#}
<p>Blocks are expressions. When labeled, {#syntax#}break{#endsyntax#} can be used
to return a value from the block:
</p>
{#code_begin|test|test_labeled_break#}
const std = @import("std");
const expect = std.testing.expect;
test "labeled break from labeled block expression" {
var y: i32 = 123;
const x = blk: {
y += 1;
break :blk y;
};
try expect(x == 124);
try expect(y == 124);
}
{#code_end#}
<p>Here, {#syntax#}blk{#endsyntax#} can be any name.</p>
{#see_also|Labeled while|Labeled for#}
{#header_open|Shadowing#}
<p>{#link|Identifiers#} are never allowed to "hide" other identifiers by using the same name:</p>
{#code_begin|test_err|test_shadowing|local variable shadows declaration#}
const pi = 3.14;
test "inside test block" {
// Let's even go inside another block
{
var pi: i32 = 1234;
}
}
{#code_end#}
<p>
Because of this, when you read Zig code you can always rely on an identifier to consistently mean
the same thing within the scope it is defined. Note that you can, however, use the same name if
the scopes are separate:
</p>
{#code_begin|test|test_scopes#}
test "separate scopes" {
{
const pi = 3.14;
_ = pi;
}
{
var pi: bool = true;
_ = &pi;
}
}
{#code_end#}
{#header_close#}
{#header_open|Empty Blocks#}
<p>An empty block is equivalent to {#syntax#}void{}{#endsyntax#}:</p>
{#code_begin|test|test_empty_block#}
const std = @import("std");
const expect = std.testing.expect;
test {
const a = {};
const b = void{};
try expect(@TypeOf(a) == void);
try expect(@TypeOf(b) == void);
try expect(a == b);
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|switch#}
{#code_begin|test|test_switch#}
const std = @import("std");
const builtin = @import("builtin");
const expect = std.testing.expect;
test "switch simple" {
const a: u64 = 10;
const zz: u64 = 103;
// All branches of a switch expression must be able to be coerced to a
// common type.
//
// Branches cannot fallthrough. If fallthrough behavior is desired, combine
// the cases and use an if.
const b = switch (a) {
// Multiple cases can be combined via a ','
1, 2, 3 => 0,
// Ranges can be specified using the ... syntax. These are inclusive
// of both ends.
5...100 => 1,
// Branches can be arbitrarily complex.
101 => blk: {
const c: u64 = 5;
break :blk c * 2 + 1;
},
// Switching on arbitrary expressions is allowed as long as the
// expression is known at compile-time.
zz => zz,
blk: {
const d: u32 = 5;
const e: u32 = 100;
break :blk d + e;
} => 107,
// The else branch catches everything not already captured.
// Else branches are mandatory unless the entire range of values
// is handled.
else => 9,
};
try expect(b == 1);
}
// Switch expressions can be used outside a function:
const os_msg = switch (builtin.target.os.tag) {
.linux => "we found a linux user",
else => "not a linux user",
};
// Inside a function, switch statements implicitly are compile-time
// evaluated if the target expression is compile-time known.
test "switch inside function" {
switch (builtin.target.os.tag) {
.fuchsia => {
// On an OS other than fuchsia, block is not even analyzed,
// so this compile error is not triggered.
// On fuchsia this compile error would be triggered.
@compileError("fuchsia not supported");
},
else => {},
}
}
{#code_end#}
<p>
{#syntax#}switch{#endsyntax#} can be used to capture the field values
of a {#link|Tagged union#}. Modifications to the field values can be
done by placing a {#syntax#}*{#endsyntax#} before the capture variable name,
turning it into a pointer.
</p>
{#code_begin|test|test_switch_tagged_union#}
const expect = @import("std").testing.expect;
test "switch on tagged union" {
const Point = struct {
x: u8,
y: u8,
};
const Item = union(enum) {
a: u32,
c: Point,
d,
e: u32,
};
var a = Item{ .c = Point{ .x = 1, .y = 2 } };
// Switching on more complex enums is allowed.
const b = switch (a) {
// A capture group is allowed on a match, and will return the enum
// value matched. If the payload types of both cases are the same
// they can be put into the same switch prong.
Item.a, Item.e => |item| item,
// A reference to the matched value can be obtained using `*` syntax.
Item.c => |*item| blk: {
item.*.x += 1;
break :blk 6;
},
// No else is required if the types cases was exhaustively handled
Item.d => 8,
};
try expect(b == 6);
try expect(a.c.x == 2);
}
{#code_end#}
{#see_also|comptime|enum|@compileError|Compile Variables#}
{#header_open|Exhaustive Switching#}
<p>
When a {#syntax#}switch{#endsyntax#} expression does not have an {#syntax#}else{#endsyntax#} clause,
it must exhaustively list all the possible values. Failure to do so is a compile error:
</p>
{#code_begin|test_err|test_unhandled_enumeration_value|unhandled enumeration value#}
const Color = enum {
auto,
off,
on,
};
test "exhaustive switching" {
const color = Color.off;
switch (color) {
Color.auto => {},
Color.on => {},
}
}
{#code_end#}
{#header_close#}
{#header_open|Switching with Enum Literals#}
<p>
{#link|Enum Literals#} can be useful to use with {#syntax#}switch{#endsyntax#} to avoid
repetitively specifying {#link|enum#} or {#link|union#} types:
</p>
{#code_begin|test|test_exhaustive_switch#}
const std = @import("std");
const expect = std.testing.expect;
const Color = enum {
auto,
off,
on,
};
test "enum literals with switch" {
const color = Color.off;
const result = switch (color) {
.auto => false,
.on => false,
.off => true,
};
try expect(result);
}
{#code_end#}
{#header_close#}
{#header_open|Inline Switch Prongs#}
<p>
Switch prongs can be marked as {#syntax#}inline{#endsyntax#} to generate
the prong's body for each possible value it could have, making the
captured value {#link|comptime#}.
</p>
{#code_begin|test|test_inline_switch#}
const std = @import("std");
const expect = std.testing.expect;
const expectError = std.testing.expectError;
fn isFieldOptional(comptime T: type, field_index: usize) !bool {
const fields = @typeInfo(T).Struct.fields;
return switch (field_index) {
// This prong is analyzed twice with `idx` being a
// comptime-known value each time.
inline 0, 1 => |idx| @typeInfo(fields[idx].type) == .Optional,
else => return error.IndexOutOfBounds,
};
}
const Struct1 = struct { a: u32, b: ?u32 };
test "using @typeInfo with runtime values" {
var index: usize = 0;
try expect(!try isFieldOptional(Struct1, index));
index += 1;
try expect(try isFieldOptional(Struct1, index));
index += 1;
try expectError(error.IndexOutOfBounds, isFieldOptional(Struct1, index));
}
// Calls to `isFieldOptional` on `Struct1` get unrolled to an equivalent
// of this function:
fn isFieldOptionalUnrolled(field_index: usize) !bool {
return switch (field_index) {
0 => false,
1 => true,
else => return error.IndexOutOfBounds,
};
}
{#code_end#}
<p>The {#syntax#}inline{#endsyntax#} keyword may also be combined with ranges:</p>
{#code_begin|syntax|inline_prong_range#}
fn isFieldOptional(comptime T: type, field_index: usize) !bool {
const fields = @typeInfo(T).Struct.fields;
return switch (field_index) {
inline 0...fields.len - 1 => |idx| @typeInfo(fields[idx].type) == .Optional,
else => return error.IndexOutOfBounds,
};
}
{#code_end#}
<p>
{#syntax#}inline else{#endsyntax#} prongs can be used as a type safe
alternative to {#syntax#}inline for{#endsyntax#} loops:
</p>
{#code_begin|test|test_inline_else#}
const std = @import("std");
const expect = std.testing.expect;
const SliceTypeA = extern struct {
len: usize,
ptr: [*]u32,
};
const SliceTypeB = extern struct {
ptr: [*]SliceTypeA,
len: usize,
};
const AnySlice = union(enum) {
a: SliceTypeA,
b: SliceTypeB,
c: []const u8,
d: []AnySlice,
};
fn withFor(any: AnySlice) usize {
const Tag = @typeInfo(AnySlice).Union.tag_type.?;
inline for (@typeInfo(Tag).Enum.fields) |field| {
// With `inline for` the function gets generated as
// a series of `if` statements relying on the optimizer
// to convert it to a switch.
if (field.value == @intFromEnum(any)) {
return @field(any, field.name).len;
}
}
// When using `inline for` the compiler doesn't know that every
// possible case has been handled requiring an explicit `unreachable`.
unreachable;
}
fn withSwitch(any: AnySlice) usize {
return switch (any) {
// With `inline else` the function is explicitly generated
// as the desired switch and the compiler can check that
// every possible case is handled.
inline else => |slice| slice.len,
};
}
test "inline for and inline else similarity" {
const any = AnySlice{ .c = "hello" };
try expect(withFor(any) == 5);
try expect(withSwitch(any) == 5);
}
{#code_end#}
<p>
When using an inline prong switching on an union an additional
capture can be used to obtain the union's enum tag value.
</p>
{#code_begin|test|test_inline_switch_union_tag#}
const std = @import("std");
const expect = std.testing.expect;
const U = union(enum) {
a: u32,
b: f32,
};
fn getNum(u: U) u32 {
switch (u) {
// Here `num` is a runtime-known value that is either
// `u.a` or `u.b` and `tag` is `u`'s comptime-known tag value.
inline else => |num, tag| {
if (tag == .b) {
return @intFromFloat(num);
}
return num;
}
}
}
test "test" {
const u = U{ .b = 42 };
try expect(getNum(u) == 42);
}
{#code_end#}
{#see_also|inline while|inline for#}
{#header_close#}
{#header_close#}
{#header_open|while#}
<p>
A while loop is used to repeatedly execute an expression until
some condition is no longer true.
</p>
{#code_begin|test|test_while#}
const expect = @import("std").testing.expect;
test "while basic" {
var i: usize = 0;
while (i < 10) {
i += 1;
}
try expect(i == 10);
}
{#code_end#}
<p>
Use {#syntax#}break{#endsyntax#} to exit a while loop early.
</p>
{#code_begin|test|test_while_break#}
const expect = @import("std").testing.expect;
test "while break" {
var i: usize = 0;
while (true) {
if (i == 10)
break;
i += 1;
}
try expect(i == 10);
}
{#code_end#}
<p>
Use {#syntax#}continue{#endsyntax#} to jump back to the beginning of the loop.
</p>
{#code_begin|test|test_while_continue#}
const expect = @import("std").testing.expect;
test "while continue" {
var i: usize = 0;
while (true) {
i += 1;
if (i < 10)
continue;
break;
}
try expect(i == 10);
}
{#code_end#}
<p>
While loops support a continue expression which is executed when the loop
is continued. The {#syntax#}continue{#endsyntax#} keyword respects this expression.
</p>
{#code_begin|test|test_while_continue_expression#}
const expect = @import("std").testing.expect;
test "while loop continue expression" {
var i: usize = 0;
while (i < 10) : (i += 1) {}
try expect(i == 10);
}
test "while loop continue expression, more complicated" {
var i: usize = 1;
var j: usize = 1;
while (i * j < 2000) : ({ i *= 2; j *= 3; }) {
const my_ij = i * j;
try expect(my_ij < 2000);
}
}
{#code_end#}
<p>
While loops are expressions. The result of the expression is the
result of the {#syntax#}else{#endsyntax#} clause of a while loop, which is executed when
the condition of the while loop is tested as false.
</p>
<p>
{#syntax#}break{#endsyntax#}, like {#syntax#}return{#endsyntax#}, accepts a value
parameter. This is the result of the {#syntax#}while{#endsyntax#} expression.
When you {#syntax#}break{#endsyntax#} from a while loop, the {#syntax#}else{#endsyntax#} branch is not
evaluated.
</p>
{#code_begin|test|test_while_else#}
const expect = @import("std").testing.expect;
test "while else" {
try expect(rangeHasNumber(0, 10, 5));
try expect(!rangeHasNumber(0, 10, 15));
}
fn rangeHasNumber(begin: usize, end: usize, number: usize) bool {
var i = begin;
return while (i < end) : (i += 1) {
if (i == number) {
break true;
}
} else false;
}
{#code_end#}
{#header_open|Labeled while#}
<p>When a {#syntax#}while{#endsyntax#} loop is labeled, it can be referenced from a {#syntax#}break{#endsyntax#}
or {#syntax#}continue{#endsyntax#} from within a nested loop:</p>
{#code_begin|test|test_while_nested_break#}
test "nested break" {
outer: while (true) {
while (true) {
break :outer;
}
}
}
test "nested continue" {
var i: usize = 0;
outer: while (i < 10) : (i += 1) {
while (true) {
continue :outer;
}
}
}
{#code_end#}
{#header_close#}
{#header_open|while with Optionals#}
<p>
Just like {#link|if#} expressions, while loops can take an optional as the
condition and capture the payload. When {#link|null#} is encountered the loop
exits.
</p>
<p>
When the {#syntax#}|x|{#endsyntax#} syntax is present on a {#syntax#}while{#endsyntax#} expression,
the while condition must have an {#link|Optional Type#}.
</p>
<p>
The {#syntax#}else{#endsyntax#} branch is allowed on optional iteration. In this case, it will
be executed on the first null value encountered.
</p>
{#code_begin|test|test_while_null_capture#}
const expect = @import("std").testing.expect;
test "while null capture" {
var sum1: u32 = 0;
numbers_left = 3;
while (eventuallyNullSequence()) |value| {
sum1 += value;
}
try expect(sum1 == 3);
// null capture with an else block
var sum2: u32 = 0;
numbers_left = 3;
while (eventuallyNullSequence()) |value| {
sum2 += value;
} else {
try expect(sum2 == 3);
}
// null capture with a continue expression
var i: u32 = 0;
var sum3: u32 = 0;
numbers_left = 3;
while (eventuallyNullSequence()) |value| : (i += 1) {
sum3 += value;
}
try expect(i == 3);
}
var numbers_left: u32 = undefined;
fn eventuallyNullSequence() ?u32 {
return if (numbers_left == 0) null else blk: {
numbers_left -= 1;
break :blk numbers_left;
};
}
{#code_end#}
{#header_close#}
{#header_open|while with Error Unions#}
<p>
Just like {#link|if#} expressions, while loops can take an error union as
the condition and capture the payload or the error code. When the
condition results in an error code the else branch is evaluated and
the loop is finished.
</p>
<p>
When the {#syntax#}else |x|{#endsyntax#} syntax is present on a {#syntax#}while{#endsyntax#} expression,
the while condition must have an {#link|Error Union Type#}.
</p>
{#code_begin|test|test_while_error_capture#}
const expect = @import("std").testing.expect;
test "while error union capture" {
var sum1: u32 = 0;
numbers_left = 3;
while (eventuallyErrorSequence()) |value| {
sum1 += value;
} else |err| {
try expect(err == error.ReachedZero);
}
}
var numbers_left: u32 = undefined;
fn eventuallyErrorSequence() anyerror!u32 {
return if (numbers_left == 0) error.ReachedZero else blk: {
numbers_left -= 1;
break :blk numbers_left;
};
}
{#code_end#}
{#header_close#}
{#header_open|inline while#}
<p>
While loops can be inlined. This causes the loop to be unrolled, which
allows the code to do some things which only work at compile time,
such as use types as first class values.
</p>
{#code_begin|test|test_inline_while#}
const expect = @import("std").testing.expect;
test "inline while loop" {
comptime var i = 0;
var sum: usize = 0;
inline while (i < 3) : (i += 1) {
const T = switch (i) {
0 => f32,
1 => i8,
2 => bool,
else => unreachable,
};
sum += typeNameLength(T);
}
try expect(sum == 9);
}
fn typeNameLength(comptime T: type) usize {
return @typeName(T).len;
}
{#code_end#}
<p>
It is recommended to use {#syntax#}inline{#endsyntax#} loops only for one of these reasons:
</p>
<ul>
<li>You need the loop to execute at {#link|comptime#} for the semantics to work.</li>
<li>
You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.
</li>
</ul>
{#header_close#}
{#see_also|if|Optionals|Errors|comptime|unreachable#}
{#header_close#}
{#header_open|for#}
{#code_begin|test|test_for#}
const expect = @import("std").testing.expect;
test "for basics" {
const items = [_]i32 { 4, 5, 3, 4, 0 };
var sum: i32 = 0;
// For loops iterate over slices and arrays.
for (items) |value| {
// Break and continue are supported.
if (value == 0) {
continue;
}
sum += value;
}
try expect(sum == 16);
// To iterate over a portion of a slice, reslice.
for (items[0..1]) |value| {
sum += value;
}
try expect(sum == 20);
// To access the index of iteration, specify a second condition as well
// as a second capture value.
var sum2: i32 = 0;
for (items, 0..) |_, i| {
try expect(@TypeOf(i) == usize);
sum2 += @as(i32, @intCast(i));
}
try expect(sum2 == 10);
// To iterate over consecutive integers, use the range syntax.
// Unbounded range is always a compile error.
var sum3 : usize = 0;
for (0..5) |i| {
sum3 += i;
}
try expect(sum3 == 10);
}
test "multi object for" {
const items = [_]usize{ 1, 2, 3 };
const items2 = [_]usize{ 4, 5, 6 };
var count: usize = 0;
// Iterate over multiple objects.
// All lengths must be equal at the start of the loop, otherwise detectable
// illegal behavior occurs.
for (items, items2) |i, j| {
count += i + j;
}
try expect(count == 21);
}
test "for reference" {
var items = [_]i32{ 3, 4, 2 };
// Iterate over the slice by reference by
// specifying that the capture value is a pointer.
for (&items) |*value| {
value.* += 1;
}
try expect(items[0] == 4);
try expect(items[1] == 5);
try expect(items[2] == 3);
}
test "for else" {
// For allows an else attached to it, the same as a while loop.
const items = [_]?i32{ 3, 4, null, 5 };
// For loops can also be used as expressions.
// Similar to while loops, when you break from a for loop, the else branch is not evaluated.
var sum: i32 = 0;
const result = for (items) |value| {
if (value != null) {
sum += value.?;
}
} else blk: {
try expect(sum == 12);
break :blk sum;
};
try expect(result == 12);
}
{#code_end#}
{#header_open|Labeled for#}
<p>When a {#syntax#}for{#endsyntax#} loop is labeled, it can be referenced from a {#syntax#}break{#endsyntax#}
or {#syntax#}continue{#endsyntax#} from within a nested loop:</p>
{#code_begin|test|test_for_nested_break#}
const std = @import("std");
const expect = std.testing.expect;
test "nested break" {
var count: usize = 0;
outer: for (1..6) |_| {
for (1..6) |_| {
count += 1;
break :outer;
}
}
try expect(count == 1);
}
test "nested continue" {
var count: usize = 0;
outer: for (1..9) |_| {
for (1..6) |_| {
count += 1;
continue :outer;
}
}
try expect(count == 8);
}
{#code_end#}
{#header_close#}
{#header_open|inline for#}
<p>
For loops can be inlined. This causes the loop to be unrolled, which
allows the code to do some things which only work at compile time,
such as use types as first class values.
The capture value and iterator value of inlined for loops are
compile-time known.
</p>
{#code_begin|test|test_inline_for#}
const expect = @import("std").testing.expect;
test "inline for loop" {
const nums = [_]i32{2, 4, 6};
var sum: usize = 0;
inline for (nums) |i| {
const T = switch (i) {
2 => f32,
4 => i8,
6 => bool,
else => unreachable,
};
sum += typeNameLength(T);
}
try expect(sum == 9);
}
fn typeNameLength(comptime T: type) usize {
return @typeName(T).len;
}
{#code_end#}
<p>
It is recommended to use {#syntax#}inline{#endsyntax#} loops only for one of these reasons:
</p>
<ul>
<li>You need the loop to execute at {#link|comptime#} for the semantics to work.</li>
<li>
You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.
</li>
</ul>
{#header_close#}
{#see_also|while|comptime|Arrays|Slices#}
{#header_close#}
{#header_open|if#}
{#code_begin|test|test_if#}
// If expressions have three uses, corresponding to the three types:
// * bool
// * ?T
// * anyerror!T
const expect = @import("std").testing.expect;
test "if expression" {
// If expressions are used instead of a ternary expression.
const a: u32 = 5;
const b: u32 = 4;
const result = if (a != b) 47 else 3089;
try expect(result == 47);
}
test "if boolean" {
// If expressions test boolean conditions.
const a: u32 = 5;
const b: u32 = 4;
if (a != b) {
try expect(true);
} else if (a == 9) {
unreachable;
} else {
unreachable;
}
}
test "if error union" {
// If expressions test for errors.
// Note the |err| capture on the else.
const a: anyerror!u32 = 0;
if (a) |value| {
try expect(value == 0);
} else |err| {
_ = err;
unreachable;
}
const b: anyerror!u32 = error.BadValue;
if (b) |value| {
_ = value;
unreachable;
} else |err| {
try expect(err == error.BadValue);
}
// The else and |err| capture is strictly required.
if (a) |value| {
try expect(value == 0);
} else |_| {}
// To check only the error value, use an empty block expression.
if (b) |_| {} else |err| {
try expect(err == error.BadValue);
}
// Access the value by reference using a pointer capture.
var c: anyerror!u32 = 3;
if (c) |*value| {
value.* = 9;
} else |_| {
unreachable;
}
if (c) |value| {
try expect(value == 9);
} else |_| {
unreachable;
}
}
{#code_end#}
{#header_open|if with Optionals#}
{#code_begin|test|test_if_optionals#}
const expect = @import("std").testing.expect;
test "if optional" {
// If expressions test for null.
const a: ?u32 = 0;
if (a) |value| {
try expect(value == 0);
} else {
unreachable;
}
const b: ?u32 = null;
if (b) |_| {
unreachable;
} else {
try expect(true);
}
// The else is not required.
if (a) |value| {
try expect(value == 0);
}
// To test against null only, use the binary equality operator.
if (b == null) {
try expect(true);
}
// Access the value by reference using a pointer capture.
var c: ?u32 = 3;
if (c) |*value| {
value.* = 2;
}
if (c) |value| {
try expect(value == 2);
} else {
unreachable;
}
}
test "if error union with optional" {
// If expressions test for errors before unwrapping optionals.
// The |optional_value| capture's type is ?u32.
const a: anyerror!?u32 = 0;
if (a) |optional_value| {
try expect(optional_value.? == 0);
} else |err| {
_ = err;
unreachable;
}
const b: anyerror!?u32 = null;
if (b) |optional_value| {
try expect(optional_value == null);
} else |_| {
unreachable;
}
const c: anyerror!?u32 = error.BadValue;
if (c) |optional_value| {
_ = optional_value;
unreachable;
} else |err| {
try expect(err == error.BadValue);
}
// Access the value by reference by using a pointer capture each time.
var d: anyerror!?u32 = 3;
if (d) |*optional_value| {
if (optional_value.*) |*value| {
value.* = 9;
}
} else |_| {
unreachable;
}
if (d) |optional_value| {
try expect(optional_value.? == 9);
} else |_| {
unreachable;
}
}
{#code_end#}
{#header_close#}
{#see_also|Optionals|Errors#}
{#header_close#}
{#header_open|defer#}
<p>Executes an expression unconditionally at scope exit.</p>
{#code_begin|test|test_defer#}
const std = @import("std");
const expect = std.testing.expect;
const print = std.debug.print;
fn deferExample() !usize {
var a: usize = 1;
{
defer a = 2;
a = 1;
}
try expect(a == 2);
a = 5;
return a;
}
test "defer basics" {
try expect((try deferExample()) == 5);
}
{#code_end#}
<p>Defer expressions are evaluated in reverse order.</p>
{#code_begin|test|defer_unwind#}
const std = @import("std");
const expect = std.testing.expect;
const print = std.debug.print;
test "defer unwinding" {
print("\n", .{});
defer {
print("1 ", .{});
}
defer {
print("2 ", .{});
}
if (false) {
// defers are not run if they are never executed.
defer {
print("3 ", .{});
}
}
}
{#code_end#}
<p>Inside a defer expression the return statement is not allowed.</p>
{#code_begin|test_err|test_invalid_defer|cannot return from defer expression#}
fn deferInvalidExample() !void {
defer {
return error.DeferError;
}
return error.DeferError;
}
{#code_end#}
{#see_also|Errors#}
{#header_close#}
{#header_open|unreachable#}
<p>
In {#link|Debug#} and {#link|ReleaseSafe#} mode
{#syntax#}unreachable{#endsyntax#} emits a call to {#syntax#}panic{#endsyntax#} with the message <code>reached unreachable code</code>.
</p>
<p>
In {#link|ReleaseFast#} and {#link|ReleaseSmall#} mode, the optimizer uses the assumption that {#syntax#}unreachable{#endsyntax#} code
will never be hit to perform optimizations.
</p>
{#header_open|Basics#}
{#code_begin|test|test_unreachable#}
// unreachable is used to assert that control flow will never reach a
// particular location:
test "basic math" {
const x = 1;
const y = 2;
if (x + y != 3) {
unreachable;
}
}
{#code_end#}
<p>In fact, this is how {#syntax#}std.debug.assert{#endsyntax#} is implemented:</p>
{#code_begin|test_err|test_assertion_failure#}
// This is how std.debug.assert is implemented
fn assert(ok: bool) void {
if (!ok) unreachable; // assertion failure
}
// This test will fail because we hit unreachable.
test "this will fail" {
assert(false);
}
{#code_end#}
{#header_close#}
{#header_open|At Compile-Time#}
{#code_begin|test_err|test_comptime_unreachable|unreachable code#}
const assert = @import("std").debug.assert;
test "type of unreachable" {
comptime {
// The type of unreachable is noreturn.
// However this assertion will still fail to compile because
// unreachable expressions are compile errors.
assert(@TypeOf(unreachable) == noreturn);
}
}
{#code_end#}
{#see_also|Zig Test|Build Mode|comptime#}
{#header_close#}
{#header_close#}
{#header_open|noreturn#}
<p>
{#syntax#}noreturn{#endsyntax#} is the type of:
</p>
<ul>
<li>{#syntax#}break{#endsyntax#}</li>
<li>{#syntax#}continue{#endsyntax#}</li>
<li>{#syntax#}return{#endsyntax#}</li>
<li>{#syntax#}unreachable{#endsyntax#}</li>
<li>{#syntax#}while (true) {}{#endsyntax#}</li>
</ul>
<p>When resolving types together, such as {#syntax#}if{#endsyntax#} clauses or {#syntax#}switch{#endsyntax#} prongs,
the {#syntax#}noreturn{#endsyntax#} type is compatible with every other type. Consider:
</p>
{#code_begin|test|test_noreturn#}
fn foo(condition: bool, b: u32) void {
const a = if (condition) b else return;
_ = a;
@panic("do something with a");
}
test "noreturn" {
foo(false, 1);
}
{#code_end#}
<p>Another use case for {#syntax#}noreturn{#endsyntax#} is the {#syntax#}exit{#endsyntax#} function:</p>
{#code_begin|test|test_noreturn_from_exit#}
{#target_windows#}
const std = @import("std");
const builtin = @import("builtin");
const native_arch = builtin.cpu.arch;
const expect = std.testing.expect;
const WINAPI: std.builtin.CallingConvention = if (native_arch == .x86) .Stdcall else .C;
extern "kernel32" fn ExitProcess(exit_code: c_uint) callconv(WINAPI) noreturn;
test "foo" {
const value = bar() catch ExitProcess(1);
try expect(value == 1234);
}
fn bar() anyerror!u32 {
return 1234;
}
{#code_end#}
{#header_close#}
{#header_open|Functions#}
{#code_begin|test|test_functions#}
const std = @import("std");
const builtin = @import("builtin");
const native_arch = builtin.cpu.arch;
const expect = std.testing.expect;
// Functions are declared like this
fn add(a: i8, b: i8) i8 {
if (a == 0) {
return b;
}
return a + b;
}
// The export specifier makes a function externally visible in the generated
// object file, and makes it use the C ABI.
export fn sub(a: i8, b: i8) i8 { return a - b; }
// The extern specifier is used to declare a function that will be resolved
// at link time, when linking statically, or at runtime, when linking
// dynamically. The quoted identifier after the extern keyword specifies
// the library that has the function. (e.g. "c" -> libc.so)
// The callconv specifier changes the calling convention of the function.
const WINAPI: std.builtin.CallingConvention = if (native_arch == .x86) .Stdcall else .C;
extern "kernel32" fn ExitProcess(exit_code: u32) callconv(WINAPI) noreturn;
extern "c" fn atan2(a: f64, b: f64) f64;
// The @setCold builtin tells the optimizer that a function is rarely called.
fn abort() noreturn {
@setCold(true);
while (true) {}
}
// The naked calling convention makes a function not have any function prologue or epilogue.
// This can be useful when integrating with assembly.
fn _start() callconv(.Naked) noreturn {
abort();
}
// The inline calling convention forces a function to be inlined at all call sites.
// If the function cannot be inlined, it is a compile-time error.
fn shiftLeftOne(a: u32) callconv(.Inline) u32 {
return a << 1;
}
// The pub specifier allows the function to be visible when importing.
// Another file can use @import and call sub2
pub fn sub2(a: i8, b: i8) i8 { return a - b; }
// Function pointers are prefixed with `*const `.
const Call2Op = *const fn (a: i8, b: i8) i8;
fn doOp(fnCall: Call2Op, op1: i8, op2: i8) i8 {
return fnCall(op1, op2);
}
test "function" {
try expect(doOp(add, 5, 6) == 11);
try expect(doOp(sub2, 5, 6) == -1);
}
{#code_end#}
<p>There is a difference between a function <em>body</em> and a function <em>pointer</em>.
Function bodies are {#link|comptime#}-only types while function {#link|Pointers#} may be
runtime-known.</p>
{#header_open|Pass-by-value Parameters#}
<p>
Primitive types such as {#link|Integers#} and {#link|Floats#} passed as parameters
are copied, and then the copy is available in the function body. This is called "passing by value".
Copying a primitive type is essentially free and typically involves nothing more than
setting a register.
</p>
<p>
Structs, unions, and arrays can sometimes be more efficiently passed as a reference, since a copy
could be arbitrarily expensive depending on the size. When these types are passed
as parameters, Zig may choose to copy and pass by value, or pass by reference, whichever way
Zig decides will be faster. This is made possible, in part, by the fact that parameters are immutable.
</p>
{#code_begin|test|test_pass_by_reference_or_value#}
const Point = struct {
x: i32,
y: i32,
};
fn foo(point: Point) i32 {
// Here, `point` could be a reference, or a copy. The function body
// can ignore the difference and treat it as a value. Be very careful
// taking the address of the parameter - it should be treated as if
// the address will become invalid when the function returns.
return point.x + point.y;
}
const expect = @import("std").testing.expect;
test "pass struct to function" {
try expect(foo(Point{ .x = 1, .y = 2 }) == 3);
}
{#code_end#}
<p>
For extern functions, Zig follows the C ABI for passing structs and unions by value.
</p>
{#header_close#}
{#header_open|Function Parameter Type Inference#}
<p>
Function parameters can be declared with {#syntax#}anytype{#endsyntax#} in place of the type.
In this case the parameter types will be inferred when the function is called.
Use {#link|@TypeOf#} and {#link|@typeInfo#} to get information about the inferred type.
</p>
{#code_begin|test|test_fn_type_inference#}
const expect = @import("std").testing.expect;
fn addFortyTwo(x: anytype) @TypeOf(x) {
return x + 42;
}
test "fn type inference" {
try expect(addFortyTwo(1) == 43);
try expect(@TypeOf(addFortyTwo(1)) == comptime_int);
const y: i64 = 2;
try expect(addFortyTwo(y) == 44);
try expect(@TypeOf(addFortyTwo(y)) == i64);
}
{#code_end#}
{#header_close#}
{#header_open|inline fn#}
<p>
Adding the {#syntax#}inline{#endsyntax#} keyword to a function definition makes that
function become <em>semantically inlined</em> at the callsite. This is
not a hint to be possibly observed by optimization passes, but has
implications on the types and values involved in the function call.
</p>
<p>
Unlike normal function calls, arguments at an inline function callsite which are
compile-time known are treated as {#link|Compile Time Parameters#}. This can potentially
propagate all the way to the return value:
</p>
{#code_begin|test|inline_call#}
test "inline function call" {
if (foo(1200, 34) != 1234) {
@compileError("bad");
}
}
inline fn foo(a: i32, b: i32) i32 {
return a + b;
}
{#code_end#}
<p>If {#syntax#}inline{#endsyntax#} is removed, the test fails with the compile error
instead of passing.</p>
<p>It is generally better to let the compiler decide when to inline a
function, except for these scenarios:</p>
<ul>
<li>To change how many stack frames are in the call stack, for debugging purposes.</li>
<li>To force comptime-ness of the arguments to propagate to the return value of the function, as in the above example.</li>
<li>Real world performance measurements demand it.</li>
</ul>
<p>Note that {#syntax#}inline{#endsyntax#} actually <em>restricts</em>
what the compiler is allowed to do. This can harm binary size,
compilation speed, and even runtime performance.</p>
{#header_close#}
{#header_open|Function Reflection#}
{#code_begin|test|test_fn_reflection#}
const std = @import("std");
const math = std.math;
const testing = std.testing;
test "fn reflection" {
try testing.expect(@typeInfo(@TypeOf(testing.expect)).Fn.params[0].type.? == bool);
try testing.expect(@typeInfo(@TypeOf(testing.tmpDir)).Fn.return_type.? == testing.TmpDir);
try testing.expect(@typeInfo(@TypeOf(math.Log2Int)).Fn.is_generic);
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Errors#}
{#header_open|Error Set Type#}
<p>
An error set is like an {#link|enum#}.
However, each error name across the entire compilation gets assigned an unsigned integer
greater than 0. You are allowed to declare the same error name more than once, and if you do, it
gets assigned the same integer value.
</p>
<p>
The error set type defaults to a {#syntax#}u16{#endsyntax#}, though if the maximum number of distinct
error values is provided via the <kbd>--error-limit [num]</kbd> command line parameter an integer type
with the minimum number of bits required to represent all of the error values will be used.
</p>
<p>
You can {#link|coerce|Type Coercion#} an error from a subset to a superset:
</p>
{#code_begin|test|test_coerce_error_subset_to_superset#}
const std = @import("std");
const FileOpenError = error {
AccessDenied,
OutOfMemory,
FileNotFound,
};
const AllocationError = error {
OutOfMemory,
};
test "coerce subset to superset" {
const err = foo(AllocationError.OutOfMemory);
try std.testing.expect(err == FileOpenError.OutOfMemory);
}
fn foo(err: AllocationError) FileOpenError {
return err;
}
{#code_end#}
<p>
But you cannot {#link|coerce|Type Coercion#} an error from a superset to a subset:
</p>
{#code_begin|test_err|test_coerce_error_superset_to_subset|not a member of destination error set#}
const FileOpenError = error {
AccessDenied,
OutOfMemory,
FileNotFound,
};
const AllocationError = error {
OutOfMemory,
};
test "coerce superset to subset" {
foo(FileOpenError.OutOfMemory) catch {};
}
fn foo(err: FileOpenError) AllocationError {
return err;
}
{#code_end#}
<p>
There is a shortcut for declaring an error set with only 1 value, and then getting that value:
</p>
{#code_begin|syntax|single_value_error_set_shortcut#}
const err = error.FileNotFound;
{#code_end#}
<p>This is equivalent to:</p>
{#code_begin|syntax|single_value_error_set#}
const err = (error {FileNotFound}).FileNotFound;
{#code_end#}
<p>
This becomes useful when using {#link|Inferred Error Sets#}.
</p>
{#header_open|The Global Error Set#}
<p>{#syntax#}anyerror{#endsyntax#} refers to the global error set.
This is the error set that contains all errors in the entire compilation unit.
It is a superset of all other error sets and a subset of none of them.
</p>
<p>
You can {#link|coerce|Type Coercion#} any error set to the global one, and you can explicitly
cast an error of the global error set to a non-global one. This inserts a language-level
assert to make sure the error value is in fact in the destination error set.
</p>
<p>
The global error set should generally be avoided because it prevents the
compiler from knowing what errors are possible at compile-time. Knowing
the error set at compile-time is better for generated documentation and
helpful error messages, such as forgetting a possible error value in a {#link|switch#}.
</p>
{#header_close#}
{#header_close#}
{#header_open|Error Union Type#}
<p>
An error set type and normal type can be combined with the {#syntax#}!{#endsyntax#}
binary operator to form an error union type. You are likely to use an
error union type more often than an error set type by itself.
</p>
<p>
Here is a function to parse a string into a 64-bit integer:
</p>
{#code_begin|test|error_union_parsing_u64#}
const std = @import("std");
const maxInt = std.math.maxInt;
pub fn parseU64(buf: []const u8, radix: u8) !u64 {
var x: u64 = 0;
for (buf) |c| {
const digit = charToDigit(c);
if (digit >= radix) {
return error.InvalidChar;
}
// x *= radix
var ov = @mulWithOverflow(x, radix);
if (ov[1] != 0) return error.OverFlow;
// x += digit
ov = @addWithOverflow(ov[0], digit);
if (ov[1] != 0) return error.OverFlow;
x = ov[0];
}
return x;
}
fn charToDigit(c: u8) u8 {
return switch (c) {
'0' ... '9' => c - '0',
'A' ... 'Z' => c - 'A' + 10,
'a' ... 'z' => c - 'a' + 10,
else => maxInt(u8),
};
}
test "parse u64" {
const result = try parseU64("1234", 10);
try std.testing.expect(result == 1234);
}
{#code_end#}
<p>
Notice the return type is {#syntax#}!u64{#endsyntax#}. This means that the function
either returns an unsigned 64 bit integer, or an error. We left off the error set
to the left of the {#syntax#}!{#endsyntax#}, so the error set is inferred.
</p>
<p>
Within the function definition, you can see some return statements that return
an error, and at the bottom a return statement that returns a {#syntax#}u64{#endsyntax#}.
Both types {#link|coerce|Type Coercion#} to {#syntax#}anyerror!u64{#endsyntax#}.
</p>
<p>
What it looks like to use this function varies depending on what you're
trying to do. One of the following:
</p>
<ul>
<li>You want to provide a default value if it returned an error.</li>
<li>If it returned an error then you want to return the same error.</li>
<li>You know with complete certainty it will not return an error, so want to unconditionally unwrap it.</li>
<li>You want to take a different action for each possible error.</li>
</ul>
{#header_open|catch#}
<p>If you want to provide a default value, you can use the {#syntax#}catch{#endsyntax#} binary operator:</p>
{#code_begin|syntax|catch#}
const parseU64 = @import("error_union_parsing_u64.zig").parseU64;
fn doAThing(str: []u8) void {
const number = parseU64(str, 10) catch 13;
_ = number; // ...
}
{#code_end#}
<p>
In this code, {#syntax#}number{#endsyntax#} will be equal to the successfully parsed string, or
a default value of 13. The type of the right hand side of the binary {#syntax#}catch{#endsyntax#} operator must
match the unwrapped error union type, or be of type {#syntax#}noreturn{#endsyntax#}.
</p>
<p>
If you want to provide a default value with
{#syntax#}catch{#endsyntax#} after performing some logic, you
can combine {#syntax#}catch{#endsyntax#} with named {#link|Blocks#}:
</p>
{#code_begin|syntax|handle_error_with_catch_block.zig#}
const parseU64 = @import("error_union_parsing_u64.zig").parseU64;
fn doAThing(str: []u8) void {
const number = parseU64(str, 10) catch blk: {
// do things
break :blk 13;
};
_ = number; // number is now initialized
}
{#code_end#}
{#header_close#}
{#header_open|try#}
<p>Let's say you wanted to return the error if you got one, otherwise continue with the
function logic:</p>
{#code_begin|syntax|catch_err_return#}
const parseU64 = @import("error_union_parsing_u64.zig").parseU64;
fn doAThing(str: []u8) !void {
const number = parseU64(str, 10) catch |err| return err;
_ = number; // ...
}
{#code_end#}
<p>
There is a shortcut for this. The {#syntax#}try{#endsyntax#} expression:
</p>
{#code_begin|syntax|try#}
const parseU64 = @import("error_union_parsing_u64.zig").parseU64;
fn doAThing(str: []u8) !void {
const number = try parseU64(str, 10);
_ = number; // ...
}
{#code_end#}
<p>
{#syntax#}try{#endsyntax#} evaluates an error union expression. If it is an error, it returns
from the current function with the same error. Otherwise, the expression results in
the unwrapped value.
</p>
{#header_close#}
<p>
Maybe you know with complete certainty that an expression will never be an error.
In this case you can do this:
</p>
{#syntax#}const number = parseU64("1234", 10) catch unreachable;{#endsyntax#}
<p>
Here we know for sure that "1234" will parse successfully. So we put the
{#syntax#}unreachable{#endsyntax#} value on the right hand side. {#syntax#}unreachable{#endsyntax#} generates
a panic in {#link|Debug#} and {#link|ReleaseSafe#} modes and undefined behavior in
{#link|ReleaseFast#} and {#link|ReleaseSmall#} modes. So, while we're debugging the
application, if there <em>was</em> a surprise error here, the application would crash
appropriately.
</p>
<p>
You may want to take a different action for every situation. For that, we combine
the {#link|if#} and {#link|switch#} expression:
</p>
{#syntax_block|zig|handle_all_error_scenarios.zig#}
fn doAThing(str: []u8) void {
if (parseU64(str, 10)) |number| {
doSomethingWithNumber(number);
} else |err| switch (err) {
error.Overflow => {
// handle overflow...
},
// we promise that InvalidChar won't happen (or crash in debug mode if it does)
error.InvalidChar => unreachable,
}
}
{#end_syntax_block#}
<p>
Finally, you may want to handle only some errors. For that, you can capture the unhandled
errors in the {#syntax#}else{#endsyntax#} case, which now contains a narrower error set:
</p>
{#syntax_block|zig|handle_some_error_scenarios.zig#}
fn doAnotherThing(str: []u8) error{InvaidChar}!void {
if (parseU64(str, 10)) |number| {
doSomethingWithNumber(number);
} else |err| switch (err) {
error.Overflow => {
// handle overflow...
},
else => |leftover_err| return leftover_err,
}
}
{#end_syntax_block#}
<p>
You must use the variable capture syntax. If you don't need the
variable, you can capture with {#syntax#}_{#endsyntax#} and avoid the
{#syntax#}switch{#endsyntax#}.
</p>
{#syntax_block|zig|handle_no_error_scenarios.zig#}
fn doADifferentThing(str: []u8) void {
if (parseU64(str, 10)) |number| {
doSomethingWithNumber(number);
} else |_| {
// do as you'd like
}
}
{#end_syntax_block#}
{#header_open|errdefer#}
<p>
The other component to error handling is defer statements.
In addition to an unconditional {#link|defer#}, Zig has {#syntax#}errdefer{#endsyntax#},
which evaluates the deferred expression on block exit path if and only if
the function returned with an error from the block.
</p>
<p>
Example:
</p>
{#syntax_block|zig|errdefer_example.zig#}
fn createFoo(param: i32) !Foo {
const foo = try tryToAllocateFoo();
// now we have allocated foo. we need to free it if the function fails.
// but we want to return it if the function succeeds.
errdefer deallocateFoo(foo);
const tmp_buf = allocateTmpBuffer() orelse return error.OutOfMemory;
// tmp_buf is truly a temporary resource, and we for sure want to clean it up
// before this block leaves scope
defer deallocateTmpBuffer(tmp_buf);
if (param > 1337) return error.InvalidParam;
// here the errdefer will not run since we're returning success from the function.
// but the defer will run!
return foo;
}
{#end_syntax_block#}
<p>
The neat thing about this is that you get robust error handling without
the verbosity and cognitive overhead of trying to make sure every exit path
is covered. The deallocation code is always directly following the allocation code.
</p>
{#header_close#}
{#header_open|Common errdefer Slip-Ups#}
<p>
It should be noted that {#syntax#}errdefer{#endsyntax#} statements only last until the end of the block
they are written in, and therefore are not run if an error is returned outside of that block:
</p>
{#code_begin|test_err|test_errdefer_slip_ups|1 tests leaked memory#}
const std = @import("std");
const Allocator = std.mem.Allocator;
const Foo = struct {
data: u32,
};
fn tryToAllocateFoo(allocator: Allocator) !*Foo {
return allocator.create(Foo);
}
fn deallocateFoo(allocator: Allocator, foo: *Foo) void {
allocator.destroy(foo);
}
fn getFooData() !u32 {
return 666;
}
fn createFoo(allocator: Allocator, param: i32) !*Foo {
const foo = getFoo: {
var foo = try tryToAllocateFoo(allocator);
errdefer deallocateFoo(allocator, foo); // Only lasts until the end of getFoo
// Calls deallocateFoo on error
foo.data = try getFooData();
break :getFoo foo;
};
// Outside of the scope of the errdefer, so
// deallocateFoo will not be called here
if (param > 1337) return error.InvalidParam;
return foo;
}
test "createFoo" {
try std.testing.expectError(error.InvalidParam, createFoo(std.testing.allocator, 2468));
}
{#code_end#}
<p>
To ensure that {#syntax#}deallocateFoo{#endsyntax#} is properly called
when returning an error, you must add an {#syntax#}errdefer{#endsyntax#} outside of the block:
</p>
{#code_begin|test|test_errdefer_block#}
const std = @import("std");
const Allocator = std.mem.Allocator;
const Foo = struct {
data: u32,
};
fn tryToAllocateFoo(allocator: Allocator) !*Foo {
return allocator.create(Foo);
}
fn deallocateFoo(allocator: Allocator, foo: *Foo) void {
allocator.destroy(foo);
}
fn getFooData() !u32 {
return 666;
}
fn createFoo(allocator: Allocator, param: i32) !*Foo {
const foo = getFoo: {
var foo = try tryToAllocateFoo(allocator);
errdefer deallocateFoo(allocator, foo);
foo.data = try getFooData();
break :getFoo foo;
};
// This lasts for the rest of the function
errdefer deallocateFoo(allocator, foo);
// Error is now properly handled by errdefer
if (param > 1337) return error.InvalidParam;
return foo;
}
test "createFoo" {
try std.testing.expectError(error.InvalidParam, createFoo(std.testing.allocator, 2468));
}
{#code_end#}
<p>
The fact that errdefers only last for the block they are declared in is
especially important when using loops:
</p>
{#code_begin|test_err|test_errdefer_loop_leak|3 errors were logged#}
const std = @import("std");
const Allocator = std.mem.Allocator;
const Foo = struct {
data: *u32
};
fn getData() !u32 {
return 666;
}
fn genFoos(allocator: Allocator, num: usize) ![]Foo {
const foos = try allocator.alloc(Foo, num);
errdefer allocator.free(foos);
for (foos, 0..) |*foo, i| {
foo.data = try allocator.create(u32);
// This errdefer does not last between iterations
errdefer allocator.destroy(foo.data);
// The data for the first 3 foos will be leaked
if(i >= 3) return error.TooManyFoos;
foo.data.* = try getData();
}
return foos;
}
test "genFoos" {
try std.testing.expectError(error.TooManyFoos, genFoos(std.testing.allocator, 5));
}
{#code_end#}
<p>
Special care must be taken with code that allocates in a loop
to make sure that no memory is leaked when returning an error:
</p>
{#code_begin|test|test_errdefer_loop#}
const std = @import("std");
const Allocator = std.mem.Allocator;
const Foo = struct {
data: *u32
};
fn getData() !u32 {
return 666;
}
fn genFoos(allocator: Allocator, num: usize) ![]Foo {
const foos = try allocator.alloc(Foo, num);
errdefer allocator.free(foos);
// Used to track how many foos have been initialized
// (including their data being allocated)
var num_allocated: usize = 0;
errdefer for (foos[0..num_allocated]) |foo| {
allocator.destroy(foo.data);
};
for (foos, 0..) |*foo, i| {
foo.data = try allocator.create(u32);
num_allocated += 1;
if (i >= 3) return error.TooManyFoos;
foo.data.* = try getData();
}
return foos;
}
test "genFoos" {
try std.testing.expectError(error.TooManyFoos, genFoos(std.testing.allocator, 5));
}
{#code_end#}
{#header_close#}
<p>
A couple of other tidbits about error handling:
</p>
<ul>
<li>These primitives give enough expressiveness that it's completely practical
to have failing to check for an error be a compile error. If you really want
to ignore the error, you can add {#syntax#}catch unreachable{#endsyntax#} and
get the added benefit of crashing in Debug and ReleaseSafe modes if your assumption was wrong.
</li>
<li>
Since Zig understands error types, it can pre-weight branches in favor of
errors not occurring. Just a small optimization benefit that is not available
in other languages.
</li>
</ul>
{#see_also|defer|if|switch#}
<p>An error union is created with the {#syntax#}!{#endsyntax#} binary operator.
You can use compile-time reflection to access the child type of an error union:</p>
{#code_begin|test|test_error_union#}
const expect = @import("std").testing.expect;
test "error union" {
var foo: anyerror!i32 = undefined;
// Coerce from child type of an error union:
foo = 1234;
// Coerce from an error set:
foo = error.SomeError;
// Use compile-time reflection to access the payload type of an error union:
try comptime expect(@typeInfo(@TypeOf(foo)).ErrorUnion.payload == i32);
// Use compile-time reflection to access the error set type of an error union:
try comptime expect(@typeInfo(@TypeOf(foo)).ErrorUnion.error_set == anyerror);
}
{#code_end#}
{#header_open|Merging Error Sets#}
<p>
Use the {#syntax#}||{#endsyntax#} operator to merge two error sets together. The resulting
error set contains the errors of both error sets. Doc comments from the left-hand
side override doc comments from the right-hand side. In this example, the doc
comments for {#syntax#}C.PathNotFound{#endsyntax#} is <code>A doc comment</code>.
</p>
<p>
This is especially useful for functions which return different error sets depending
on {#link|comptime#} branches. For example, the Zig standard library uses
{#syntax#}LinuxFileOpenError || WindowsFileOpenError{#endsyntax#} for the error set of opening
files.
</p>
{#code_begin|test|test_merging_error_sets#}
const A = error{
NotDir,
/// A doc comment
PathNotFound,
};
const B = error{
OutOfMemory,
/// B doc comment
PathNotFound,
};
const C = A || B;
fn foo() C!void {
return error.NotDir;
}
test "merge error sets" {
if (foo()) {
@panic("unexpected");
} else |err| switch (err) {
error.OutOfMemory => @panic("unexpected"),
error.PathNotFound => @panic("unexpected"),
error.NotDir => {},
}
}
{#code_end#}
{#header_close#}
{#header_open|Inferred Error Sets#}
<p>
Because many functions in Zig return a possible error, Zig supports inferring the error set.
To infer the error set for a function, prepend the {#syntax#}!{#endsyntax#} operator to the function’s return type, like {#syntax#}!T{#endsyntax#}:
</p>
{#code_begin|test|test_inferred_error_sets#}
// With an inferred error set
pub fn add_inferred(comptime T: type, a: T, b: T) !T {
const ov = @addWithOverflow(a, b);
if (ov[1] != 0) return error.Overflow;
return ov[0];
}
// With an explicit error set
pub fn add_explicit(comptime T: type, a: T, b: T) Error!T {
const ov = @addWithOverflow(a, b);
if (ov[1] != 0) return error.Overflow;
return ov[0];
}
const Error = error {
Overflow,
};
const std = @import("std");
test "inferred error set" {
if (add_inferred(u8, 255, 1)) |_| unreachable else |err| switch (err) {
error.Overflow => {}, // ok
}
}
{#code_end#}
<p>
When a function has an inferred error set, that function becomes generic and thus it becomes
trickier to do certain things with it, such as obtain a function pointer, or have an error
set that is consistent across different build targets. Additionally, inferred error sets
are incompatible with recursion.
</p>
<p>
In these situations, it is recommended to use an explicit error set. You can generally start
with an empty error set and let compile errors guide you toward completing the set.
</p>
<p>
These limitations may be overcome in a future version of Zig.
</p>
{#header_close#}
{#header_close#}
{#header_open|Error Return Traces#}
<p>
Error Return Traces show all the points in the code that an error was returned to the calling function. This makes it practical to use {#link|try#} everywhere and then still be able to know what happened if an error ends up bubbling all the way out of your application.
</p>
{#code_begin|exe_err|error_return_trace#}
pub fn main() !void {
try foo(12);
}
fn foo(x: i32) !void {
if (x >= 5) {
try bar();
} else {
try bang2();
}
}
fn bar() !void {
if (baz()) {
try quux();
} else |err| switch (err) {
error.FileNotFound => try hello(),
}
}
fn baz() !void {
try bang1();
}
fn quux() !void {
try bang2();
}
fn hello() !void {
try bang2();
}
fn bang1() !void {
return error.FileNotFound;
}
fn bang2() !void {
return error.PermissionDenied;
}
{#code_end#}
<p>
Look closely at this example. This is no stack trace.
</p>
<p>
You can see that the final error bubbled up was {#syntax#}PermissionDenied{#endsyntax#},
but the original error that started this whole thing was {#syntax#}FileNotFound{#endsyntax#}. In the {#syntax#}bar{#endsyntax#} function, the code handles the original error code,
and then returns another one, from the switch statement. Error Return Traces make this clear, whereas a stack trace would look like this:
</p>
{#code_begin|exe_err|stack_trace#}
pub fn main() void {
foo(12);
}
fn foo(x: i32) void {
if (x >= 5) {
bar();
} else {
bang2();
}
}
fn bar() void {
if (baz()) {
quux();
} else {
hello();
}
}
fn baz() bool {
return bang1();
}
fn quux() void {
bang2();
}
fn hello() void {
bang2();
}
fn bang1() bool {
return false;
}
fn bang2() void {
@panic("PermissionDenied");
}
{#code_end#}
<p>
Here, the stack trace does not explain how the control
flow in {#syntax#}bar{#endsyntax#} got to the {#syntax#}hello(){#endsyntax#} call.
One would have to open a debugger or further instrument the application
in order to find out. The error return trace, on the other hand,
shows exactly how the error bubbled up.
</p>
<p>
This debugging feature makes it easier to iterate quickly on code that
robustly handles all error conditions. This means that Zig developers
will naturally find themselves writing correct, robust code in order
to increase their development pace.
</p>
<p>
Error Return Traces are enabled by default in {#link|Debug#} and {#link|ReleaseSafe#} builds and disabled by default in {#link|ReleaseFast#} and {#link|ReleaseSmall#} builds.
</p>
<p>
There are a few ways to activate this error return tracing feature:
</p>
<ul>
<li>Return an error from main</li>
<li>An error makes its way to {#syntax#}catch unreachable{#endsyntax#} and you have not overridden the default panic handler</li>
<li>Use {#link|errorReturnTrace#} to access the current return trace. You can use {#syntax#}std.debug.dumpStackTrace{#endsyntax#} to print it. This function returns comptime-known {#link|null#} when building without error return tracing support.</li>
</ul>
{#header_open|Implementation Details#}
<p>
To analyze performance cost, there are two cases:
</p>
<ul>
<li>when no errors are returned</li>
<li>when returning errors</li>
</ul>
<p>
For the case when no errors are returned, the cost is a single memory write operation, only in the first non-failable function in the call graph that calls a failable function, i.e. when a function returning {#syntax#}void{#endsyntax#} calls a function returning {#syntax#}error{#endsyntax#}.
This is to initialize this struct in the stack memory:
</p>
{#syntax_block|zig|stack_trace_struct.zig#}
pub const StackTrace = struct {
index: usize,
instruction_addresses: [N]usize,
};
{#end_syntax_block#}
<p>
Here, N is the maximum function call depth as determined by call graph analysis. Recursion is ignored and counts for 2.
</p>
<p>
A pointer to {#syntax#}StackTrace{#endsyntax#} is passed as a secret parameter to every function that can return an error, but it's always the first parameter, so it can likely sit in a register and stay there.
</p>
<p>
That's it for the path when no errors occur. It's practically free in terms of performance.
</p>
<p>
When generating the code for a function that returns an error, just before the {#syntax#}return{#endsyntax#} statement (only for the {#syntax#}return{#endsyntax#} statements that return errors), Zig generates a call to this function:
</p>
{#syntax_block|zig|zig_return_error_fn.zig#}
// marked as "no-inline" in LLVM IR
fn __zig_return_error(stack_trace: *StackTrace) void {
stack_trace.instruction_addresses[stack_trace.index] = @returnAddress();
stack_trace.index = (stack_trace.index + 1) % N;
}
{#end_syntax_block#}
<p>
The cost is 2 math operations plus some memory reads and writes. The memory accessed is constrained and should remain cached for the duration of the error return bubbling.
</p>
<p>
As for code size cost, 1 function call before a return statement is no big deal. Even so,
I have <a href="https://github.com/ziglang/zig/issues/690">a plan</a> to make the call to
{#syntax#}__zig_return_error{#endsyntax#} a tail call, which brings the code size cost down to actually zero. What is a return statement in code without error return tracing can become a jump instruction in code with error return tracing.
</p>
{#header_close#}
{#header_close#}
{#header_close#}
{#header_open|Optionals#}
<p>
One area that Zig provides safety without compromising efficiency or
readability is with the optional type.
</p>
<p>
The question mark symbolizes the optional type. You can convert a type to an optional
type by putting a question mark in front of it, like this:
</p>
{#code_begin|syntax|optional_integer#}
// normal integer
const normal_int: i32 = 1234;
// optional integer
const optional_int: ?i32 = 5678;
{#code_end#}
<p>
Now the variable {#syntax#}optional_int{#endsyntax#} could be an {#syntax#}i32{#endsyntax#}, or {#syntax#}null{#endsyntax#}.
</p>
<p>
Instead of integers, let's talk about pointers. Null references are the source of many runtime
exceptions, and even stand accused of being
<a href="https://www.lucidchart.com/techblog/2015/08/31/the-worst-mistake-of-computer-science/">the worst mistake of computer science</a>.
</p>
<p>Zig does not have them.</p>
<p>
Instead, you can use an optional pointer. This secretly compiles down to a normal pointer,
since we know we can use 0 as the null value for the optional type. But the compiler
can check your work and make sure you don't assign null to something that can't be null.
</p>
<p>
Typically the downside of not having null is that it makes the code more verbose to
write. But, let's compare some equivalent C code and Zig code.
</p>
<p>
Task: call malloc, if the result is null, return null.
</p>
<p>C code</p>
{#syntax_block|c|call_malloc_in_c.c#}
// malloc prototype included for reference
void *malloc(size_t size);
struct Foo *do_a_thing(void) {
char *ptr = malloc(1234);
if (!ptr) return NULL;
// ...
}
{#end_syntax_block#}
<p>Zig code</p>
{#syntax_block|zig|call_malloc_from_zig.zig#}
// malloc prototype included for reference
extern fn malloc(size: usize) ?[*]u8;
fn doAThing() ?*Foo {
const ptr = malloc(1234) orelse return null;
_ = ptr; // ...
}
{#end_syntax_block#}
<p>
Here, Zig is at least as convenient, if not more, than C. And, the type of "ptr"
is {#syntax#}[*]u8{#endsyntax#} <em>not</em> {#syntax#}?[*]u8{#endsyntax#}. The {#syntax#}orelse{#endsyntax#} keyword
unwrapped the optional type and therefore {#syntax#}ptr{#endsyntax#} is guaranteed to be non-null everywhere
it is used in the function.
</p>
<p>
The other form of checking against NULL you might see looks like this:
</p>
{#syntax_block|c|checking_null_in_c.c#}
void do_a_thing(struct Foo *foo) {
// do some stuff
if (foo) {
do_something_with_foo(foo);
}
// do some stuff
}
{#end_syntax_block#}
<p>
In Zig you can accomplish the same thing:
</p>
{#code_begin|syntax|checking_null_in_zig#}
const Foo = struct{};
fn doSomethingWithFoo(foo: *Foo) void { _ = foo; }
fn doAThing(optional_foo: ?*Foo) void {
// do some stuff
if (optional_foo) |foo| {
doSomethingWithFoo(foo);
}
// do some stuff
}
{#code_end#}
<p>
Once again, the notable thing here is that inside the if block,
{#syntax#}foo{#endsyntax#} is no longer an optional pointer, it is a pointer, which
cannot be null.
</p>
<p>
One benefit to this is that functions which take pointers as arguments can
be annotated with the "nonnull" attribute - <code>__attribute__((nonnull))</code> in
<a href="https://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Function-Attributes.html">GCC</a>.
The optimizer can sometimes make better decisions knowing that pointer arguments
cannot be null.
</p>
{#header_open|Optional Type#}
<p>An optional is created by putting {#syntax#}?{#endsyntax#} in front of a type. You can use compile-time
reflection to access the child type of an optional:</p>
{#code_begin|test|test_optional_type#}
const expect = @import("std").testing.expect;
test "optional type" {
// Declare an optional and coerce from null:
var foo: ?i32 = null;
// Coerce from child type of an optional
foo = 1234;
// Use compile-time reflection to access the child type of the optional:
try comptime expect(@typeInfo(@TypeOf(foo)).Optional.child == i32);
}
{#code_end#}
{#header_close#}
{#header_open|null#}
<p>
Just like {#link|undefined#}, {#syntax#}null{#endsyntax#} has its own type, and the only way to use it is to
cast it to a different type:
</p>
{#code_begin|syntax|null#}
const optional_value: ?i32 = null;
{#code_end#}
{#header_close#}
{#header_open|Optional Pointers#}
<p>An optional pointer is guaranteed to be the same size as a pointer. The {#syntax#}null{#endsyntax#} of
the optional is guaranteed to be address 0.</p>
{#code_begin|test|test_optional_pointer#}
const expect = @import("std").testing.expect;
test "optional pointers" {
// Pointers cannot be null. If you want a null pointer, use the optional
// prefix `?` to make the pointer type optional.
var ptr: ?*i32 = null;
var x: i32 = 1;
ptr = &x;
try expect(ptr.?.* == 1);
// Optional pointers are the same size as normal pointers, because pointer
// value 0 is used as the null value.
try expect(@sizeOf(?*i32) == @sizeOf(*i32));
}
{#code_end#}
{#header_close#}
{#see_also|while with Optionals|if with Optionals#}
{#header_close#}
{#header_open|Casting#}
<p>
A <strong>type cast</strong> converts a value of one type to another.
Zig has {#link|Type Coercion#} for conversions that are known to be completely safe and unambiguous,
and {#link|Explicit Casts#} for conversions that one would not want to happen on accident.
There is also a third kind of type conversion called {#link|Peer Type Resolution#} for
the case when a result type must be decided given multiple operand types.
</p>
{#header_open|Type Coercion#}
<p>
Type coercion occurs when one type is expected, but different type is provided:
</p>
{#code_begin|test|test_type_coercion#}
test "type coercion - variable declaration" {
const a: u8 = 1;
const b: u16 = a;
_ = b;
}
test "type coercion - function call" {
const a: u8 = 1;
foo(a);
}
fn foo(b: u16) void {
_ = b;
}
test "type coercion - @as builtin" {
const a: u8 = 1;
const b = @as(u16, a);
_ = b;
}
{#code_end#}
<p>
Type coercions are only allowed when it is completely unambiguous how to get from one type to another,
and the transformation is guaranteed to be safe. There is one exception, which is {#link|C Pointers#}.
</p>
{#header_open|Type Coercion: Stricter Qualification#}
<p>
Values which have the same representation at runtime can be cast to increase the strictness
of the qualifiers, no matter how nested the qualifiers are:
</p>
<ul>
<li>{#syntax#}const{#endsyntax#} - non-const to const is allowed</li>
<li>{#syntax#}volatile{#endsyntax#} - non-volatile to volatile is allowed</li>
<li>{#syntax#}align{#endsyntax#} - bigger to smaller alignment is allowed </li>
<li>{#link|error sets|Error Set Type#} to supersets is allowed</li>
</ul>
<p>
These casts are no-ops at runtime since the value representation does not change.
</p>
{#code_begin|test|test_no_op_casts#}
test "type coercion - const qualification" {
var a: i32 = 1;
const b: *i32 = &a;
foo(b);
}
fn foo(_: *const i32) void {}
{#code_end#}
<p>
In addition, pointers coerce to const optional pointers:
</p>
{#code_begin|test|test_pointer_coerce_const_optional#}
const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;
test "cast *[1][*]const u8 to [*]const ?[*]const u8" {
const window_name = [1][*]const u8{"window name"};
const x: [*]const ?[*]const u8 = &window_name;
try expect(mem.eql(u8, std.mem.sliceTo(@as([*:0]const u8, @ptrCast(x[0].?)), 0), "window name"));
}
{#code_end#}
{#header_close#}
{#header_open|Type Coercion: Integer and Float Widening#}
<p>
{#link|Integers#} coerce to integer types which can represent every value of the old type, and likewise
{#link|Floats#} coerce to float types which can represent every value of the old type.
</p>
{#code_begin|test|test_integer_widening#}
const std = @import("std");
const builtin = @import("builtin");
const expect = std.testing.expect;
const mem = std.mem;
test "integer widening" {
const a: u8 = 250;
const b: u16 = a;
const c: u32 = b;
const d: u64 = c;
const e: u64 = d;
const f: u128 = e;
try expect(f == a);
}
test "implicit unsigned integer to signed integer" {
const a: u8 = 250;
const b: i16 = a;
try expect(b == 250);
}
test "float widening" {
const a: f16 = 12.34;
const b: f32 = a;
const c: f64 = b;
const d: f128 = c;
try expect(d == a);
}
{#code_end#}
{#header_close#}
{#header_open|Type Coercion: Float to Int#}
<p>
A compiler error is appropriate because this ambiguous expression leaves the compiler
two choices about the coercion.
</p>
<ul>
<li>Cast {#syntax#}54.0{#endsyntax#} to {#syntax#}comptime_int{#endsyntax#} resulting in {#syntax#}@as(comptime_int, 10){#endsyntax#}, which is casted to {#syntax#}@as(f32, 10){#endsyntax#}</li>
<li>Cast {#syntax#}5{#endsyntax#} to {#syntax#}comptime_float{#endsyntax#} resulting in {#syntax#}@as(comptime_float, 10.8){#endsyntax#}, which is casted to {#syntax#}@as(f32, 10.8){#endsyntax#}</li>
</ul>
{#code_begin|test_err|test_ambiguous_coercion#}
// Compile time coercion of float to int
test "implicit cast to comptime_int" {
const f: f32 = 54.0 / 5;
_ = f;
}
{#code_end#}
{#header_close#}
{#header_open|Type Coercion: Slices, Arrays and Pointers#}
{#code_begin|test|test_coerce_slices_arrays_and_pointers#}
const std = @import("std");
const expect = std.testing.expect;
// You can assign constant pointers to arrays to a slice with
// const modifier on the element type. Useful in particular for
// String literals.
test "*const [N]T to []const T" {
const x1: []const u8 = "hello";
const x2: []const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
try expect(std.mem.eql(u8, x1, x2));
const y: []const f32 = &[2]f32{ 1.2, 3.4 };
try expect(y[0] == 1.2);
}
// Likewise, it works when the destination type is an error union.
test "*const [N]T to E![]const T" {
const x1: anyerror![]const u8 = "hello";
const x2: anyerror![]const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
try expect(std.mem.eql(u8, try x1, try x2));
const y: anyerror![]const f32 = &[2]f32{ 1.2, 3.4 };
try expect((try y)[0] == 1.2);
}
// Likewise, it works when the destination type is an optional.
test "*const [N]T to ?[]const T" {
const x1: ?[]const u8 = "hello";
const x2: ?[]const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
try expect(std.mem.eql(u8, x1.?, x2.?));
const y: ?[]const f32 = &[2]f32{ 1.2, 3.4 };
try expect(y.?[0] == 1.2);
}
// In this cast, the array length becomes the slice length.
test "*[N]T to []T" {
var buf: [5]u8 = "hello".*;
const x: []u8 = &buf;
try expect(std.mem.eql(u8, x, "hello"));
const buf2 = [2]f32{ 1.2, 3.4 };
const x2: []const f32 = &buf2;
try expect(std.mem.eql(f32, x2, &[2]f32{ 1.2, 3.4 }));
}
// Single-item pointers to arrays can be coerced to many-item pointers.
test "*[N]T to [*]T" {
var buf: [5]u8 = "hello".*;
const x: [*]u8 = &buf;
try expect(x[4] == 'o');
// x[5] would be an uncaught out of bounds pointer dereference!
}
// Likewise, it works when the destination type is an optional.
test "*[N]T to ?[*]T" {
var buf: [5]u8 = "hello".*;
const x: ?[*]u8 = &buf;
try expect(x.?[4] == 'o');
}
// Single-item pointers can be cast to len-1 single-item arrays.
test "*T to *[1]T" {
var x: i32 = 1234;
const y: *[1]i32 = &x;
const z: [*]i32 = y;
try expect(z[0] == 1234);
}
{#code_end#}
{#see_also|C Pointers#}
{#header_close#}
{#header_open|Type Coercion: Optionals#}
<p>
The payload type of {#link|Optionals#}, as well as {#link|null#}, coerce to the optional type.
</p>
{#code_begin|test|test_coerce_optionals#}
const std = @import("std");
const expect = std.testing.expect;
test "coerce to optionals" {
const x: ?i32 = 1234;
const y: ?i32 = null;
try expect(x.? == 1234);
try expect(y == null);
}
{#code_end#}
<p>Optionals work nested inside the {#link|Error Union Type#}, too:</p>
{#code_begin|test|test_coerce_optional_wrapped_error_union#}
const std = @import("std");
const expect = std.testing.expect;
test "coerce to optionals wrapped in error union" {
const x: anyerror!?i32 = 1234;
const y: anyerror!?i32 = null;
try expect((try x).? == 1234);
try expect((try y) == null);
}
{#code_end#}
{#header_close#}
{#header_open|Type Coercion: Error Unions#}
<p>The payload type of an {#link|Error Union Type#} as well as the {#link|Error Set Type#}
coerce to the error union type:
</p>
{#code_begin|test|test_coerce_to_error_union#}
const std = @import("std");
const expect = std.testing.expect;
test "coercion to error unions" {
const x: anyerror!i32 = 1234;
const y: anyerror!i32 = error.Failure;
try expect((try x) == 1234);
try std.testing.expectError(error.Failure, y);
}
{#code_end#}
{#header_close#}
{#header_open|Type Coercion: Compile-Time Known Numbers#}
<p>When a number is {#link|comptime#}-known to be representable in the destination type,
it may be coerced:
</p>
{#code_begin|test|test_coerce_large_to_small#}
const std = @import("std");
const expect = std.testing.expect;
test "coercing large integer type to smaller one when value is comptime-known to fit" {
const x: u64 = 255;
const y: u8 = x;
try expect(y == 255);
}
{#code_end#}
{#header_close#}
{#header_open|Type Coercion: Unions and Enums#}
<p>Tagged unions can be coerced to enums, and enums can be coerced to tagged unions
when they are {#link|comptime#}-known to be a field of the union that has only one possible value, such as
{#link|void#}:
</p>
{#code_begin|test|test_coerce_unions_enums#}
const std = @import("std");
const expect = std.testing.expect;
const E = enum {
one,
two,
three,
};
const U = union(E) {
one: i32,
two: f32,
three,
};
const U2 = union(enum) {
a: void,
b: f32,
fn tag(self: U2) usize {
switch (self) {
.a => return 1,
.b => return 2,
}
}
};
test "coercion between unions and enums" {
const u = U{ .two = 12.34 };
const e: E = u; // coerce union to enum
try expect(e == E.two);
const three = E.three;
const u_2: U = three; // coerce enum to union
try expect(u_2 == E.three);
const u_3: U = .three; // coerce enum literal to union
try expect(u_3 == E.three);
const u_4: U2 = .a; // coerce enum literal to union with inferred enum tag type.
try expect(u_4.tag() == 1);
// The following example is invalid.
// error: coercion from enum '@TypeOf(.enum_literal)' to union 'test_coerce_unions_enum.U2' must initialize 'f32' field 'b'
//var u_5: U2 = .b;
//try expect(u_5.tag() == 2);
}
{#code_end#}
{#see_also|union|enum#}
{#header_close#}
{#header_open|Type Coercion: undefined#}
<p>{#link|undefined#} can be coerced to any type.</p>
{#header_close#}
{#header_open|Type Coercion: Tuples to Arrays#}
<p>{#link|Tuples#} can be coerced to arrays, if all of the fields have the same type.</p>
{#code_begin|test|test_coerce_tuples_arrays#}
const std = @import("std");
const expect = std.testing.expect;
const Tuple = struct{ u8, u8 };
test "coercion from homogenous tuple to array" {
const tuple: Tuple = .{5, 6};
const array: [2]u8 = tuple;
_ = array;
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Explicit Casts#}
<p>
Explicit casts are performed via {#link|Builtin Functions#}.
Some explicit casts are safe; some are not.
Some explicit casts perform language-level assertions; some do not.
Some explicit casts are no-ops at runtime; some are not.
</p>
<ul>
<li>{#link|@bitCast#} - change type but maintain bit representation</li>
<li>{#link|@alignCast#} - make a pointer have more alignment</li>
<li>{#link|@enumFromInt#} - obtain an enum value based on its integer tag value</li>
<li>{#link|@errorFromInt#} - obtain an error code based on its integer value</li>
<li>{#link|@errorCast#} - convert to a smaller error set</li>
<li>{#link|@floatCast#} - convert a larger float to a smaller float</li>
<li>{#link|@floatFromInt#} - convert an integer to a float value</li>
<li>{#link|@intCast#} - convert between integer types</li>
<li>{#link|@intFromBool#} - convert true to 1 and false to 0</li>
<li>{#link|@intFromEnum#} - obtain the integer tag value of an enum or tagged union</li>
<li>{#link|@intFromError#} - obtain the integer value of an error code</li>
<li>{#link|@intFromFloat#} - obtain the integer part of a float value</li>
<li>{#link|@intFromPtr#} - obtain the address of a pointer</li>
<li>{#link|@ptrFromInt#} - convert an address to a pointer</li>
<li>{#link|@ptrCast#} - convert between pointer types</li>
<li>{#link|@truncate#} - convert between integer types, chopping off bits</li>
</ul>
{#header_close#}
{#header_open|Peer Type Resolution#}
<p>Peer Type Resolution occurs in these places:</p>
<ul>
<li>{#link|switch#} expressions</li>
<li>{#link|if#} expressions</li>
<li>{#link|while#} expressions</li>
<li>{#link|for#} expressions</li>
<li>Multiple break statements in a block</li>
<li>Some {#link|binary operations|Table of Operators#}</li>
</ul>
<p>
This kind of type resolution chooses a type that all peer types can coerce into. Here are
some examples:
</p>
{#code_begin|test|test_peer_type_resolution#}
const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;
test "peer resolve int widening" {
const a: i8 = 12;
const b: i16 = 34;
const c = a + b;
try expect(c == 46);
try expect(@TypeOf(c) == i16);
}
test "peer resolve arrays of different size to const slice" {
try expect(mem.eql(u8, boolToStr(true), "true"));
try expect(mem.eql(u8, boolToStr(false), "false"));
try comptime expect(mem.eql(u8, boolToStr(true), "true"));
try comptime expect(mem.eql(u8, boolToStr(false), "false"));
}
fn boolToStr(b: bool) []const u8 {
return if (b) "true" else "false";
}
test "peer resolve array and const slice" {
try testPeerResolveArrayConstSlice(true);
try comptime testPeerResolveArrayConstSlice(true);
}
fn testPeerResolveArrayConstSlice(b: bool) !void {
const value1 = if (b) "aoeu" else @as([]const u8, "zz");
const value2 = if (b) @as([]const u8, "zz") else "aoeu";
try expect(mem.eql(u8, value1, "aoeu"));
try expect(mem.eql(u8, value2, "zz"));
}
test "peer type resolution: ?T and T" {
try expect(peerTypeTAndOptionalT(true, false).? == 0);
try expect(peerTypeTAndOptionalT(false, false).? == 3);
comptime {
try expect(peerTypeTAndOptionalT(true, false).? == 0);
try expect(peerTypeTAndOptionalT(false, false).? == 3);
}
}
fn peerTypeTAndOptionalT(c: bool, b: bool) ?usize {
if (c) {
return if (b) null else @as(usize, 0);
}
return @as(usize, 3);
}
test "peer type resolution: *[0]u8 and []const u8" {
try expect(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
try expect(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
comptime {
try expect(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
try expect(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
}
}
fn peerTypeEmptyArrayAndSlice(a: bool, slice: []const u8) []const u8 {
if (a) {
return &[_]u8{};
}
return slice[0..1];
}
test "peer type resolution: *[0]u8, []const u8, and anyerror![]u8" {
{
var data = "hi".*;
const slice = data[0..];
try expect((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
try expect((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
}
comptime {
var data = "hi".*;
const slice = data[0..];
try expect((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
try expect((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
}
}
fn peerTypeEmptyArrayAndSliceAndError(a: bool, slice: []u8) anyerror![]u8 {
if (a) {
return &[_]u8{};
}
return slice[0..1];
}
test "peer type resolution: *const T and ?*T" {
const a: *const usize = @ptrFromInt(0x123456780);
const b: ?*usize = @ptrFromInt(0x123456780);
try expect(a == b);
try expect(b == a);
}
test "peer type resolution: error union switch" {
// The non-error and error cases are only peers if the error case is just a switch expression;
// the pattern `if (x) {...} else |err| blk: { switch (err) {...} }` does not consider the
// non-error and error case to be peers.
var a: error{ A, B, C }!u32 = 0;
_ = &a;
const b = if (a) |x|
x + 3
else |err| switch (err) {
error.A => 0,
error.B => 1,
error.C => null,
};
try expect(@TypeOf(b) == ?u32);
// The non-error and error cases are only peers if the error case is just a switch expression;
// the pattern `x catch |err| blk: { switch (err) {...} }` does not consider the unwrapped `x`
// and error case to be peers.
const c = a catch |err| switch (err) {
error.A => 0,
error.B => 1,
error.C => null,
};
try expect(@TypeOf(c) == ?u32);
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Zero Bit Types#}
<p>For some types, {#link|@sizeOf#} is 0:</p>
<ul>
<li>{#link|void#}</li>
<li>The {#link|Integers#} {#syntax#}u0{#endsyntax#} and {#syntax#}i0{#endsyntax#}.</li>
<li>{#link|Arrays#} and {#link|Vectors#} with len 0, or with an element type that is a zero bit type.</li>
<li>An {#link|enum#} with only 1 tag.</li>
<li>A {#link|struct#} with all fields being zero bit types.</li>
<li>A {#link|union#} with only 1 field which is a zero bit type.</li>
</ul>
<p>
These types can only ever have one possible value, and thus
require 0 bits to represent. Code that makes use of these types is
not included in the final generated code:
</p>
{#code_begin|syntax|zero_bit_types#}
export fn entry() void {
var x: void = {};
var y: void = {};
x = y;
y = x;
}
{#code_end#}
<p>When this turns into machine code, there is no code generated in the
body of {#syntax#}entry{#endsyntax#}, even in {#link|Debug#} mode. For example, on x86_64:</p>
<pre><code>0000000000000010 &lt;entry&gt;:
10: 55 push %rbp
11: 48 89 e5 mov %rsp,%rbp
14: 5d pop %rbp
15: c3 retq </code></pre>
<p>These assembly instructions do not have any code associated with the void values -
they only perform the function call prologue and epilogue.</p>
{#header_open|void#}
<p>
{#syntax#}void{#endsyntax#} can be useful for instantiating generic types. For example, given a
{#syntax#}Map(Key, Value){#endsyntax#}, one can pass {#syntax#}void{#endsyntax#} for the {#syntax#}Value{#endsyntax#}
type to make it into a {#syntax#}Set{#endsyntax#}:
</p>
{#code_begin|test|test_void_in_hashmap#}
const std = @import("std");
const expect = std.testing.expect;
test "turn HashMap into a set with void" {
var map = std.AutoHashMap(i32, void).init(std.testing.allocator);
defer map.deinit();
try map.put(1, {});
try map.put(2, {});
try expect(map.contains(2));
try expect(!map.contains(3));
_ = map.remove(2);
try expect(!map.contains(2));
}
{#code_end#}
<p>Note that this is different from using a dummy value for the hash map value.
By using {#syntax#}void{#endsyntax#} as the type of the value, the hash map entry type has no value field, and
thus the hash map takes up less space. Further, all the code that deals with storing and loading the
value is deleted, as seen above.
</p>
<p>
{#syntax#}void{#endsyntax#} is distinct from {#syntax#}anyopaque{#endsyntax#}.
{#syntax#}void{#endsyntax#} has a known size of 0 bytes, and {#syntax#}anyopaque{#endsyntax#} has an unknown, but non-zero, size.
</p>
<p>
Expressions of type {#syntax#}void{#endsyntax#} are the only ones whose value can be ignored. For example, ignoring
a non-{#syntax#}void{#endsyntax#} expression is a compile error:
</p>
{#code_begin|test_err|test_expression_ignored|ignored#}
test "ignoring expression value" {
foo();
}
fn foo() i32 {
return 1234;
}
{#code_end#}
<p>However, if the expression has type {#syntax#}void{#endsyntax#}, there will be no error. Expression results can be explicitly ignored by assigning them to {#syntax#}_{#endsyntax#}. </p>
{#code_begin|test|test_void_ignored#}
test "void is ignored" {
returnsVoid();
}
test "explicitly ignoring expression value" {
_ = foo();
}
fn returnsVoid() void {}
fn foo() i32 {
return 1234;
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Result Location Semantics#}
<p>
During compilation, every Zig expression and sub-expression is assigned optional result location
information. This information dictates what type the expression should have (its result type), and
where the resulting value should be placed in memory (its result location). The information is
optional in the sense that not every expression has this information: assignment to
{#syntax#}_{#endsyntax#}, for instance, does not provide any information about the type of an
expression, nor does it provide a concrete memory location to place it in.
</p>
<p>
As a motivating example, consider the statement {#syntax#}const x: u32 = 42;{#endsyntax#}. The type
annotation here provides a result type of {#syntax#}u32{#endsyntax#} to the initialization expression
{#syntax#}42{#endsyntax#}, instructing the compiler to coerce this integer (initally of type
{#syntax#}comptime_int{#endsyntax#}) to this type. We will see more examples shortly.
</p>
<p>
This is not an implementation detail: the logic outlined above is codified into the Zig language
specification, and is the primary mechanism of type inference in the language. This system is
collectively referred to as "Result Location Semantics".
</p>
{#header_open|Result Types#}
<p>
Result types are propagated recursively through expressions where possible. For instance, if the
expression {#syntax#}&e{#endsyntax#} has result type {#syntax#}*u32{#endsyntax#}, then
{#syntax#}e{#endsyntax#} is given a result type of {#syntax#}u32{#endsyntax#}, allowing the
language to perform this coercion before taking a reference.
</p>
<p>
The result type mechanism is utilized by casting builtins such as {#syntax#}@intCast{#endsyntax#}.
Rather than taking as an argument the type to cast to, these builtins use their result type to
determine this information. The result type is often known from context; where it is not, the
{#syntax#}@as{#endsyntax#} builtin can be used to explicitly provide a result type.
</p>
<p>
We can break down the result types for each component of a simple expression as follows:
</p>
{#code_begin|test|result_type_propagation#}
const expectEqual = @import("std").testing.expectEqual;
test "result type propagates through struct initializer" {
const S = struct { x: u32 };
const val: u64 = 123;
const s: S = .{ .x = @intCast(val) };
// .{ .x = @intCast(val) } has result type `S` due to the type annotation
// @intCast(val) has result type `u32` due to the type of the field `S.x`
// val has no result type, as it is permitted to be any integer type
try expectEqual(@as(u32, 123), s.x);
}
{#code_end#}
<p>
This result type information is useful for the aforementioned cast builtins, as well as to avoid
the construction of pre-coercion values, and to avoid the need for explicit type coercions in some
cases. The following table details how some common expressions propagate result types, where
{#syntax#}x{#endsyntax#} and {#syntax#}y{#endsyntax#} are arbitrary sub-expressions.
</p>
<div class="table-wrapper">
<table>
<thead>
<tr>
<th scope="col">Expression</th>
<th scope="col">Parent Result Type</th>
<th scope="col">Sub-expression Result Type</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">{#syntax#}const val: T = x{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}T{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}var val: T = x{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}T{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}val = x{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}@TypeOf(val){#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}@as(T, x){#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}T{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}&x{#endsyntax#}</th>
<td>{#syntax#}*T{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}T{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}&x{#endsyntax#}</th>
<td>{#syntax#}[]T{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} is some array of {#syntax#}T{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}f(x){#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} has the type of the first parameter of {#syntax#}f{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}.{x}{#endsyntax#}</th>
<td>{#syntax#}T{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}std.meta.FieldType(T, .@"0"){#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}.{ .a = x }{#endsyntax#}</th>
<td>{#syntax#}T{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}std.meta.FieldType(T, .a){#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}T{x}{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}std.meta.FieldType(T, .@"0"){#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}T{ .a = x }{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}std.meta.FieldType(T, .a){#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}@Type(x){#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}std.builtin.Type{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}@typeInfo(x){#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} is a {#syntax#}type{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}x << y{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}y{#endsyntax#} is a {#syntax#}std.math.Log2IntCeil(@TypeOf(x)){#endsyntax#}</td>
</tr>
</tbody>
</table>
</div>
{#header_close#}
{#header_open|Result Locations#}
<p>
In addition to result type information, every expression may be optionally assigned a result
location: a pointer to which the value must be directly written. This system can be used to prevent
intermediate copies when initializing data structures, which can be important for types which must
have a fixed memory address ("pinned" types).
</p>
<p>
When compiling the simple assignment expression {#syntax#}x = e{#endsyntax#}, many languages would
create the temporary value {#syntax#}e{#endsyntax#} on the stack, and then assign it to
{#syntax#}x{#endsyntax#}, potentially performing a type coercion in the process. Zig approaches this
differently. The expression {#syntax#}e{#endsyntax#} is given a result type matching the type of
{#syntax#}x{#endsyntax#}, and a result location of {#syntax#}&x{#endsyntax#}. For many syntactic
forms of {#syntax#}e{#endsyntax#}, this has no practical impact. However, it can have important
semantic effects when working with more complex syntax forms.
</p>
<p>
For instance, if the expression {#syntax#}.{ .a = x, .b = y }{#endsyntax#} has a result location of
{#syntax#}ptr{#endsyntax#}, then {#syntax#}x{#endsyntax#} is given a result location of
{#syntax#}&ptr.a{#endsyntax#}, and {#syntax#}y{#endsyntax#} a result location of {#syntax#}&ptr.b{#endsyntax#}.
Without this system, this expression would construct a temporary struct value entirely on the stack, and
only then copy it to the destination address. In essence, Zig desugars the assignment
{#syntax#}foo = .{ .a = x, .b = y }{#endsyntax#} to the two statements {#syntax#}foo.a = x; foo.b = y;{#endsyntax#}.
</p>
<p>
This can sometimes be important when assigning an aggregate value where the initialization
expression depends on the previous value of the aggregate. The easiest way to demonstrate this is by
attempting to swap fields of a struct or array - the following logic looks sound, but in fact is not:
</p>
{#code_begin|test_err|result_location_interfering_with_swap#}
const expect = @import("std").testing.expect;
test "attempt to swap array elements with array initializer" {
var arr: [2]u32 = .{ 1, 2 };
arr = .{ arr[1], arr[0] };
// The previous line is equivalent to the following two lines:
// arr[0] = arr[1];
// arr[1] = arr[0];
// So this fails!
try expect(arr[0] == 2); // succeeds
try expect(arr[1] == 1); // fails
}
{#code_end#}
<p>
The following table details how some common expressions propagate result locations, where
{#syntax#}x{#endsyntax#} and {#syntax#}y{#endsyntax#} are arbitrary sub-expressions. Note that
some expressions cannot provide meaningful result locations to sub-expressions, even if they
themselves have a result location.
</p>
<div class="table-wrapper">
<table>
<thead>
<tr>
<th scope="col">Expression</th>
<th scope="col">Result Location</th>
<th scope="col">Sub-expression Result Locations</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">{#syntax#}const val: T = x{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} has result location {#syntax#}&val{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}var val: T = x{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} has result location {#syntax#}&val{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}val = x{#endsyntax#}</th>
<td>-</td>
<td>{#syntax#}x{#endsyntax#} has result location {#syntax#}&val{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}@as(T, x){#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location</td>
</tr>
<tr>
<th scope="row">{#syntax#}&x{#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location</td>
</tr>
<tr>
<th scope="row">{#syntax#}f(x){#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location</td>
</tr>
<tr>
<th scope="row">{#syntax#}.{x}{#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has result location {#syntax#}&ptr[0]{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}.{ .a = x }{#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has result location {#syntax#}&ptr.a{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}T{x}{#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location (typed initializers do not propagate result locations)</td>
</tr>
<tr>
<th scope="row">{#syntax#}T{ .a = x }{#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location (typed initializers do not propagate result locations)</td>
</tr>
<tr>
<th scope="row">{#syntax#}@Type(x){#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location</td>
</tr>
<tr>
<th scope="row">{#syntax#}@typeInfo(x){#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} has no result location</td>
</tr>
<tr>
<th scope="row">{#syntax#}x << y{#endsyntax#}</th>
<td>{#syntax#}ptr{#endsyntax#}</td>
<td>{#syntax#}x{#endsyntax#} and {#syntax#}y{#endsyntax#} do not have result locations</td>
</tr>
</tbody>
</table>
</div>
{#header_close#}
{#header_close#}
{#header_open|usingnamespace#}
<p>
{#syntax#}usingnamespace{#endsyntax#} is a declaration that mixes all the public
declarations of the operand, which must be a {#link|struct#}, {#link|union#}, {#link|enum#},
or {#link|opaque#}, into the namespace:
</p>
{#code_begin|test|test_usingnamespace#}
test "using std namespace" {
const S = struct {
usingnamespace @import("std");
};
try S.testing.expect(true);
}
{#code_end#}
<p>
{#syntax#}usingnamespace{#endsyntax#} has an important use case when organizing the public
API of a file or package. For example, one might have <code class="file">c.zig</code> with all of the
{#link|C imports|Import from C Header File#}:
</p>
{#syntax_block|zig|c.zig#}
pub usingnamespace @cImport({
@cInclude("epoxy/gl.h");
@cInclude("GLFW/glfw3.h");
@cDefine("STBI_ONLY_PNG", "");
@cDefine("STBI_NO_STDIO", "");
@cInclude("stb_image.h");
});
{#end_syntax_block#}
<p>
The above example demonstrates using {#syntax#}pub{#endsyntax#} to qualify the
{#syntax#}usingnamespace{#endsyntax#} additionally makes the imported declarations
{#syntax#}pub{#endsyntax#}. This can be used to forward declarations, giving precise control
over what declarations a given file exposes.
</p>
{#header_close#}
{#header_open|comptime#}
<p>
Zig places importance on the concept of whether an expression is known at compile-time.
There are a few different places this concept is used, and these building blocks are used
to keep the language small, readable, and powerful.
</p>
{#header_open|Introducing the Compile-Time Concept#}
{#header_open|Compile-Time Parameters#}
<p>
Compile-time parameters is how Zig implements generics. It is compile-time duck typing.
</p>
{#code_begin|syntax|compile-time_duck_typing#}
fn max(comptime T: type, a: T, b: T) T {
return if (a > b) a else b;
}
fn gimmeTheBiggerFloat(a: f32, b: f32) f32 {
return max(f32, a, b);
}
fn gimmeTheBiggerInteger(a: u64, b: u64) u64 {
return max(u64, a, b);
}
{#code_end#}
<p>
In Zig, types are first-class citizens. They can be assigned to variables, passed as parameters to functions,
and returned from functions. However, they can only be used in expressions which are known at <em>compile-time</em>,
which is why the parameter {#syntax#}T{#endsyntax#} in the above snippet must be marked with {#syntax#}comptime{#endsyntax#}.
</p>
<p>
A {#syntax#}comptime{#endsyntax#} parameter means that:
</p>
<ul>
<li>At the callsite, the value must be known at compile-time, or it is a compile error.</li>
<li>In the function definition, the value is known at compile-time.</li>
</ul>
<p>
For example, if we were to introduce another function to the above snippet:
</p>
{#code_begin|test_err|test_unresolved_comptime_value|unable to resolve comptime value#}
fn max(comptime T: type, a: T, b: T) T {
return if (a > b) a else b;
}
test "try to pass a runtime type" {
foo(false);
}
fn foo(condition: bool) void {
const result = max(
if (condition) f32 else u64,
1234,
5678);
_ = result;
}
{#code_end#}
<p>
This is an error because the programmer attempted to pass a value only known at run-time
to a function which expects a value known at compile-time.
</p>
<p>
Another way to get an error is if we pass a type that violates the type checker when the
function is analyzed. This is what it means to have <em>compile-time duck typing</em>.
</p>
<p>
For example:
</p>
{#code_begin|test_err|test_comptime_mismatched_type|operator > not allowed for type 'bool'#}
fn max(comptime T: type, a: T, b: T) T {
return if (a > b) a else b;
}
test "try to compare bools" {
_ = max(bool, true, false);
}
{#code_end#}
<p>
On the flip side, inside the function definition with the {#syntax#}comptime{#endsyntax#} parameter, the
value is known at compile-time. This means that we actually could make this work for the bool type
if we wanted to:
</p>
{#code_begin|test|test_comptime_max_with_bool#}
fn max(comptime T: type, a: T, b: T) T {
if (T == bool) {
return a or b;
} else if (a > b) {
return a;
} else {
return b;
}
}
test "try to compare bools" {
try @import("std").testing.expect(max(bool, false, true) == true);
}
{#code_end#}
<p>
This works because Zig implicitly inlines {#syntax#}if{#endsyntax#} expressions when the condition
is known at compile-time, and the compiler guarantees that it will skip analysis of
the branch not taken.
</p>
<p>
This means that the actual function generated for {#syntax#}max{#endsyntax#} in this situation looks like
this:
</p>
{#code_begin|syntax|compiler_generated_function#}
fn max(a: bool, b: bool) bool {
{
return a or b;
}
}
{#code_end#}
<p>
All the code that dealt with compile-time known values is eliminated and we are left with only
the necessary run-time code to accomplish the task.
</p>
<p>
This works the same way for {#syntax#}switch{#endsyntax#} expressions - they are implicitly inlined
when the target expression is compile-time known.
</p>
{#header_close#}
{#header_open|Compile-Time Variables#}
<p>
In Zig, the programmer can label variables as {#syntax#}comptime{#endsyntax#}. This guarantees to the compiler
that every load and store of the variable is performed at compile-time. Any violation of this results in a
compile error.
</p>
<p>
This combined with the fact that we can {#syntax#}inline{#endsyntax#} loops allows us to write
a function which is partially evaluated at compile-time and partially at run-time.
</p>
<p>
For example:
</p>
{#code_begin|test|test_comptime_evaluation#}
const expect = @import("std").testing.expect;
const CmdFn = struct {
name: []const u8,
func: fn(i32) i32,
};
const cmd_fns = [_]CmdFn{
CmdFn {.name = "one", .func = one},
CmdFn {.name = "two", .func = two},
CmdFn {.name = "three", .func = three},
};
fn one(value: i32) i32 { return value + 1; }
fn two(value: i32) i32 { return value + 2; }
fn three(value: i32) i32 { return value + 3; }
fn performFn(comptime prefix_char: u8, start_value: i32) i32 {
var result: i32 = start_value;
comptime var i = 0;
inline while (i < cmd_fns.len) : (i += 1) {
if (cmd_fns[i].name[0] == prefix_char) {
result = cmd_fns[i].func(result);
}
}
return result;
}
test "perform fn" {
try expect(performFn('t', 1) == 6);
try expect(performFn('o', 0) == 1);
try expect(performFn('w', 99) == 99);
}
{#code_end#}
<p>
This example is a bit contrived, because the compile-time evaluation component is unnecessary;
this code would work fine if it was all done at run-time. But it does end up generating
different code. In this example, the function {#syntax#}performFn{#endsyntax#} is generated three different times,
for the different values of {#syntax#}prefix_char{#endsyntax#} provided:
</p>
{#syntax_block|zig|performFn_1#}
// From the line:
// expect(performFn('t', 1) == 6);
fn performFn(start_value: i32) i32 {
var result: i32 = start_value;
result = two(result);
result = three(result);
return result;
}
{#end_syntax_block#}
{#syntax_block|zig|performFn_2#}
// From the line:
// expect(performFn('o', 0) == 1);
fn performFn(start_value: i32) i32 {
var result: i32 = start_value;
result = one(result);
return result;
}
{#end_syntax_block#}
{#syntax_block|zig|performFn_3#}
// From the line:
// expect(performFn('w', 99) == 99);
fn performFn(start_value: i32) i32 {
var result: i32 = start_value;
_ = &result;
return result;
}
{#end_syntax_block#}
<p>
Note that this happens even in a debug build.
This is not a way to write more optimized code, but it is a way to make sure that what <em>should</em> happen
at compile-time, <em>does</em> happen at compile-time. This catches more errors and allows expressiveness
that in other languages requires using macros, generated code, or a preprocessor to accomplish.
</p>
{#header_close#}
{#header_open|Compile-Time Expressions#}
<p>
In Zig, it matters whether a given expression is known at compile-time or run-time. A programmer can
use a {#syntax#}comptime{#endsyntax#} expression to guarantee that the expression will be evaluated at compile-time.
If this cannot be accomplished, the compiler will emit an error. For example:
</p>
{#code_begin|test_err|test_comptime_call_extern_function|comptime call of extern function#}
extern fn exit() noreturn;
test "foo" {
comptime {
exit();
}
}
{#code_end#}
<p>
It doesn't make sense that a program could call {#syntax#}exit(){#endsyntax#} (or any other external function)
at compile-time, so this is a compile error. However, a {#syntax#}comptime{#endsyntax#} expression does much
more than sometimes cause a compile error.
</p>
<p>
Within a {#syntax#}comptime{#endsyntax#} expression:
</p>
<ul>
<li>All variables are {#syntax#}comptime{#endsyntax#} variables.</li>
<li>All {#syntax#}if{#endsyntax#}, {#syntax#}while{#endsyntax#}, {#syntax#}for{#endsyntax#}, and {#syntax#}switch{#endsyntax#}
expressions are evaluated at compile-time, or emit a compile error if this is not possible.</li>
<li>All {#syntax#}return{#endsyntax#} and {#syntax#}try{#endsyntax#} expressions are invalid (unless the function itself is called at compile-time).</li>
<li>All code with runtime side effects or depending on runtime values emits a compile error.</li>
<li>All function calls cause the compiler to interpret the function at compile-time, emitting a
compile error if the function tries to do something that has global runtime side effects.</li>
</ul>
<p>
This means that a programmer can create a function which is called both at compile-time and run-time, with
no modification to the function required.
</p>
<p>
Let's look at an example:
</p>
{#code_begin|test|test_fibonacci_recursion#}
const expect = @import("std").testing.expect;
fn fibonacci(index: u32) u32 {
if (index < 2) return index;
return fibonacci(index - 1) + fibonacci(index - 2);
}
test "fibonacci" {
// test fibonacci at run-time
try expect(fibonacci(7) == 13);
// test fibonacci at compile-time
try comptime expect(fibonacci(7) == 13);
}
{#code_end#}
<p>
Imagine if we had forgotten the base case of the recursive function and tried to run the tests:
</p>
{#code_begin|test_err|test_fibonacci_comptime_overflow|overflow of integer type#}
const expect = @import("std").testing.expect;
fn fibonacci(index: u32) u32 {
//if (index < 2) return index;
return fibonacci(index - 1) + fibonacci(index - 2);
}
test "fibonacci" {
try comptime expect(fibonacci(7) == 13);
}
{#code_end#}
<p>
The compiler produces an error which is a stack trace from trying to evaluate the
function at compile-time.
</p>
<p>
Luckily, we used an unsigned integer, and so when we tried to subtract 1 from 0, it triggered
undefined behavior, which is always a compile error if the compiler knows it happened.
But what would have happened if we used a signed integer?
</p>
{#code_begin|syntax|fibonacci_comptime_infinite_recursion#}
const assert = @import("std").debug.assert;
fn fibonacci(index: i32) i32 {
//if (index < 2) return index;
return fibonacci(index - 1) + fibonacci(index - 2);
}
test "fibonacci" {
try comptime assert(fibonacci(7) == 13);
}
{#code_end#}
<p>
The compiler is supposed to notice that evaluating this function at
compile-time took more than 1000 branches, and thus emits an error and
gives up. If the programmer wants to increase the budget for compile-time
computation, they can use a built-in function called
{#link|@setEvalBranchQuota#} to change the default number 1000 to
something else.
</p>
<p>
However, there is a <a href="https://github.com/ziglang/zig/issues/13724">design
flaw in the compiler</a> causing it to stack overflow instead of having the proper
behavior here. I'm terribly sorry about that. I hope to get this resolved
before the next release.
</p>
<p>
What if we fix the base case, but put the wrong value in the
{#syntax#}expect{#endsyntax#} line?
</p>
{#code_begin|test_err|test_fibonacci_comptime_unreachable|reached unreachable#}
const assert = @import("std").debug.assert;
fn fibonacci(index: i32) i32 {
if (index < 2) return index;
return fibonacci(index - 1) + fibonacci(index - 2);
}
test "fibonacci" {
try comptime assert(fibonacci(7) == 99999);
}
{#code_end#}
<p>
At {#link|container|Containers#} level (outside of any function), all expressions are implicitly
{#syntax#}comptime{#endsyntax#} expressions. This means that we can use functions to
initialize complex static data. For example:
</p>
{#code_begin|test|test_container-level_comptime_expressions#}
const first_25_primes = firstNPrimes(25);
const sum_of_first_25_primes = sum(&first_25_primes);
fn firstNPrimes(comptime n: usize) [n]i32 {
var prime_list: [n]i32 = undefined;
var next_index: usize = 0;
var test_number: i32 = 2;
while (next_index < prime_list.len) : (test_number += 1) {
var test_prime_index: usize = 0;
var is_prime = true;
while (test_prime_index < next_index) : (test_prime_index += 1) {
if (test_number % prime_list[test_prime_index] == 0) {
is_prime = false;
break;
}
}
if (is_prime) {
prime_list[next_index] = test_number;
next_index += 1;
}
}
return prime_list;
}
fn sum(numbers: []const i32) i32 {
var result: i32 = 0;
for (numbers) |x| {
result += x;
}
return result;
}
test "variable values" {
try @import("std").testing.expect(sum_of_first_25_primes == 1060);
}
{#code_end#}
<p>
When we compile this program, Zig generates the constants
with the answer pre-computed. Here are the lines from the generated LLVM IR:
</p>
<pre><code class="llvm">@0 = internal unnamed_addr constant [25 x i32] [i32 2, i32 3, i32 5, i32 7, i32 11, i32 13, i32 17, i32 19, i32 23, i32 29, i32 31, i32 37, i32 41, i32 43, i32 47, i32 53, i32 59, i32 61, i32 67, i32 71, i32 73, i32 79, i32 83, i32 89, i32 97]
@1 = internal unnamed_addr constant i32 1060</code></pre>
<p>
Note that we did not have to do anything special with the syntax of these functions. For example,
we could call the {#syntax#}sum{#endsyntax#} function as is with a slice of numbers whose length and values were
only known at run-time.
</p>
{#header_close#}
{#header_close#}
{#header_open|Generic Data Structures#}
<p>
Zig uses comptime capabilities to implement generic data structures without introducing any
special-case syntax.
</p>
<p>
Here is an example of a generic {#syntax#}List{#endsyntax#} data structure.
</p>
{#code_begin|syntax|generic_data_structure#}
fn List(comptime T: type) type {
return struct {
items: []T,
len: usize,
};
}
// The generic List data structure can be instantiated by passing in a type:
var buffer: [10]i32 = undefined;
var list = List(i32){
.items = &buffer,
.len = 0,
};
{#code_end#}
<p>
That's it. It's a function that returns an anonymous {#syntax#}struct{#endsyntax#}.
For the purposes of error messages and debugging, Zig infers the name
{#syntax#}"List(i32)"{#endsyntax#} from the function name and parameters invoked when creating
the anonymous struct.
</p>
<p>
To explicitly give a type a name, we assign it to a constant.
</p>
{#code_begin|syntax|anonymous_struct_name#}
const Node = struct {
next: ?*Node,
name: []const u8,
};
var node_a = Node{
.next = null,
.name = "Node A",
};
var node_b = Node{
.next = &node_a,
.name = "Node B",
};
{#code_end#}
<p>
In this example, the {#syntax#}Node{#endsyntax#} struct refers to itself.
This works because all top level declarations are order-independent.
As long as the compiler can determine the size of the struct, it is free to refer to itself.
In this case, {#syntax#}Node{#endsyntax#} refers to itself as a pointer, which has a
well-defined size at compile time, so it works fine.
</p>
{#header_close#}
{#header_open|Case Study: print in Zig#}
<p>
Putting all of this together, let's see how {#syntax#}print{#endsyntax#} works in Zig.
</p>
{#code_begin|exe|print#}
const print = @import("std").debug.print;
const a_number: i32 = 1234;
const a_string = "foobar";
pub fn main() void {
print("here is a string: '{s}' here is a number: {}\n", .{a_string, a_number});
}
{#code_end#}
<p>
Let's crack open the implementation of this and see how it works:
</p>
{#code_begin|syntax|poc_print_fn#}
const Writer = struct {
/// Calls print and then flushes the buffer.
pub fn print(self: *Writer, comptime format: []const u8, args: anytype) anyerror!void {
const State = enum {
start,
open_brace,
close_brace,
};
comptime var start_index: usize = 0;
comptime var state = State.start;
comptime var next_arg: usize = 0;
inline for (format, 0..) |c, i| {
switch (state) {
State.start => switch (c) {
'{' => {
if (start_index < i) try self.write(format[start_index..i]);
state = State.open_brace;
},
'}' => {
if (start_index < i) try self.write(format[start_index..i]);
state = State.close_brace;
},
else => {},
},
State.open_brace => switch (c) {
'{' => {
state = State.start;
start_index = i;
},
'}' => {
try self.printValue(args[next_arg]);
next_arg += 1;
state = State.start;
start_index = i + 1;
},
's' => {
continue;
},
else => @compileError("Unknown format character: " ++ [1]u8{c}),
},
State.close_brace => switch (c) {
'}' => {
state = State.start;
start_index = i;
},
else => @compileError("Single '}' encountered in format string"),
},
}
}
comptime {
if (args.len != next_arg) {
@compileError("Unused arguments");
}
if (state != State.start) {
@compileError("Incomplete format string: " ++ format);
}
}
if (start_index < format.len) {
try self.write(format[start_index..format.len]);
}
try self.flush();
}
fn write(self: *Writer, value: []const u8) !void {
_ = self;
_ = value;
}
pub fn printValue(self: *Writer, value: anytype) !void {
_ = self;
_ = value;
}
fn flush(self: *Writer) !void {
_ = self;
}
};
{#code_end#}
<p>
This is a proof of concept implementation; the actual function in the standard library has more
formatting capabilities.
</p>
<p>
Note that this is not hard-coded into the Zig compiler; this is userland code in the standard library.
</p>
<p>
When this function is analyzed from our example code above, Zig partially evaluates the function
and emits a function that actually looks like this:
</p>
{#syntax_block|zig|Emitted print Function#}
pub fn print(self: *Writer, arg0: []const u8, arg1: i32) !void {
try self.write("here is a string: '");
try self.printValue(arg0);
try self.write("' here is a number: ");
try self.printValue(arg1);
try self.write("\n");
try self.flush();
}
{#end_syntax_block#}
<p>
{#syntax#}printValue{#endsyntax#} is a function that takes a parameter of any type, and does different things depending
on the type:
</p>
{#code_begin|syntax|poc_printValue_fn#}
const Writer = struct {
pub fn printValue(self: *Writer, value: anytype) !void {
switch (@typeInfo(@TypeOf(value))) {
.Int => {
return self.writeInt(value);
},
.Float => {
return self.writeFloat(value);
},
.Pointer => {
return self.write(value);
},
else => {
@compileError("Unable to print type '" ++ @typeName(@TypeOf(value)) ++ "'");
},
}
}
fn write(self: *Writer, value: []const u8) !void {
_ = self;
_ = value;
}
fn writeInt(self: *Writer, value: anytype) !void {
_ = self;
_ = value;
}
fn writeFloat(self: *Writer, value: anytype) !void {
_ = self;
_ = value;
}
};
{#code_end#}
<p>
And now, what happens if we give too many arguments to {#syntax#}print{#endsyntax#}?
</p>
{#code_begin|test_err|test_print_too_many_args|unused argument in 'here is a string: '{s}' here is a number: {}#}
const print = @import("std").debug.print;
const a_number: i32 = 1234;
const a_string = "foobar";
test "print too many arguments" {
print("here is a string: '{s}' here is a number: {}\n", .{
a_string,
a_number,
a_number,
});
}
{#code_end#}
<p>
Zig gives programmers the tools needed to protect themselves against their own mistakes.
</p>
<p>
Zig doesn't care whether the format argument is a string literal,
only that it is a compile-time known value that can be coerced to a {#syntax#}[]const u8{#endsyntax#}:
</p>
{#code_begin|exe|print_comptime-known_format#}
const print = @import("std").debug.print;
const a_number: i32 = 1234;
const a_string = "foobar";
const fmt = "here is a string: '{s}' here is a number: {}\n";
pub fn main() void {
print(fmt, .{a_string, a_number});
}
{#code_end#}
<p>
This works fine.
</p>
<p>
Zig does not special case string formatting in the compiler and instead exposes enough power to accomplish this
task in userland. It does so without introducing another language on top of Zig, such as
a macro language or a preprocessor language. It's Zig all the way down.
</p>
{#header_close#}
{#see_also|inline while|inline for#}
{#header_close#}
{#header_open|Assembly#}
<p>
For some use cases, it may be necessary to directly control the machine code generated
by Zig programs, rather than relying on Zig's code generation. For these cases, one
can use inline assembly. Here is an example of implementing Hello, World on x86_64 Linux
using inline assembly:
</p>
{#code_begin|exe|inline_assembly#}
{#target_linux_x86_64#}
pub fn main() noreturn {
const msg = "hello world\n";
_ = syscall3(SYS_write, STDOUT_FILENO, @intFromPtr(msg), msg.len);
_ = syscall1(SYS_exit, 0);
unreachable;
}
pub const SYS_write = 1;
pub const SYS_exit = 60;
pub const STDOUT_FILENO = 1;
pub fn syscall1(number: usize, arg1: usize) usize {
return asm volatile ("syscall"
: [ret] "={rax}" (-> usize),
: [number] "{rax}" (number),
[arg1] "{rdi}" (arg1),
: "rcx", "r11"
);
}
pub fn syscall3(number: usize, arg1: usize, arg2: usize, arg3: usize) usize {
return asm volatile ("syscall"
: [ret] "={rax}" (-> usize),
: [number] "{rax}" (number),
[arg1] "{rdi}" (arg1),
[arg2] "{rsi}" (arg2),
[arg3] "{rdx}" (arg3),
: "rcx", "r11"
);
}
{#code_end#}
<p>
Dissecting the syntax:
</p>
{#code_begin|syntax|Assembly Syntax Explained#}
pub fn syscall1(number: usize, arg1: usize) usize {
// Inline assembly is an expression which returns a value.
// the `asm` keyword begins the expression.
return asm
// `volatile` is an optional modifier that tells Zig this
// inline assembly expression has side-effects. Without
// `volatile`, Zig is allowed to delete the inline assembly
// code if the result is unused.
volatile (
// Next is a comptime string which is the assembly code.
// Inside this string one may use `%[ret]`, `%[number]`,
// or `%[arg1]` where a register is expected, to specify
// the register that Zig uses for the argument or return value,
// if the register constraint strings are used. However in
// the below code, this is not used. A literal `%` can be
// obtained by escaping it with a double percent: `%%`.
// Often multiline string syntax comes in handy here.
\\syscall
// Next is the output. It is possible in the future Zig will
// support multiple outputs, depending on how
// https://github.com/ziglang/zig/issues/215 is resolved.
// It is allowed for there to be no outputs, in which case
// this colon would be directly followed by the colon for the inputs.
:
// This specifies the name to be used in `%[ret]` syntax in
// the above assembly string. This example does not use it,
// but the syntax is mandatory.
[ret]
// Next is the output constraint string. This feature is still
// considered unstable in Zig, and so LLVM/GCC documentation
// must be used to understand the semantics.
// http://releases.llvm.org/10.0.0/docs/LangRef.html#inline-asm-constraint-string
// https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
// In this example, the constraint string means "the result value of
// this inline assembly instruction is whatever is in $rax".
"={rax}"
// Next is either a value binding, or `->` and then a type. The
// type is the result type of the inline assembly expression.
// If it is a value binding, then `%[ret]` syntax would be used
// to refer to the register bound to the value.
(-> usize),
// Next is the list of inputs.
// The constraint for these inputs means, "when the assembly code is
// executed, $rax shall have the value of `number` and $rdi shall have
// the value of `arg1`". Any number of input parameters is allowed,
// including none.
: [number] "{rax}" (number),
[arg1] "{rdi}" (arg1),
// Next is the list of clobbers. These declare a set of registers whose
// values will not be preserved by the execution of this assembly code.
// These do not include output or input registers. The special clobber
// value of "memory" means that the assembly writes to arbitrary undeclared
// memory locations - not only the memory pointed to by a declared indirect
// output. In this example we list $rcx and $r11 because it is known the
// kernel syscall does not preserve these registers.
: "rcx", "r11"
);
}
{#code_end#}
<p>
For x86 and x86_64 targets, the syntax is AT&amp;T syntax, rather than the more
popular Intel syntax. This is due to technical constraints; assembly parsing is
provided by LLVM and its support for Intel syntax is buggy and not well tested.
</p>
<p>
Some day Zig may have its own assembler. This would allow it to integrate more seamlessly
into the language, as well as be compatible with the popular NASM syntax. This documentation
section will be updated before 1.0.0 is released, with a conclusive statement about the status
of AT&amp;T vs Intel/NASM syntax.
</p>
{#header_open|Output Constraints#}
<p>
Output constraints are still considered to be unstable in Zig, and
so
<a href="http://releases.llvm.org/10.0.0/docs/LangRef.html#inline-asm-constraint-string">LLVM documentation</a>
and
<a href="https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html">GCC documentation</a>
must be used to understand the semantics.
</p>
<p>
Note that some breaking changes to output constraints are planned with
<a href="https://github.com/ziglang/zig/issues/215">issue #215</a>.
</p>
{#header_close#}
{#header_open|Input Constraints#}
<p>
Input constraints are still considered to be unstable in Zig, and
so
<a href="http://releases.llvm.org/10.0.0/docs/LangRef.html#inline-asm-constraint-string">LLVM documentation</a>
and
<a href="https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html">GCC documentation</a>
must be used to understand the semantics.
</p>
<p>
Note that some breaking changes to input constraints are planned with
<a href="https://github.com/ziglang/zig/issues/215">issue #215</a>.
</p>
{#header_close#}
{#header_open|Clobbers#}
<p>
Clobbers are the set of registers whose values will not be preserved by the execution of
the assembly code. These do not include output or input registers. The special clobber
value of {#syntax#}"memory"{#endsyntax#} means that the assembly causes writes to
arbitrary undeclared memory locations - not only the memory pointed to by a declared
indirect output.
</p>
<p>
Failure to declare the full set of clobbers for a given inline assembly
expression is unchecked {#link|Undefined Behavior#}.
</p>
{#header_close#}
{#header_open|Global Assembly#}
<p>
When an assembly expression occurs in a {#link|container|Containers#} level {#link|comptime#} block, this is
<strong>global assembly</strong>.
</p>
<p>
This kind of assembly has different rules than inline assembly. First, {#syntax#}volatile{#endsyntax#}
is not valid because all global assembly is unconditionally included.
Second, there are no inputs, outputs, or clobbers. All global assembly is concatenated
verbatim into one long string and assembled together. There are no template substitution rules regarding
<code>%</code> as there are in inline assembly expressions.
</p>
{#code_begin|test|test_global_assembly#}
{#target_linux_x86_64#}
const std = @import("std");
const expect = std.testing.expect;
comptime {
asm (
\\.global my_func;
\\.type my_func, @function;
\\my_func:
\\ lea (%rdi,%rsi,1),%eax
\\ retq
);
}
extern fn my_func(a: i32, b: i32) i32;
test "global assembly" {
try expect(my_func(12, 34) == 46);
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Atomics#}
<p>TODO: @fence()</p>
<p>TODO: @atomic rmw</p>
<p>TODO: builtin atomic memory ordering enum</p>
{#see_also|@atomicLoad|@atomicStore|@atomicRmw|@fence|@cmpxchgWeak|@cmpxchgStrong#}
{#header_close#}
{#header_open|Async Functions#}
<p>Async functions are being temporarily regressed and will be
<a href="https://github.com/ziglang/zig/issues/6025">restored before Zig
0.11.0 is tagged</a>. I apologize for the instability. Please use Zig 0.10.0 with
the <code>-fstage1</code> flag for now if you need this feature.</p>
{#header_close#}
{#header_open|Builtin Functions|2col#}
<p>
Builtin functions are provided by the compiler and are prefixed with <code>@</code>.
The {#syntax#}comptime{#endsyntax#} keyword on a parameter means that the parameter must be known
at compile time.
</p>
{#header_open|@addrSpaceCast#}
<pre>{#syntax#}@addrSpaceCast(ptr: anytype) anytype{#endsyntax#}</pre>
<p>
Converts a pointer from one address space to another. The new address space is inferred
based on the result type. Depending on the current target and address spaces, this cast
may be a no-op, a complex operation, or illegal. If the cast is legal, then the resulting
pointer points to the same memory location as the pointer operand. It is always valid to
cast a pointer between the same address spaces.
</p>
{#header_close#}
{#header_open|@addWithOverflow#}
<pre>{#syntax#}@addWithOverflow(a: anytype, b: anytype) struct { @TypeOf(a, b), u1 }{#endsyntax#}</pre>
<p>
Performs {#syntax#}a + b{#endsyntax#} and returns a tuple with the result and a possible overflow bit.
</p>
{#header_close#}
{#header_open|@alignCast#}
<pre>{#syntax#}@alignCast(ptr: anytype) anytype{#endsyntax#}</pre>
<p>
{#syntax#}ptr{#endsyntax#} can be {#syntax#}*T{#endsyntax#}, {#syntax#}?*T{#endsyntax#}, or {#syntax#}[]T{#endsyntax#}.
Changes the alignment of a pointer. The alignment to use is inferred based on the result type.
</p>
<p>A {#link|pointer alignment safety check|Incorrect Pointer Alignment#} is added
to the generated code to make sure the pointer is aligned as promised.</p>
{#header_close#}
{#header_open|@alignOf#}
<pre>{#syntax#}@alignOf(comptime T: type) comptime_int{#endsyntax#}</pre>
<p>
This function returns the number of bytes that this type should be aligned to
for the current target to match the C ABI. When the child type of a pointer has
this alignment, the alignment can be omitted from the type.
</p>
<pre>{#syntax#}const assert = @import("std").debug.assert;
comptime {
assert(*u32 == *align(@alignOf(u32)) u32);
}{#endsyntax#}</pre>
<p>
The result is a target-specific compile time constant. It is guaranteed to be
less than or equal to {#link|@sizeOf(T)|@sizeOf#}.
</p>
{#see_also|Alignment#}
{#header_close#}
{#header_open|@as#}
<pre>{#syntax#}@as(comptime T: type, expression) T{#endsyntax#}</pre>
<p>
Performs {#link|Type Coercion#}. This cast is allowed when the conversion is unambiguous and safe,
and is the preferred way to convert between types, whenever possible.
</p>
{#header_close#}
{#header_open|@atomicLoad#}
<pre>{#syntax#}@atomicLoad(comptime T: type, ptr: *const T, comptime ordering: AtomicOrder) T{#endsyntax#}</pre>
<p>
This builtin function atomically dereferences a pointer to a {#syntax#}T{#endsyntax#} and returns the value.
</p>
<p>
{#syntax#}T{#endsyntax#} must be a pointer, a {#syntax#}bool{#endsyntax#}, a float,
an integer or an enum.
</p>
<p>{#syntax#}AtomicOrder{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicOrder{#endsyntax#}.</p>
{#see_also|@atomicStore|@atomicRmw|@fence|@cmpxchgWeak|@cmpxchgStrong#}
{#header_close#}
{#header_open|@atomicRmw#}
<pre>{#syntax#}@atomicRmw(comptime T: type, ptr: *T, comptime op: AtomicRmwOp, operand: T, comptime ordering: AtomicOrder) T{#endsyntax#}</pre>
<p>
This builtin function dereferences a pointer to a {#syntax#}T{#endsyntax#} and atomically
modifies the value and returns the previous value.
</p>
<p>
{#syntax#}T{#endsyntax#} must be a pointer, a {#syntax#}bool{#endsyntax#}, a float,
an integer or an enum.
</p>
<p>{#syntax#}AtomicOrder{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicOrder{#endsyntax#}.</p>
<p>{#syntax#}AtomicRmwOp{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicRmwOp{#endsyntax#}.</p>
{#see_also|@atomicStore|@atomicLoad|@fence|@cmpxchgWeak|@cmpxchgStrong#}
{#header_close#}
{#header_open|@atomicStore#}
<pre>{#syntax#}@atomicStore(comptime T: type, ptr: *T, value: T, comptime ordering: AtomicOrder) void{#endsyntax#}</pre>
<p>
This builtin function dereferences a pointer to a {#syntax#}T{#endsyntax#} and atomically stores the given value.
</p>
<p>
{#syntax#}T{#endsyntax#} must be a pointer, a {#syntax#}bool{#endsyntax#}, a float,
an integer or an enum.
</p>
<p>{#syntax#}AtomicOrder{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicOrder{#endsyntax#}.</p>
{#see_also|@atomicLoad|@atomicRmw|@fence|@cmpxchgWeak|@cmpxchgStrong#}
{#header_close#}
{#header_open|@bitCast#}
<pre>{#syntax#}@bitCast(value: anytype) anytype{#endsyntax#}</pre>
<p>
Converts a value of one type to another type. The return type is the
inferred result type.
</p>
<p>
Asserts that {#syntax#}@sizeOf(@TypeOf(value)) == @sizeOf(DestType){#endsyntax#}.
</p>
<p>
Asserts that {#syntax#}@typeInfo(DestType) != .Pointer{#endsyntax#}. Use {#syntax#}@ptrCast{#endsyntax#} or {#syntax#}@ptrFromInt{#endsyntax#} if you need this.
</p>
<p>
Can be used for these things for example:
</p>
<ul>
<li>Convert {#syntax#}f32{#endsyntax#} to {#syntax#}u32{#endsyntax#} bits</li>
<li>Convert {#syntax#}i32{#endsyntax#} to {#syntax#}u32{#endsyntax#} preserving twos complement</li>
</ul>
<p>
Works at compile-time if {#syntax#}value{#endsyntax#} is known at compile time. It's a compile error to bitcast a value of undefined layout; this means that, besides the restriction from types which possess dedicated casting builtins (enums, pointers, error sets), bare structs, error unions, slices, optionals, and any other type without a well-defined memory layout, also cannot be used in this operation.
</p>
{#header_close#}
{#header_open|@bitOffsetOf#}
<pre>{#syntax#}@bitOffsetOf(comptime T: type, comptime field_name: []const u8) comptime_int{#endsyntax#}</pre>
<p>
Returns the bit offset of a field relative to its containing struct.
</p>
<p>
For non {#link|packed structs|packed struct#}, this will always be divisible by {#syntax#}8{#endsyntax#}.
For packed structs, non-byte-aligned fields will share a byte offset, but they will have different
bit offsets.
</p>
{#see_also|@offsetOf#}
{#header_close#}
{#header_open|@bitSizeOf#}
<pre>{#syntax#}@bitSizeOf(comptime T: type) comptime_int{#endsyntax#}</pre>
<p>
This function returns the number of bits it takes to store {#syntax#}T{#endsyntax#} in memory if the type
were a field in a packed struct/union.
The result is a target-specific compile time constant.
</p>
<p>
This function measures the size at runtime. For types that are disallowed at runtime, such as
{#syntax#}comptime_int{#endsyntax#} and {#syntax#}type{#endsyntax#}, the result is {#syntax#}0{#endsyntax#}.
</p>
{#see_also|@sizeOf|@typeInfo#}
{#header_close#}
{#header_open|@breakpoint#}
<pre>{#syntax#}@breakpoint() void{#endsyntax#}</pre>
<p>
This function inserts a platform-specific debug trap instruction which causes
debuggers to break there.
Unlike for {#syntax#}@trap(){#endsyntax#}, execution may continue after this point if the program is resumed.
</p>
<p>
This function is only valid within function scope.
</p>
{#see_also|@trap#}
{#header_close#}
{#header_open|@mulAdd#}
<pre>{#syntax#}@mulAdd(comptime T: type, a: T, b: T, c: T) T{#endsyntax#}</pre>
<p>
Fused multiply-add, similar to {#syntax#}(a * b) + c{#endsyntax#}, except
only rounds once, and is thus more accurate.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@byteSwap#}
<pre>{#syntax#}@byteSwap(operand: anytype) T{#endsyntax#}</pre>
<p>{#syntax#}@TypeOf(operand){#endsyntax#} must be an integer type or an integer vector type with bit count evenly divisible by 8.</p>
<p>{#syntax#}operand{#endsyntax#} may be an {#link|integer|Integers#} or {#link|vector|Vectors#}.</p>
<p>
Swaps the byte order of the integer. This converts a big endian integer to a little endian integer,
and converts a little endian integer to a big endian integer.
</p>
<p>
Note that for the purposes of memory layout with respect to endianness, the integer type should be
related to the number of bytes reported by {#link|@sizeOf#} bytes. This is demonstrated with
{#syntax#}u24{#endsyntax#}. {#syntax#}@sizeOf(u24) == 4{#endsyntax#}, which means that a
{#syntax#}u24{#endsyntax#} stored in memory takes 4 bytes, and those 4 bytes are what are swapped on
a little vs big endian system. On the other hand, if {#syntax#}T{#endsyntax#} is specified to
be {#syntax#}u24{#endsyntax#}, then only 3 bytes are reversed.
</p>
{#header_close#}
{#header_open|@bitReverse#}
<pre>{#syntax#}@bitReverse(integer: anytype) T{#endsyntax#}</pre>
<p>{#syntax#}@TypeOf(anytype){#endsyntax#} accepts any integer type or integer vector type.</p>
<p>
Reverses the bitpattern of an integer value, including the sign bit if applicable.
</p>
<p>
For example 0b10110110 ({#syntax#}u8 = 182{#endsyntax#}, {#syntax#}i8 = -74{#endsyntax#})
becomes 0b01101101 ({#syntax#}u8 = 109{#endsyntax#}, {#syntax#}i8 = 109{#endsyntax#}).
</p>
{#header_close#}
{#header_open|@offsetOf#}
<pre>{#syntax#}@offsetOf(comptime T: type, comptime field_name: []const u8) comptime_int{#endsyntax#}</pre>
<p>
Returns the byte offset of a field relative to its containing struct.
</p>
{#see_also|@bitOffsetOf#}
{#header_close#}
{#header_open|@call#}
<pre>{#syntax#}@call(modifier: std.builtin.CallModifier, function: anytype, args: anytype) anytype{#endsyntax#}</pre>
<p>
Calls a function, in the same way that invoking an expression with parentheses does:
</p>
{#code_begin|test|test_call_builtin#}
const expect = @import("std").testing.expect;
test "noinline function call" {
try expect(@call(.auto, add, .{3, 9}) == 12);
}
fn add(a: i32, b: i32) i32 {
return a + b;
}
{#code_end#}
<p>
{#syntax#}@call{#endsyntax#} allows more flexibility than normal function call syntax does. The
{#syntax#}CallModifier{#endsyntax#} enum is reproduced here:
</p>
{#code_begin|syntax|builtin.CallModifier struct#}
pub const CallModifier = enum {
/// Equivalent to function call syntax.
auto,
/// Equivalent to async keyword used with function call syntax.
async_kw,
/// Prevents tail call optimization. This guarantees that the return
/// address will point to the callsite, as opposed to the callsite's
/// callsite. If the call is otherwise required to be tail-called
/// or inlined, a compile error is emitted instead.
never_tail,
/// Guarantees that the call will not be inlined. If the call is
/// otherwise required to be inlined, a compile error is emitted instead.
never_inline,
/// Asserts that the function call will not suspend. This allows a
/// non-async function to call an async function.
no_async,
/// Guarantees that the call will be generated with tail call optimization.
/// If this is not possible, a compile error is emitted instead.
always_tail,
/// Guarantees that the call will inlined at the callsite.
/// If this is not possible, a compile error is emitted instead.
always_inline,
/// Evaluates the call at compile-time. If the call cannot be completed at
/// compile-time, a compile error is emitted instead.
compile_time,
};
{#code_end#}
{#header_close#}
{#header_open|@cDefine#}
<pre>{#syntax#}@cDefine(comptime name: []const u8, value) void{#endsyntax#}</pre>
<p>
This function can only occur inside {#syntax#}@cImport{#endsyntax#}.
</p>
<p>
This appends <code>#define $name $value</code> to the {#syntax#}@cImport{#endsyntax#}
temporary buffer.
</p>
<p>
To define without a value, like this:
</p>
<pre><code class="c">#define _GNU_SOURCE</code></pre>
<p>
Use the void value, like this:
</p>
<pre>{#syntax#}@cDefine("_GNU_SOURCE", {}){#endsyntax#}</pre>
{#see_also|Import from C Header File|@cInclude|@cImport|@cUndef|void#}
{#header_close#}
{#header_open|@cImport#}
<pre>{#syntax#}@cImport(expression) type{#endsyntax#}</pre>
<p>
This function parses C code and imports the functions, types, variables,
and compatible macro definitions into a new empty struct type, and then
returns that type.
</p>
<p>
{#syntax#}expression{#endsyntax#} is interpreted at compile time. The builtin functions
{#syntax#}@cInclude{#endsyntax#}, {#syntax#}@cDefine{#endsyntax#}, and {#syntax#}@cUndef{#endsyntax#} work
within this expression, appending to a temporary buffer which is then parsed as C code.
</p>
<p>
Usually you should only have one {#syntax#}@cImport{#endsyntax#} in your entire application, because it saves the compiler
from invoking clang multiple times, and prevents inline functions from being duplicated.
</p>
<p>
Reasons for having multiple {#syntax#}@cImport{#endsyntax#} expressions would be:
</p>
<ul>
<li>To avoid a symbol collision, for example if foo.h and bar.h both <code>#define CONNECTION_COUNT</code></li>
<li>To analyze the C code with different preprocessor defines</li>
</ul>
{#see_also|Import from C Header File|@cInclude|@cDefine|@cUndef#}
{#header_close#}
{#header_open|@cInclude#}
<pre>{#syntax#}@cInclude(comptime path: []const u8) void{#endsyntax#}</pre>
<p>
This function can only occur inside {#syntax#}@cImport{#endsyntax#}.
</p>
<p>
This appends <code>#include &lt;$path&gt;\n</code> to the {#syntax#}c_import{#endsyntax#}
temporary buffer.
</p>
{#see_also|Import from C Header File|@cImport|@cDefine|@cUndef#}
{#header_close#}
{#header_open|@clz#}
<pre>{#syntax#}@clz(operand: anytype) anytype{#endsyntax#}</pre>
<p>{#syntax#}@TypeOf(operand){#endsyntax#} must be an integer type or an integer vector type.</p>
<p>{#syntax#}operand{#endsyntax#} may be an {#link|integer|Integers#} or {#link|vector|Vectors#}.</p>
<p>
Counts the number of most-significant (leading in a big-endian sense) zeroes in an integer - "count leading zeroes".
</p>
<p>
If {#syntax#}operand{#endsyntax#} is a {#link|comptime#}-known integer,
the return type is {#syntax#}comptime_int{#endsyntax#}.
Otherwise, the return type is an unsigned integer or vector of unsigned integers with the minimum number
of bits that can represent the bit count of the integer type.
</p>
<p>
If {#syntax#}operand{#endsyntax#} is zero, {#syntax#}@clz{#endsyntax#} returns the bit width
of integer type {#syntax#}T{#endsyntax#}.
</p>
{#see_also|@ctz|@popCount#}
{#header_close#}
{#header_open|@cmpxchgStrong#}
<pre>{#syntax#}@cmpxchgStrong(comptime T: type, ptr: *T, expected_value: T, new_value: T, success_order: AtomicOrder, fail_order: AtomicOrder) ?T{#endsyntax#}</pre>
<p>
This function performs a strong atomic compare-and-exchange operation, returning {#syntax#}null{#endsyntax#}
if the current value is not the given expected value. It's the equivalent of this code,
except atomic:
</p>
{#code_begin|syntax|not_atomic_cmpxchgStrong#}
fn cmpxchgStrongButNotAtomic(comptime T: type, ptr: *T, expected_value: T, new_value: T) ?T {
const old_value = ptr.*;
if (old_value == expected_value) {
ptr.* = new_value;
return null;
} else {
return old_value;
}
}
{#code_end#}
<p>
If you are using cmpxchg in a retry loop, {#link|@cmpxchgWeak#} is the better choice, because it can be implemented
more efficiently in machine instructions.
</p>
<p>
{#syntax#}T{#endsyntax#} must be a pointer, a {#syntax#}bool{#endsyntax#}, a float,
an integer or an enum.
</p>
<p>{#syntax#}@typeInfo(@TypeOf(ptr)).Pointer.alignment{#endsyntax#} must be {#syntax#}>= @sizeOf(T).{#endsyntax#}</p>
<p>{#syntax#}AtomicOrder{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicOrder{#endsyntax#}.</p>
{#see_also|@atomicStore|@atomicLoad|@atomicRmw|@fence|@cmpxchgWeak#}
{#header_close#}
{#header_open|@cmpxchgWeak#}
<pre>{#syntax#}@cmpxchgWeak(comptime T: type, ptr: *T, expected_value: T, new_value: T, success_order: AtomicOrder, fail_order: AtomicOrder) ?T{#endsyntax#}</pre>
<p>
This function performs a weak atomic compare-and-exchange operation, returning {#syntax#}null{#endsyntax#}
if the current value is not the given expected value. It's the equivalent of this code,
except atomic:
</p>
{#syntax_block|zig|cmpxchgWeakButNotAtomic#}
fn cmpxchgWeakButNotAtomic(comptime T: type, ptr: *T, expected_value: T, new_value: T) ?T {
const old_value = ptr.*;
if (old_value == expected_value and usuallyTrueButSometimesFalse()) {
ptr.* = new_value;
return null;
} else {
return old_value;
}
}
{#end_syntax_block#}
<p>
If you are using cmpxchg in a retry loop, the sporadic failure will be no problem, and {#syntax#}cmpxchgWeak{#endsyntax#}
is the better choice, because it can be implemented more efficiently in machine instructions.
However if you need a stronger guarantee, use {#link|@cmpxchgStrong#}.
</p>
<p>
{#syntax#}T{#endsyntax#} must be a pointer, a {#syntax#}bool{#endsyntax#}, a float,
an integer or an enum.
</p>
<p>{#syntax#}@typeInfo(@TypeOf(ptr)).Pointer.alignment{#endsyntax#} must be {#syntax#}>= @sizeOf(T).{#endsyntax#}</p>
<p>{#syntax#}AtomicOrder{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicOrder{#endsyntax#}.</p>
{#see_also|@atomicStore|@atomicLoad|@atomicRmw|@fence|@cmpxchgStrong#}
{#header_close#}
{#header_open|@compileError#}
<pre>{#syntax#}@compileError(comptime msg: []const u8) noreturn{#endsyntax#}</pre>
<p>
This function, when semantically analyzed, causes a compile error with the
message {#syntax#}msg{#endsyntax#}.
</p>
<p>
There are several ways that code avoids being semantically checked, such as
using {#syntax#}if{#endsyntax#} or {#syntax#}switch{#endsyntax#} with compile time constants,
and {#syntax#}comptime{#endsyntax#} functions.
</p>
{#header_close#}
{#header_open|@compileLog#}
<pre>{#syntax#}@compileLog(args: ...) void{#endsyntax#}</pre>
<p>
This function prints the arguments passed to it at compile-time.
</p>
<p>
To prevent accidentally leaving compile log statements in a codebase,
a compilation error is added to the build, pointing to the compile
log statement. This error prevents code from being generated, but
does not otherwise interfere with analysis.
</p>
<p>
This function can be used to do "printf debugging" on
compile-time executing code.
</p>
{#code_begin|test_err|test_compileLog_builtin|found compile log statement#}
const print = @import("std").debug.print;
const num1 = blk: {
var val1: i32 = 99;
@compileLog("comptime val1 = ", val1);
val1 = val1 + 1;
break :blk val1;
};
test "main" {
@compileLog("comptime in main");
print("Runtime in main, num1 = {}.\n", .{num1});
}
{#code_end#}
{#header_close#}
{#header_open|@constCast#}
<pre>{#syntax#}@constCast(value: anytype) DestType{#endsyntax#}</pre>
<p>
Remove {#syntax#}const{#endsyntax#} qualifier from a pointer.
</p>
{#header_close#}
{#header_open|@ctz#}
<pre>{#syntax#}@ctz(operand: anytype) anytype{#endsyntax#}</pre>
<p>{#syntax#}@TypeOf(operand){#endsyntax#} must be an integer type or an integer vector type.</p>
<p>{#syntax#}operand{#endsyntax#} may be an {#link|integer|Integers#} or {#link|vector|Vectors#}.</p>
<p>
Counts the number of least-significant (trailing in a big-endian sense) zeroes in an integer - "count trailing zeroes".
</p>
<p>
If {#syntax#}operand{#endsyntax#} is a {#link|comptime#}-known integer,
the return type is {#syntax#}comptime_int{#endsyntax#}.
Otherwise, the return type is an unsigned integer or vector of unsigned integers with the minimum number
of bits that can represent the bit count of the integer type.
</p>
<p>
If {#syntax#}operand{#endsyntax#} is zero, {#syntax#}@ctz{#endsyntax#} returns
the bit width of integer type {#syntax#}T{#endsyntax#}.
</p>
{#see_also|@clz|@popCount#}
{#header_close#}
{#header_open|@cUndef#}
<pre>{#syntax#}@cUndef(comptime name: []const u8) void{#endsyntax#}</pre>
<p>
This function can only occur inside {#syntax#}@cImport{#endsyntax#}.
</p>
<p>
This appends <code>#undef $name</code> to the {#syntax#}@cImport{#endsyntax#}
temporary buffer.
</p>
{#see_also|Import from C Header File|@cImport|@cDefine|@cInclude#}
{#header_close#}
{#header_open|@cVaArg#}
<pre>{#syntax#}@cVaArg(operand: *std.builtin.VaList, comptime T: type) T{#endsyntax#}</pre>
<p>
Implements the C macro {#syntax#}va_arg{#endsyntax#}.
</p>
{#see_also|@cVaCopy|@cVaEnd|@cVaStart#}
{#header_close#}
{#header_open|@cVaCopy#}
<pre>{#syntax#}@cVaCopy(src: *std.builtin.VaList) std.builtin.VaList{#endsyntax#}</pre>
<p>
Implements the C macro {#syntax#}va_copy{#endsyntax#}.
</p>
{#see_also|@cVaArg|@cVaEnd|@cVaStart#}
{#header_close#}
{#header_open|@cVaEnd#}
<pre>{#syntax#}@cVaEnd(src: *std.builtin.VaList) void{#endsyntax#}</pre>
<p>
Implements the C macro {#syntax#}va_end{#endsyntax#}.
</p>
{#see_also|@cVaArg|@cVaCopy|@cVaStart#}
{#header_close#}
{#header_open|@cVaStart#}
<pre>{#syntax#}@cVaStart() std.builtin.VaList{#endsyntax#}</pre>
<p>
Implements the C macro {#syntax#}va_start{#endsyntax#}. Only valid inside a variadic function.
</p>
{#see_also|@cVaArg|@cVaCopy|@cVaEnd#}
{#header_close#}
{#header_open|@divExact#}
<pre>{#syntax#}@divExact(numerator: T, denominator: T) T{#endsyntax#}</pre>
<p>
Exact division. Caller guarantees {#syntax#}denominator != 0{#endsyntax#} and
{#syntax#}@divTrunc(numerator, denominator) * denominator == numerator{#endsyntax#}.
</p>
<ul>
<li>{#syntax#}@divExact(6, 3) == 2{#endsyntax#}</li>
<li>{#syntax#}@divExact(a, b) * b == a{#endsyntax#}</li>
</ul>
<p>For a function that returns a possible error code, use {#syntax#}@import("std").math.divExact{#endsyntax#}.</p>
{#see_also|@divTrunc|@divFloor#}
{#header_close#}
{#header_open|@divFloor#}
<pre>{#syntax#}@divFloor(numerator: T, denominator: T) T{#endsyntax#}</pre>
<p>
Floored division. Rounds toward negative infinity. For unsigned integers it is
the same as {#syntax#}numerator / denominator{#endsyntax#}. Caller guarantees {#syntax#}denominator != 0{#endsyntax#} and
{#syntax#}!(@typeInfo(T) == .Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1){#endsyntax#}.
</p>
<ul>
<li>{#syntax#}@divFloor(-5, 3) == -2{#endsyntax#}</li>
<li>{#syntax#}(@divFloor(a, b) * b) + @mod(a, b) == a{#endsyntax#}</li>
</ul>
<p>For a function that returns a possible error code, use {#syntax#}@import("std").math.divFloor{#endsyntax#}.</p>
{#see_also|@divTrunc|@divExact#}
{#header_close#}
{#header_open|@divTrunc#}
<pre>{#syntax#}@divTrunc(numerator: T, denominator: T) T{#endsyntax#}</pre>
<p>
Truncated division. Rounds toward zero. For unsigned integers it is
the same as {#syntax#}numerator / denominator{#endsyntax#}. Caller guarantees {#syntax#}denominator != 0{#endsyntax#} and
{#syntax#}!(@typeInfo(T) == .Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1){#endsyntax#}.
</p>
<ul>
<li>{#syntax#}@divTrunc(-5, 3) == -1{#endsyntax#}</li>
<li>{#syntax#}(@divTrunc(a, b) * b) + @rem(a, b) == a{#endsyntax#}</li>
</ul>
<p>For a function that returns a possible error code, use {#syntax#}@import("std").math.divTrunc{#endsyntax#}.</p>
{#see_also|@divFloor|@divExact#}
{#header_close#}
{#header_open|@embedFile#}
<pre>{#syntax#}@embedFile(comptime path: []const u8) *const [N:0]u8{#endsyntax#}</pre>
<p>
This function returns a compile time constant pointer to null-terminated,
fixed-size array with length equal to the byte count of the file given by
{#syntax#}path{#endsyntax#}. The contents of the array are the contents of the file.
This is equivalent to a {#link|string literal|String Literals and Unicode Code Point Literals#}
with the file contents.
</p>
<p>
{#syntax#}path{#endsyntax#} is absolute or relative to the current file, just like {#syntax#}@import{#endsyntax#}.
</p>
{#see_also|@import#}
{#header_close#}
{#header_open|@enumFromInt#}
<pre>{#syntax#}@enumFromInt(integer: anytype) anytype{#endsyntax#}</pre>
<p>
Converts an integer into an {#link|enum#} value. The return type is the inferred result type.
</p>
<p>
Attempting to convert an integer which represents no value in the chosen enum type invokes
safety-checked {#link|Undefined Behavior#}.
</p>
{#see_also|@intFromEnum#}
{#header_close#}
{#header_open|@errorFromInt#}
<pre>{#syntax#}@errorFromInt(value: std.meta.Int(.unsigned, @bitSizeOf(anyerror))) anyerror{#endsyntax#}</pre>
<p>
Converts from the integer representation of an error into {#link|The Global Error Set#} type.
</p>
<p>
It is generally recommended to avoid this
cast, as the integer representation of an error is not stable across source code changes.
</p>
<p>
Attempting to convert an integer that does not correspond to any error results in
safety-protected {#link|Undefined Behavior#}.
</p>
{#see_also|@intFromError#}
{#header_close#}
{#header_open|@errorName#}
<pre>{#syntax#}@errorName(err: anyerror) [:0]const u8{#endsyntax#}</pre>
<p>
This function returns the string representation of an error. The string representation
of {#syntax#}error.OutOfMem{#endsyntax#} is {#syntax#}"OutOfMem"{#endsyntax#}.
</p>
<p>
If there are no calls to {#syntax#}@errorName{#endsyntax#} in an entire application,
or all calls have a compile-time known value for {#syntax#}err{#endsyntax#}, then no
error name table will be generated.
</p>
{#header_close#}
{#header_open|@errorReturnTrace#}
<pre>{#syntax#}@errorReturnTrace() ?*builtin.StackTrace{#endsyntax#}</pre>
<p>
If the binary is built with error return tracing, and this function is invoked in a
function that calls a function with an error or error union return type, returns a
stack trace object. Otherwise returns {#link|null#}.
</p>
{#header_close#}
{#header_open|@errorCast#}
<pre>{#syntax#}@errorCast(value: anytype) anytype{#endsyntax#}</pre>
<p>
Converts an error set or error union value from one error set to another error set. The return type is the
inferred result type. Attempting to convert an error which is not in the destination error
set results in safety-protected {#link|Undefined Behavior#}.
</p>
{#header_close#}
{#header_open|@export#}
<pre>{#syntax#}@export(declaration, comptime options: std.builtin.ExportOptions) void{#endsyntax#}</pre>
<p>
Creates a symbol in the output object file.
</p>
<p>
<code>declaration</code> must be one of two things:
</p>
<ul>
<li>An identifier ({#syntax#}x{#endsyntax#}) identifying a {#link|function|Functions#} or a
{#link|variable|Container Level Variables#}.</li>
<li>Field access ({#syntax#}x.y{#endsyntax#}) looking up a {#link|function|Functions#} or a
{#link|variable|Container Level Variables#}.</li>
</ul>
<p>
This builtin can be called from a {#link|comptime#} block to conditionally export symbols.
When <code>declaration</code> is a function with the C calling convention and
{#syntax#}options.linkage{#endsyntax#} is {#syntax#}Strong{#endsyntax#}, this is equivalent to
the {#syntax#}export{#endsyntax#} keyword used on a function:
</p>
{#code_begin|obj|export_builtin#}
comptime {
@export(internalName, .{ .name = "foo", .linkage = .strong });
}
fn internalName() callconv(.C) void {}
{#code_end#}
<p>This is equivalent to:</p>
{#code_begin|obj|export_builtin_equivalent_code#}
export fn foo() void {}
{#code_end#}
<p>Note that even when using {#syntax#}export{#endsyntax#}, the {#syntax#}@"foo"{#endsyntax#} syntax for
{#link|identifiers|Identifiers#} can be used to choose any string for the symbol name:</p>
{#code_begin|obj|export_any_symbol_name#}
export fn @"A function name that is a complete sentence."() void {}
{#code_end#}
<p>
When looking at the resulting object, you can see the symbol is used verbatim:
</p>
<pre><code>00000000000001f0 T A function name that is a complete sentence.</code></pre>
{#see_also|Exporting a C Library#}
{#header_close#}
{#header_open|@extern#}
<pre>{#syntax#}@extern(T: type, comptime options: std.builtin.ExternOptions) T{#endsyntax#}</pre>
<p>
Creates a reference to an external symbol in the output object file.
T must be a pointer type.
</p>
{#see_also|@export#}
{#header_close#}
{#header_open|@fence#}
<pre>{#syntax#}@fence(order: AtomicOrder) void{#endsyntax#}</pre>
<p>
The {#syntax#}fence{#endsyntax#} function is used to introduce happens-before edges between operations.
</p>
<p>{#syntax#}AtomicOrder{#endsyntax#} can be found with {#syntax#}@import("std").builtin.AtomicOrder{#endsyntax#}.</p>
{#see_also|@atomicStore|@atomicLoad|@atomicRmw|@cmpxchgWeak|@cmpxchgStrong#}
{#header_close#}
{#header_open|@field#}
<pre>{#syntax#}@field(lhs: anytype, comptime field_name: []const u8) (field){#endsyntax#}</pre>
<p>Performs field access by a compile-time string. Works on both fields and declarations.
</p>
{#code_begin|test|test_field_builtin#}
const std = @import("std");
const Point = struct {
x: u32,
y: u32,
pub var z: u32 = 1;
};
test "field access by string" {
const expect = std.testing.expect;
var p = Point{ .x = 0, .y = 0 };
@field(p, "x") = 4;
@field(p, "y") = @field(p, "x") + 1;
try expect(@field(p, "x") == 4);
try expect(@field(p, "y") == 5);
}
test "decl access by string" {
const expect = std.testing.expect;
try expect(@field(Point, "z") == 1);
@field(Point, "z") = 2;
try expect(@field(Point, "z") == 2);
}
{#code_end#}
{#header_close#}
{#header_open|@fieldParentPtr#}
<pre>{#syntax#}@fieldParentPtr(comptime field_name: []const u8, field_ptr: *T) anytype{#endsyntax#}</pre>
<p>
Given a pointer to a field, returns the base pointer of a struct.
</p>
{#header_close#}
{#header_open|@floatCast#}
<pre>{#syntax#}@floatCast(value: anytype) anytype{#endsyntax#}</pre>
<p>
Convert from one float type to another. This cast is safe, but may cause the
numeric value to lose precision. The return type is the inferred result type.
</p>
{#header_close#}
{#header_open|@floatFromInt#}
<pre>{#syntax#}@floatFromInt(int: anytype) anytype{#endsyntax#}</pre>
<p>
Converts an integer to the closest floating point representation. The return type is the inferred result type.
To convert the other way, use {#link|@intFromFloat#}. This operation is legal
for all values of all integer types.
</p>
{#header_close#}
{#header_open|@frameAddress#}
<pre>{#syntax#}@frameAddress() usize{#endsyntax#}</pre>
<p>
This function returns the base pointer of the current stack frame.
</p>
<p>
The implications of this are target-specific and not consistent across all
platforms. The frame address may not be available in release mode due to
aggressive optimizations.
</p>
<p>
This function is only valid within function scope.
</p>
{#header_close#}
{#header_open|@hasDecl#}
<pre>{#syntax#}@hasDecl(comptime Container: type, comptime name: []const u8) bool{#endsyntax#}</pre>
<p>
Returns whether or not a {#link|container|Containers#} has a declaration
matching {#syntax#}name{#endsyntax#}.
</p>
{#code_begin|test|test_hasDecl_builtin#}
const std = @import("std");
const expect = std.testing.expect;
const Foo = struct {
nope: i32,
pub var blah = "xxx";
const hi = 1;
};
test "@hasDecl" {
try expect(@hasDecl(Foo, "blah"));
// Even though `hi` is private, @hasDecl returns true because this test is
// in the same file scope as Foo. It would return false if Foo was declared
// in a different file.
try expect(@hasDecl(Foo, "hi"));
// @hasDecl is for declarations; not fields.
try expect(!@hasDecl(Foo, "nope"));
try expect(!@hasDecl(Foo, "nope1234"));
}
{#code_end#}
{#see_also|@hasField#}
{#header_close#}
{#header_open|@hasField#}
<pre>{#syntax#}@hasField(comptime Container: type, comptime name: []const u8) bool{#endsyntax#}</pre>
<p>Returns whether the field name of a struct, union, or enum exists.</p>
<p>
The result is a compile time constant.
</p>
<p>
It does not include functions, variables, or constants.
</p>
{#see_also|@hasDecl#}
{#header_close#}
{#header_open|@import#}
<pre>{#syntax#}@import(comptime path: []const u8) type{#endsyntax#}</pre>
<p>
This function finds a zig file corresponding to {#syntax#}path{#endsyntax#} and adds it to the build,
if it is not already added.
</p>
<p>
Zig source files are implicitly structs, with a name equal to the file's basename with the extension
truncated. {#syntax#}@import{#endsyntax#} returns the struct type corresponding to the file.
</p>
<p>
Declarations which have the {#syntax#}pub{#endsyntax#} keyword may be referenced from a different
source file than the one they are declared in.
</p>
<p>
{#syntax#}path{#endsyntax#} can be a relative path or it can be the name of a package.
If it is a relative path, it is relative to the file that contains the {#syntax#}@import{#endsyntax#}
function call.
</p>
<p>
The following packages are always available:
</p>
<ul>
<li>{#syntax#}@import("std"){#endsyntax#} - Zig Standard Library</li>
<li>{#syntax#}@import("builtin"){#endsyntax#} - Target-specific information
The command <code>zig build-exe --show-builtin</code> outputs the source to stdout for reference.
</li>
<li>{#syntax#}@import("root"){#endsyntax#} - Root source file
This is usually <code>src/main.zig</code> but depends on what file is built.
</li>
</ul>
{#see_also|Compile Variables|@embedFile#}
{#header_close#}
{#header_open|@inComptime#}
<pre>{#syntax#}@inComptime() bool{#endsyntax#}</pre>
<p>
Returns whether the builtin was run in a {#syntax#}comptime{#endsyntax#} context. The result is a compile-time constant.
</p>
<p>
This can be used to provide alternative, comptime-friendly implementations of functions. It should not be used, for instance, to exclude certain functions from being evaluated at comptime.
</p>
{#see_also|comptime#}
{#header_close#}
{#header_open|@intCast#}
<pre>{#syntax#}@intCast(int: anytype) anytype{#endsyntax#}</pre>
<p>
Converts an integer to another integer while keeping the same numerical value.
The return type is the inferred result type.
Attempting to convert a number which is out of range of the destination type results in
safety-protected {#link|Undefined Behavior#}.
</p>
{#code_begin|test_err|test_intCast_builtin|cast truncated bits#}
test "integer cast panic" {
var a: u16 = 0xabcd; // runtime-known
_ = &a;
const b: u8 = @intCast(a);
_ = b;
}
{#code_end#}
<p>
To truncate the significant bits of a number out of range of the destination type, use {#link|@truncate#}.
</p>
<p>
If {#syntax#}T{#endsyntax#} is {#syntax#}comptime_int{#endsyntax#},
then this is semantically equivalent to {#link|Type Coercion#}.
</p>
{#header_close#}
{#header_open|@intFromBool#}
<pre>{#syntax#}@intFromBool(value: bool) u1{#endsyntax#}</pre>
<p>
Converts {#syntax#}true{#endsyntax#} to {#syntax#}@as(u1, 1){#endsyntax#} and {#syntax#}false{#endsyntax#} to
{#syntax#}@as(u1, 0){#endsyntax#}.
</p>
{#header_close#}
{#header_open|@intFromEnum#}
<pre>{#syntax#}@intFromEnum(enum_or_tagged_union: anytype) anytype{#endsyntax#}</pre>
<p>
Converts an enumeration value into its integer tag type. When a tagged union is passed,
the tag value is used as the enumeration value.
</p>
<p>
If there is only one possible enum value, the result is a {#syntax#}comptime_int{#endsyntax#}
known at {#link|comptime#}.
</p>
{#see_also|@enumFromInt#}
{#header_close#}
{#header_open|@intFromError#}
<pre>{#syntax#}@intFromError(err: anytype) std.meta.Int(.unsigned, @bitSizeOf(anyerror)){#endsyntax#}</pre>
<p>
Supports the following types:
</p>
<ul>
<li>{#link|The Global Error Set#}</li>
<li>{#link|Error Set Type#}</li>
<li>{#link|Error Union Type#}</li>
</ul>
<p>
Converts an error to the integer representation of an error.
</p>
<p>
It is generally recommended to avoid this
cast, as the integer representation of an error is not stable across source code changes.
</p>
{#see_also|@errorFromInt#}
{#header_close#}
{#header_open|@intFromFloat#}
<pre>{#syntax#}@intFromFloat(float: anytype) anytype{#endsyntax#}</pre>
<p>
Converts the integer part of a floating point number to the inferred result type.
</p>
<p>
If the integer part of the floating point number cannot fit in the destination type,
it invokes safety-checked {#link|Undefined Behavior#}.
</p>
{#see_also|@floatFromInt#}
{#header_close#}
{#header_open|@intFromPtr#}
<pre>{#syntax#}@intFromPtr(value: anytype) usize{#endsyntax#}</pre>
<p>
Converts {#syntax#}value{#endsyntax#} to a {#syntax#}usize{#endsyntax#} which is the address of the pointer.
{#syntax#}value{#endsyntax#} can be {#syntax#}*T{#endsyntax#} or {#syntax#}?*T{#endsyntax#}.
</p>
<p>To convert the other way, use {#link|@ptrFromInt#}</p>
{#header_close#}
{#header_open|@max#}
<pre>{#syntax#}@max(a: T, b: T) T{#endsyntax#}</pre>
<p>
Returns the maximum value of {#syntax#}a{#endsyntax#} and {#syntax#}b{#endsyntax#}. This builtin accepts integers, floats, and vectors of either. In the latter case, the operation is performed element wise.
</p>
<p>
NaNs are handled as follows: if one of the operands of a (pairwise) operation is NaN, the other operand is returned. If both operands are NaN, NaN is returned.
</p>
{#see_also|@min|Vectors#}
{#header_close#}
{#header_open|@memcpy#}
<pre>{#syntax#}@memcpy(noalias dest, noalias source) void{#endsyntax#}</pre>
<p>This function copies bytes from one region of memory to another.</p>
<p>{#syntax#}dest{#endsyntax#} must be a mutable slice, a mutable pointer to an array, or
a mutable many-item {#link|pointer|Pointers#}. It may have any
alignment, and it may have any element type.</p>
<p>{#syntax#}source{#endsyntax#} must be a slice, a pointer to
an array, or a many-item {#link|pointer|Pointers#}. It may
have any alignment, and it may have any element type.</p>
<p>The {#syntax#}source{#endsyntax#} element type must support {#link|Type Coercion#}
into the {#syntax#}dest{#endsyntax#} element type. The element types may have
different ABI size, however, that may incur a performance penalty.</p>
<p>Similar to {#link|for#} loops, at least one of {#syntax#}source{#endsyntax#} and
{#syntax#}dest{#endsyntax#} must provide a length, and if two lengths are provided,
they must be equal.</p>
<p>Finally, the two memory regions must not overlap.</p>
{#header_close#}
{#header_open|@memset#}
<pre>{#syntax#}@memset(dest, elem) void{#endsyntax#}</pre>
<p>This function sets all the elements of a memory region to {#syntax#}elem{#endsyntax#}.</p>
<p>{#syntax#}dest{#endsyntax#} must be a mutable slice or a mutable pointer to an array.
It may have any alignment, and it may have any element type.</p>
<p>{#syntax#}elem{#endsyntax#} is coerced to the element type of {#syntax#}dest{#endsyntax#}.</p>
<p>For securely zeroing out sensitive contents from memory, you should use
{#syntax#}std.crypto.utils.secureZero{#endsyntax#}</p>
{#header_close#}
{#header_open|@min#}
<pre>{#syntax#}@min(a: T, b: T) T{#endsyntax#}</pre>
<p>
Returns the minimum value of {#syntax#}a{#endsyntax#} and {#syntax#}b{#endsyntax#}. This builtin accepts integers, floats, and vectors of either. In the latter case, the operation is performed element wise.
</p>
<p>
NaNs are handled as follows: if one of the operands of a (pairwise) operation is NaN, the other operand is returned. If both operands are NaN, NaN is returned.
</p>
{#see_also|@max|Vectors#}
{#header_close#}
{#header_open|@wasmMemorySize#}
<pre>{#syntax#}@wasmMemorySize(index: u32) u32{#endsyntax#}</pre>
<p>
This function returns the size of the Wasm memory identified by {#syntax#}index{#endsyntax#} as
an unsigned value in units of Wasm pages. Note that each Wasm page is 64KB in size.
</p>
<p>
This function is a low level intrinsic with no safety mechanisms usually useful for allocator
designers targeting Wasm. So unless you are writing a new allocator from scratch, you should use
something like {#syntax#}@import("std").heap.WasmPageAllocator{#endsyntax#}.
</p>
{#see_also|@wasmMemoryGrow#}
{#header_close#}
{#header_open|@wasmMemoryGrow#}
<pre>{#syntax#}@wasmMemoryGrow(index: u32, delta: u32) i32{#endsyntax#}</pre>
<p>
This function increases the size of the Wasm memory identified by {#syntax#}index{#endsyntax#} by
{#syntax#}delta{#endsyntax#} in units of unsigned number of Wasm pages. Note that each Wasm page
is 64KB in size. On success, returns previous memory size; on failure, if the allocation fails,
returns -1.
</p>
<p>
This function is a low level intrinsic with no safety mechanisms usually useful for allocator
designers targeting Wasm. So unless you are writing a new allocator from scratch, you should use
something like {#syntax#}@import("std").heap.WasmPageAllocator{#endsyntax#}.
</p>
{#code_begin|test|test_wasmMemoryGrow_builtin#}
const std = @import("std");
const native_arch = @import("builtin").target.cpu.arch;
const expect = std.testing.expect;
test "@wasmMemoryGrow" {
if (native_arch != .wasm32) return error.SkipZigTest;
const prev = @wasmMemorySize(0);
try expect(prev == @wasmMemoryGrow(0, 1));
try expect(prev + 1 == @wasmMemorySize(0));
}
{#code_end#}
{#see_also|@wasmMemorySize#}
{#header_close#}
{#header_open|@mod#}
<pre>{#syntax#}@mod(numerator: T, denominator: T) T{#endsyntax#}</pre>
<p>
Modulus division. For unsigned integers this is the same as
{#syntax#}numerator % denominator{#endsyntax#}. Caller guarantees {#syntax#}denominator > 0{#endsyntax#}, otherwise the
operation will result in a {#link|Remainder Division by Zero#} when runtime safety checks are enabled.
</p>
<ul>
<li>{#syntax#}@mod(-5, 3) == 1{#endsyntax#}</li>
<li>{#syntax#}(@divFloor(a, b) * b) + @mod(a, b) == a{#endsyntax#}</li>
</ul>
<p>For a function that returns an error code, see {#syntax#}@import("std").math.mod{#endsyntax#}.</p>
{#see_also|@rem#}
{#header_close#}
{#header_open|@mulWithOverflow#}
<pre>{#syntax#}@mulWithOverflow(a: anytype, b: anytype) struct { @TypeOf(a, b), u1 }{#endsyntax#}</pre>
<p>
Performs {#syntax#}a * b{#endsyntax#} and returns a tuple with the result and a possible overflow bit.
</p>
{#header_close#}
{#header_open|@panic#}
<pre>{#syntax#}@panic(message: []const u8) noreturn{#endsyntax#}</pre>
<p>
Invokes the panic handler function. By default the panic handler function
calls the public {#syntax#}panic{#endsyntax#} function exposed in the root source file, or
if there is not one specified, the {#syntax#}std.builtin.default_panic{#endsyntax#}
function from {#syntax#}std/builtin.zig{#endsyntax#}.
</p>
<p>Generally it is better to use {#syntax#}@import("std").debug.panic{#endsyntax#}.
However, {#syntax#}@panic{#endsyntax#} can be useful for 2 scenarios:
</p>
<ul>
<li>From library code, calling the programmer's panic function if they exposed one in the root source file.</li>
<li>When mixing C and Zig code, calling the canonical panic implementation across multiple .o files.</li>
</ul>
{#see_also|Root Source File#}
{#header_close#}
{#header_open|@popCount#}
<pre>{#syntax#}@popCount(operand: anytype) anytype{#endsyntax#}</pre>
<p>{#syntax#}@TypeOf(operand){#endsyntax#} must be an integer type.</p>
<p>{#syntax#}operand{#endsyntax#} may be an {#link|integer|Integers#} or {#link|vector|Vectors#}.</p>
<p>
Counts the number of bits set in an integer - "population count".
</p>
<p>
If {#syntax#}operand{#endsyntax#} is a {#link|comptime#}-known integer,
the return type is {#syntax#}comptime_int{#endsyntax#}.
Otherwise, the return type is an unsigned integer or vector of unsigned integers with the minimum number
of bits that can represent the bit count of the integer type.
</p>
{#see_also|@ctz|@clz#}
{#header_close#}
{#header_open|@prefetch#}
<pre>{#syntax#}@prefetch(ptr: anytype, comptime options: PrefetchOptions) void{#endsyntax#}</pre>
<p>
This builtin tells the compiler to emit a prefetch instruction if supported by the
target CPU. If the target CPU does not support the requested prefetch instruction,
this builtin is a no-op. This function has no effect on the behavior of the program,
only on the performance characteristics.
</p>
<p>
The {#syntax#}ptr{#endsyntax#} argument may be any pointer type and determines the memory
address to prefetch. This function does not dereference the pointer, it is perfectly legal
to pass a pointer to invalid memory to this function and no illegal behavior will result.
</p>
<p>{#syntax#}PrefetchOptions{#endsyntax#} can be found with {#syntax#}@import("std").builtin.PrefetchOptions{#endsyntax#}.</p>
{#header_close#}
{#header_open|@ptrCast#}
<pre>{#syntax#}@ptrCast(value: anytype) anytype{#endsyntax#}</pre>
<p>
Converts a pointer of one type to a pointer of another type. The return type is the inferred result type.
</p>
<p>
{#link|Optional Pointers#} are allowed. Casting an optional pointer which is {#link|null#}
to a non-optional pointer invokes safety-checked {#link|Undefined Behavior#}.
</p>
<p>
{#syntax#}@ptrCast{#endsyntax#} cannot be used for:
</p>
<ul>
<li>Removing {#syntax#}const{#endsyntax#} qualifier, use {#link|@constCast#}.</li>
<li>Removing {#syntax#}volatile{#endsyntax#} qualifier, use {#link|@volatileCast#}.</li>
<li>Changing pointer address space, use {#link|@addrSpaceCast#}.</li>
<li>Increasing pointer alignment, use {#link|@alignCast#}.</li>
<li>Casting a non-slice pointer to a slice, use slicing syntax {#syntax#}ptr[start..end]{#endsyntax#}.</li>
</ul>
{#header_close#}
{#header_open|@ptrFromInt#}
<pre>{#syntax#}@ptrFromInt(address: usize) anytype{#endsyntax#}</pre>
<p>
Converts an integer to a {#link|pointer|Pointers#}. The return type is the inferred result type.
To convert the other way, use {#link|@intFromPtr#}. Casting an address of 0 to a destination type
which in not {#link|optional|Optional Pointers#} and does not have the {#syntax#}allowzero{#endsyntax#} attribute will result in a
{#link|Pointer Cast Invalid Null#} panic when runtime safety checks are enabled.
</p>
<p>
If the destination pointer type does not allow address zero and {#syntax#}address{#endsyntax#}
is zero, this invokes safety-checked {#link|Undefined Behavior#}.
</p>
{#header_close#}
{#header_open|@rem#}
<pre>{#syntax#}@rem(numerator: T, denominator: T) T{#endsyntax#}</pre>
<p>
Remainder division. For unsigned integers this is the same as
{#syntax#}numerator % denominator{#endsyntax#}. Caller guarantees {#syntax#}denominator > 0{#endsyntax#}, otherwise the
operation will result in a {#link|Remainder Division by Zero#} when runtime safety checks are enabled.
</p>
<ul>
<li>{#syntax#}@rem(-5, 3) == -2{#endsyntax#}</li>
<li>{#syntax#}(@divTrunc(a, b) * b) + @rem(a, b) == a{#endsyntax#}</li>
</ul>
<p>For a function that returns an error code, see {#syntax#}@import("std").math.rem{#endsyntax#}.</p>
{#see_also|@mod#}
{#header_close#}
{#header_open|@returnAddress#}
<pre>{#syntax#}@returnAddress() usize{#endsyntax#}</pre>
<p>
This function returns the address of the next machine code instruction that will be executed
when the current function returns.
</p>
<p>
The implications of this are target-specific and not consistent across
all platforms.
</p>
<p>
This function is only valid within function scope. If the function gets inlined into
a calling function, the returned address will apply to the calling function.
</p>
{#header_close#}
{#header_open|@select#}
<pre>{#syntax#}@select(comptime T: type, pred: @Vector(len, bool), a: @Vector(len, T), b: @Vector(len, T)) @Vector(len, T){#endsyntax#}</pre>
<p>
Selects values element-wise from {#syntax#}a{#endsyntax#} or {#syntax#}b{#endsyntax#} based on {#syntax#}pred{#endsyntax#}. If {#syntax#}pred[i]{#endsyntax#} is {#syntax#}true{#endsyntax#}, the corresponding element in the result will be {#syntax#}a[i]{#endsyntax#} and otherwise {#syntax#}b[i]{#endsyntax#}.
</p>
{#see_also|Vectors#}
{#header_close#}
{#header_open|@setAlignStack#}
<pre>{#syntax#}@setAlignStack(comptime alignment: u29) void{#endsyntax#}</pre>
<p>
Ensures that a function will have a stack alignment of at least {#syntax#}alignment{#endsyntax#} bytes.
</p>
{#header_close#}
{#header_open|@setCold#}
<pre>{#syntax#}@setCold(comptime is_cold: bool) void{#endsyntax#}</pre>
<p>
Tells the optimizer that the current function is (or is not) rarely called.
This function is only valid within function scope.
</p>
{#header_close#}
{#header_open|@setEvalBranchQuota#}
<pre>{#syntax#}@setEvalBranchQuota(comptime new_quota: u32) void{#endsyntax#}</pre>
<p>
Increase the maximum number of backwards branches that compile-time code
execution can use before giving up and making a compile error.
</p>
<p>
If the {#syntax#}new_quota{#endsyntax#} is smaller than the default quota ({#syntax#}1000{#endsyntax#}) or
a previously explicitly set quota, it is ignored.
</p>
<p>
Example:
</p>
{#code_begin|test_err|test_without_setEvalBranchQuota_builtin|evaluation exceeded 1000 backwards branches#}
test "foo" {
comptime {
var i = 0;
while (i < 1001) : (i += 1) {}
}
}
{#code_end#}
<p>Now we use {#syntax#}@setEvalBranchQuota{#endsyntax#}:</p>
{#code_begin|test|test_setEvalBranchQuota_builtin#}
test "foo" {
comptime {
@setEvalBranchQuota(1001);
var i = 0;
while (i < 1001) : (i += 1) {}
}
}
{#code_end#}
{#see_also|comptime#}
{#header_close#}
{#header_open|@setFloatMode#}
<pre>{#syntax#}@setFloatMode(comptime mode: FloatMode) void{#endsyntax#}</pre>
<p>Changes the current scope's rules about how floating point operations are defined.</p>
<ul>
<li>
{#syntax#}Strict{#endsyntax#} (default) - Floating point operations follow strict IEEE compliance.
</li>
<li>
{#syntax#}Optimized{#endsyntax#} - Floating point operations may do all of the following:
<ul>
<li>Assume the arguments and result are not NaN. Optimizations are required to retain defined behavior over NaNs, but the value of the result is undefined.</li>
<li>Assume the arguments and result are not +/-Inf. Optimizations are required to retain defined behavior over +/-Inf, but the value of the result is undefined.</li>
<li>Treat the sign of a zero argument or result as insignificant.</li>
<li>Use the reciprocal of an argument rather than perform division.</li>
<li>Perform floating-point contraction (e.g. fusing a multiply followed by an addition into a fused multiply-add).</li>
<li>Perform algebraically equivalent transformations that may change results in floating point (e.g. reassociate).</li>
</ul>
This is equivalent to <code>-ffast-math</code> in GCC.
</li>
</ul>
<p>
The floating point mode is inherited by child scopes, and can be overridden in any scope.
You can set the floating point mode in a struct or module scope by using a comptime block.
</p>
<p>{#syntax#}FloatMode{#endsyntax#} can be found with {#syntax#}@import("std").builtin.FloatMode{#endsyntax#}.</p>
{#see_also|Floating Point Operations#}
{#header_close#}
{#header_open|@setRuntimeSafety#}
<pre>{#syntax#}@setRuntimeSafety(comptime safety_on: bool) void{#endsyntax#}</pre>
<p>
Sets whether runtime safety checks are enabled for the scope that contains the function call.
</p>
{#code_begin|test_safety|test_setRuntimeSafety_builtin|integer overflow#}
{#code_release_fast#}
test "@setRuntimeSafety" {
// The builtin applies to the scope that it is called in. So here, integer overflow
// will not be caught in ReleaseFast and ReleaseSmall modes:
// var x: u8 = 255;
// x += 1; // undefined behavior in ReleaseFast/ReleaseSmall modes.
{
// However this block has safety enabled, so safety checks happen here,
// even in ReleaseFast and ReleaseSmall modes.
@setRuntimeSafety(true);
var x: u8 = 255;
x += 1;
{
// The value can be overridden at any scope. So here integer overflow
// would not be caught in any build mode.
@setRuntimeSafety(false);
// var x: u8 = 255;
// x += 1; // undefined behavior in all build modes.
}
}
}
{#code_end#}
<p>Note: it is <a href="https://github.com/ziglang/zig/issues/978">planned</a> to replace
{#syntax#}@setRuntimeSafety{#endsyntax#} with <code>@optimizeFor</code></p>
{#header_close#}
{#header_open|@shlExact#}
<pre>{#syntax#}@shlExact(value: T, shift_amt: Log2T) T{#endsyntax#}</pre>
<p>
Performs the left shift operation ({#syntax#}<<{#endsyntax#}).
For unsigned integers, the result is {#link|undefined#} if any 1 bits
are shifted out. For signed integers, the result is {#link|undefined#} if
any bits that disagree with the resultant sign bit are shifted out.
</p>
<p>
The type of {#syntax#}shift_amt{#endsyntax#} is an unsigned integer with {#syntax#}log2(@typeInfo(T).Int.bits){#endsyntax#} bits.
This is because {#syntax#}shift_amt >= @typeInfo(T).Int.bits{#endsyntax#} is undefined behavior.
</p>
<p>
{#syntax#}comptime_int{#endsyntax#} is modeled as an integer with an infinite number of bits,
meaning that in such case, {#syntax#}@shlExact{#endsyntax#} always produces a result and
cannot produce a compile error.
</p>
{#see_also|@shrExact|@shlWithOverflow#}
{#header_close#}
{#header_open|@shlWithOverflow#}
<pre>{#syntax#}@shlWithOverflow(a: anytype, shift_amt: Log2T) struct { @TypeOf(a), u1 }{#endsyntax#}</pre>
<p>
Performs {#syntax#}a << b{#endsyntax#} and returns a tuple with the result and a possible overflow bit.
</p>
<p>
The type of {#syntax#}shift_amt{#endsyntax#} is an unsigned integer with {#syntax#}log2(@typeInfo(@TypeOf(a)).Int.bits){#endsyntax#} bits.
This is because {#syntax#}shift_amt >= @typeInfo(@TypeOf(a)).Int.bits{#endsyntax#} is undefined behavior.
</p>
{#see_also|@shlExact|@shrExact#}
{#header_close#}
{#header_open|@shrExact#}
<pre>{#syntax#}@shrExact(value: T, shift_amt: Log2T) T{#endsyntax#}</pre>
<p>
Performs the right shift operation ({#syntax#}>>{#endsyntax#}). Caller guarantees
that the shift will not shift any 1 bits out.
</p>
<p>
The type of {#syntax#}shift_amt{#endsyntax#} is an unsigned integer with {#syntax#}log2(@typeInfo(T).Int.bits){#endsyntax#} bits.
This is because {#syntax#}shift_amt >= @typeInfo(T).Int.bits{#endsyntax#} is undefined behavior.
</p>
{#see_also|@shlExact|@shlWithOverflow#}
{#header_close#}
{#header_open|@shuffle#}
<pre>{#syntax#}@shuffle(comptime E: type, a: @Vector(a_len, E), b: @Vector(b_len, E), comptime mask: @Vector(mask_len, i32)) @Vector(mask_len, E){#endsyntax#}</pre>
<p>
Constructs a new {#link|vector|Vectors#} by selecting elements from {#syntax#}a{#endsyntax#} and
{#syntax#}b{#endsyntax#} based on {#syntax#}mask{#endsyntax#}.
</p>
<p>
Each element in {#syntax#}mask{#endsyntax#} selects an element from either {#syntax#}a{#endsyntax#} or
{#syntax#}b{#endsyntax#}. Positive numbers select from {#syntax#}a{#endsyntax#} starting at 0.
Negative values select from {#syntax#}b{#endsyntax#}, starting at {#syntax#}-1{#endsyntax#} and going down.
It is recommended to use the {#syntax#}~{#endsyntax#} operator for indexes from {#syntax#}b{#endsyntax#}
so that both indexes can start from {#syntax#}0{#endsyntax#} (i.e. {#syntax#}~@as(i32, 0){#endsyntax#} is
{#syntax#}-1{#endsyntax#}).
</p>
<p>
For each element of {#syntax#}mask{#endsyntax#}, if it or the selected value from
{#syntax#}a{#endsyntax#} or {#syntax#}b{#endsyntax#} is {#syntax#}undefined{#endsyntax#},
then the resulting element is {#syntax#}undefined{#endsyntax#}.
</p>
<p>
{#syntax#}a_len{#endsyntax#} and {#syntax#}b_len{#endsyntax#} may differ in length. Out-of-bounds element
indexes in {#syntax#}mask{#endsyntax#} result in compile errors.
</p>
<p>
If {#syntax#}a{#endsyntax#} or {#syntax#}b{#endsyntax#} is {#syntax#}undefined{#endsyntax#}, it
is equivalent to a vector of all {#syntax#}undefined{#endsyntax#} with the same length as the other vector.
If both vectors are {#syntax#}undefined{#endsyntax#}, {#syntax#}@shuffle{#endsyntax#} returns
a vector with all elements {#syntax#}undefined{#endsyntax#}.
</p>
<p>
{#syntax#}E{#endsyntax#} must be an {#link|integer|Integers#}, {#link|float|Floats#},
{#link|pointer|Pointers#}, or {#syntax#}bool{#endsyntax#}. The mask may be any vector length, and its
length determines the result length.
</p>
{#code_begin|test|test_shuffle_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "vector @shuffle" {
const a = @Vector(7, u8){ 'o', 'l', 'h', 'e', 'r', 'z', 'w' };
const b = @Vector(4, u8){ 'w', 'd', '!', 'x' };
// To shuffle within a single vector, pass undefined as the second argument.
// Notice that we can re-order, duplicate, or omit elements of the input vector
const mask1 = @Vector(5, i32){ 2, 3, 1, 1, 0 };
const res1: @Vector(5, u8) = @shuffle(u8, a, undefined, mask1);
try expect(std.mem.eql(u8, &@as([5]u8, res1), "hello"));
// Combining two vectors
const mask2 = @Vector(6, i32){ -1, 0, 4, 1, -2, -3 };
const res2: @Vector(6, u8) = @shuffle(u8, a, b, mask2);
try expect(std.mem.eql(u8, &@as([6]u8, res2), "world!"));
}
{#code_end#}
{#see_also|Vectors#}
{#header_close#}
{#header_open|@sizeOf#}
<pre>{#syntax#}@sizeOf(comptime T: type) comptime_int{#endsyntax#}</pre>
<p>
This function returns the number of bytes it takes to store {#syntax#}T{#endsyntax#} in memory.
The result is a target-specific compile time constant.
</p>
<p>
This size may contain padding bytes. If there were two consecutive T in memory, the padding would be the offset
in bytes between element at index 0 and the element at index 1. For {#link|integer|Integers#},
consider whether you want to use {#syntax#}@sizeOf(T){#endsyntax#} or
{#syntax#}@typeInfo(T).Int.bits{#endsyntax#}.
</p>
<p>
This function measures the size at runtime. For types that are disallowed at runtime, such as
{#syntax#}comptime_int{#endsyntax#} and {#syntax#}type{#endsyntax#}, the result is {#syntax#}0{#endsyntax#}.
</p>
{#see_also|@bitSizeOf|@typeInfo#}
{#header_close#}
{#header_open|@splat#}
<pre>{#syntax#}@splat(scalar: anytype) anytype{#endsyntax#}</pre>
<p>
Produces a vector where each element is the value {#syntax#}scalar{#endsyntax#}.
The return type and thus the length of the vector is inferred.
</p>
{#code_begin|test|test_splat_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "vector @splat" {
const scalar: u32 = 5;
const result: @Vector(4, u32) = @splat(scalar);
try expect(std.mem.eql(u32, &@as([4]u32, result), &[_]u32{ 5, 5, 5, 5 }));
}
{#code_end#}
<p>
{#syntax#}scalar{#endsyntax#} must be an {#link|integer|Integers#}, {#link|bool|Primitive Types#},
{#link|float|Floats#}, or {#link|pointer|Pointers#}.
</p>
{#see_also|Vectors|@shuffle#}
{#header_close#}
{#header_open|@reduce#}
<pre>{#syntax#}@reduce(comptime op: std.builtin.ReduceOp, value: anytype) E{#endsyntax#}</pre>
<p>
Transforms a {#link|vector|Vectors#} into a scalar value (of type <code>E</code>)
by performing a sequential horizontal reduction of its elements using the
specified operator {#syntax#}op{#endsyntax#}.
</p>
<p>
Not every operator is available for every vector element type:
</p>
<ul>
<li>Every operator is available for {#link|integer|Integers#} vectors.</li>
<li>{#syntax#}.And{#endsyntax#}, {#syntax#}.Or{#endsyntax#},
{#syntax#}.Xor{#endsyntax#} are additionally available for
{#syntax#}bool{#endsyntax#} vectors,</li>
<li>{#syntax#}.Min{#endsyntax#}, {#syntax#}.Max{#endsyntax#},
{#syntax#}.Add{#endsyntax#}, {#syntax#}.Mul{#endsyntax#} are
additionally available for {#link|floating point|Floats#} vectors,</li>
</ul>
<p>
Note that {#syntax#}.Add{#endsyntax#} and {#syntax#}.Mul{#endsyntax#}
reductions on integral types are wrapping; when applied on floating point
types the operation associativity is preserved, unless the float mode is
set to {#syntax#}Optimized{#endsyntax#}.
</p>
{#code_begin|test|test_reduce_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "vector @reduce" {
const V = @Vector(4, i32);
const value = V{ 1, -1, 1, -1 };
const result = value > @as(V, @splat(0));
// result is { true, false, true, false };
try comptime expect(@TypeOf(result) == @Vector(4, bool));
const is_all_true = @reduce(.And, result);
try comptime expect(@TypeOf(is_all_true) == bool);
try expect(is_all_true == false);
}
{#code_end#}
{#see_also|Vectors|@setFloatMode#}
{#header_close#}
{#header_open|@src#}
<pre>{#syntax#}@src() std.builtin.SourceLocation{#endsyntax#}</pre>
<p>
Returns a {#syntax#}SourceLocation{#endsyntax#} struct representing the function's name and location in the source code. This must be called in a function.
</p>
{#code_begin|test|test_src_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "@src" {
try doTheTest();
}
fn doTheTest() !void {
const src = @src();
try expect(src.line == 9);
try expect(src.column == 17);
try expect(std.mem.endsWith(u8, src.fn_name, "doTheTest"));
try expect(std.mem.endsWith(u8, src.file, "test_src_builtin.zig"));
}
{#code_end#}
{#header_close#}
{#header_open|@sqrt#}
<pre>{#syntax#}@sqrt(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Performs the square root of a floating point number. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@sin#}
<pre>{#syntax#}@sin(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Sine trigonometric function on a floating point number in radians. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@cos#}
<pre>{#syntax#}@cos(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Cosine trigonometric function on a floating point number in radians. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@tan#}
<pre>{#syntax#}@tan(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Tangent trigonometric function on a floating point number in radians.
Uses a dedicated hardware instruction when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@exp#}
<pre>{#syntax#}@exp(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Base-e exponential function on a floating point number. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@exp2#}
<pre>{#syntax#}@exp2(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Base-2 exponential function on a floating point number. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@log#}
<pre>{#syntax#}@log(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Returns the natural logarithm of a floating point number. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@log2#}
<pre>{#syntax#}@log2(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Returns the logarithm to the base 2 of a floating point number. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@log10#}
<pre>{#syntax#}@log10(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Returns the logarithm to the base 10 of a floating point number. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@abs#}
<pre>{#syntax#}@abs(value: anytype) anytype{#endsyntax#}</pre>
<p>
Returns the absolute value of an integer or a floating point number. Uses a dedicated hardware instruction
when available.
The return type is always an unsigned integer of the same bit width as the operand if the operand is an integer.
Unsigned integer operands are supported. The builtin cannot overflow for signed integer operands.
</p>
<p>
Supports {#link|Floats#}, {#link|Integers#} and {#link|Vectors#} of floats or integers.
</p>
{#header_close#}
{#header_open|@floor#}
<pre>{#syntax#}@floor(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Returns the largest integral value not greater than the given floating point number.
Uses a dedicated hardware instruction when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@ceil#}
<pre>{#syntax#}@ceil(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Returns the smallest integral value not less than the given floating point number.
Uses a dedicated hardware instruction when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@trunc#}
<pre>{#syntax#}@trunc(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Rounds the given floating point number to an integer, towards zero.
Uses a dedicated hardware instruction when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@round#}
<pre>{#syntax#}@round(value: anytype) @TypeOf(value){#endsyntax#}</pre>
<p>
Rounds the given floating point number to an integer, away from zero. Uses a dedicated hardware instruction
when available.
</p>
<p>
Supports {#link|Floats#} and {#link|Vectors#} of floats.
</p>
{#header_close#}
{#header_open|@subWithOverflow#}
<pre>{#syntax#}@subWithOverflow(a: anytype, b: anytype) struct { @TypeOf(a, b), u1 }{#endsyntax#}</pre>
<p>
Performs {#syntax#}a - b{#endsyntax#} and returns a tuple with the result and a possible overflow bit.
</p>
{#header_close#}
{#header_open|@tagName#}
<pre>{#syntax#}@tagName(value: anytype) [:0]const u8{#endsyntax#}</pre>
<p>
Converts an enum value or union value to a string literal representing the name.</p><p>If the enum is non-exhaustive and the tag value does not map to a name, it invokes safety-checked {#link|Undefined Behavior#}.
</p>
{#header_close#}
{#header_open|@This#}
<pre>{#syntax#}@This() type{#endsyntax#}</pre>
<p>
Returns the innermost struct, enum, or union that this function call is inside.
This can be useful for an anonymous struct that needs to refer to itself:
</p>
{#code_begin|test|test_this_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "@This()" {
var items = [_]i32{ 1, 2, 3, 4 };
const list = List(i32){ .items = items[0..] };
try expect(list.length() == 4);
}
fn List(comptime T: type) type {
return struct {
const Self = @This();
items: []T,
fn length(self: Self) usize {
return self.items.len;
}
};
}
{#code_end#}
<p>
When {#syntax#}@This(){#endsyntax#} is used at file scope, it returns a reference to the
struct that corresponds to the current file.
</p>
{#header_close#}
{#header_open|@trap#}
<pre>{#syntax#}@trap() noreturn{#endsyntax#}</pre>
<p>
This function inserts a platform-specific trap/jam instruction which can be used to exit the program abnormally.
This may be implemented by explicitly emitting an invalid instruction which may cause an illegal instruction exception of some sort.
Unlike for {#syntax#}@breakpoint(){#endsyntax#}, execution does not continue after this point.
</p>
<p>
Outside function scope, this builtin causes a compile error.
</p>
{#see_also|@breakpoint#}
{#header_close#}
{#header_open|@truncate#}
<pre>{#syntax#}@truncate(integer: anytype) anytype{#endsyntax#}</pre>
<p>
This function truncates bits from an integer type, resulting in a smaller
or same-sized integer type. The return type is the inferred result type.
</p>
<p>
This function always truncates the significant bits of the integer, regardless
of endianness on the target platform.
</p>
<p>
Calling {#syntax#}@truncate{#endsyntax#} on a number out of range of the destination type is well defined and working code:
</p>
{#code_begin|test|test_truncate_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "integer truncation" {
const a: u16 = 0xabcd;
const b: u8 = @truncate(a);
try expect(b == 0xcd);
}
{#code_end#}
<p>
Use {#link|@intCast#} to convert numbers guaranteed to fit the destination type.
</p>
{#header_close#}
{#header_open|@Type#}
<pre>{#syntax#}@Type(comptime info: std.builtin.Type) type{#endsyntax#}</pre>
<p>
This function is the inverse of {#link|@typeInfo#}. It reifies type information
into a {#syntax#}type{#endsyntax#}.
</p>
<p>
It is available for the following types:
</p>
<ul>
<li>{#syntax#}type{#endsyntax#}</li>
<li>{#syntax#}noreturn{#endsyntax#}</li>
<li>{#syntax#}void{#endsyntax#}</li>
<li>{#syntax#}bool{#endsyntax#}</li>
<li>{#link|Integers#} - The maximum bit count for an integer type is {#syntax#}65535{#endsyntax#}.</li>
<li>{#link|Floats#}</li>
<li>{#link|Pointers#}</li>
<li>{#syntax#}comptime_int{#endsyntax#}</li>
<li>{#syntax#}comptime_float{#endsyntax#}</li>
<li>{#syntax#}@TypeOf(undefined){#endsyntax#}</li>
<li>{#syntax#}@TypeOf(null){#endsyntax#}</li>
<li>{#link|Arrays#}</li>
<li>{#link|Optionals#}</li>
<li>{#link|Error Set Type#}</li>
<li>{#link|Error Union Type#}</li>
<li>{#link|Vectors#}</li>
<li>{#link|opaque#}</li>
<li>{#syntax#}anyframe{#endsyntax#}</li>
<li>{#link|struct#}</li>
<li>{#link|enum#}</li>
<li>{#link|Enum Literals#}</li>
<li>{#link|union#}</li>
<li>{#link|Functions#}</li>
</ul>
{#header_close#}
{#header_open|@typeInfo#}
<pre>{#syntax#}@typeInfo(comptime T: type) std.builtin.Type{#endsyntax#}</pre>
<p>
Provides type reflection.
</p>
<p>
Type information of {#link|structs|struct#}, {#link|unions|union#}, {#link|enums|enum#}, and
{#link|error sets|Error Set Type#} has fields which are guaranteed to be in the same
order as appearance in the source file.
</p>
<p>
Type information of {#link|structs|struct#}, {#link|unions|union#}, {#link|enums|enum#}, and
{#link|opaques|opaque#} has declarations, which are also guaranteed to be in the same
order as appearance in the source file.
</p>
{#header_close#}
{#header_open|@typeName#}
<pre>{#syntax#}@typeName(T: type) *const [N:0]u8{#endsyntax#}</pre>
<p>
This function returns the string representation of a type, as
an array. It is equivalent to a string literal of the type name.
The returned type name is fully qualified with the parent namespace included
as part of the type name with a series of dots.
</p>
{#header_close#}
{#header_open|@TypeOf#}
<pre>{#syntax#}@TypeOf(...) type{#endsyntax#}</pre>
<p>
{#syntax#}@TypeOf{#endsyntax#} is a special builtin function that takes any (nonzero) number of expressions
as parameters and returns the type of the result, using {#link|Peer Type Resolution#}.
</p>
<p>
The expressions are evaluated, however they are guaranteed to have no <em>runtime</em> side-effects:
</p>
{#code_begin|test|test_TypeOf_builtin#}
const std = @import("std");
const expect = std.testing.expect;
test "no runtime side effects" {
var data: i32 = 0;
const T = @TypeOf(foo(i32, &data));
try comptime expect(T == i32);
try expect(data == 0);
}
fn foo(comptime T: type, ptr: *T) T {
ptr.* += 1;
return ptr.*;
}
{#code_end#}
{#header_close#}
{#header_open|@unionInit#}
<pre>{#syntax#}@unionInit(comptime Union: type, comptime active_field_name: []const u8, init_expr) Union{#endsyntax#}</pre>
<p>
This is the same thing as {#link|union#} initialization syntax, except that the field name is a
{#link|comptime#}-known value rather than an identifier token.
</p>
<p>
{#syntax#}@unionInit{#endsyntax#} forwards its {#link|result location|Result Location Semantics#} to {#syntax#}init_expr{#endsyntax#}.
</p>
{#header_close#}
{#header_open|@Vector#}
<pre>{#syntax#}@Vector(len: comptime_int, Element: type) type{#endsyntax#}</pre>
<p>Creates {#link|Vectors#}.</p>
{#header_close#}
{#header_open|@volatileCast#}
<pre>{#syntax#}@volatileCast(value: anytype) DestType{#endsyntax#}</pre>
<p>
Remove {#syntax#}volatile{#endsyntax#} qualifier from a pointer.
</p>
{#header_close#}
{#header_open|@workGroupId#}
<pre>{#syntax#}@workGroupId(comptime dimension: u32) u32{#endsyntax#}</pre>
<p>
Returns the index of the work group in the current kernel invocation in dimension {#syntax#}dimension{#endsyntax#}.
</p>
{#header_close#}
{#header_open|@workGroupSize#}
<pre>{#syntax#}@workGroupSize(comptime dimension: u32) u32{#endsyntax#}</pre>
<p>
Returns the number of work items that a work group has in dimension {#syntax#}dimension{#endsyntax#}.
</p>
{#header_close#}
{#header_open|@workItemId#}
<pre>{#syntax#}@workItemId(comptime dimension: u32) u32{#endsyntax#}</pre>
<p>
Returns the index of the work item in the work group in dimension {#syntax#}dimension{#endsyntax#}. This function returns values between {#syntax#}0{#endsyntax#} (inclusive) and {#syntax#}@workGroupSize(dimension){#endsyntax#} (exclusive).
</p>
{#header_close#}
{#header_close#}
{#header_open|Build Mode#}
<p>
Zig has four build modes:
</p>
<ul>
<li>{#link|Debug#} (default)</li>
<li>{#link|ReleaseFast#}</li>
<li>{#link|ReleaseSafe#}</li>
<li>{#link|ReleaseSmall#}</li>
</ul>
<p>
To add standard build options to a <code class="file">build.zig</code> file:
</p>
{#code_begin|syntax|build#}
const std = @import("std");
pub fn build(b: *std.Build) void {
const optimize = b.standardOptimizeOption(.{});
const exe = b.addExecutable(.{
.name = "example",
.root_source_file = .{ .path = "example.zig" },
.optimize = optimize,
});
b.default_step.dependOn(&exe.step);
}
{#code_end#}
<p>
This causes these options to be available:
</p>
<dl>
<dt><kbd>-Doptimize=Debug</kbd></dt><dd>Optimizations off and safety on (default)</dd>
<dt><kbd>-Doptimize=ReleaseSafe</kbd></dt><dd>Optimizations on and safety on</dd>
<dt><kbd>-Doptimize=ReleaseFast</kbd></dt><dd>Optimizations on and safety off</dd>
<dt><kbd>-Doptimize=ReleaseSmall</kbd></dt><dd>Size optimizations on and safety off</dd>
</dl>
{#header_open|Debug#}
{#shell_samp#}$ zig build-exe example.zig{#end_shell_samp#}
<ul>
<li>Fast compilation speed</li>
<li>Safety checks enabled</li>
<li>Slow runtime performance</li>
<li>Large binary size</li>
<li>No reproducible build requirement</li>
</ul>
{#header_close#}
{#header_open|ReleaseFast#}
{#shell_samp#}$ zig build-exe example.zig -O ReleaseFast{#end_shell_samp#}
<ul>
<li>Fast runtime performance</li>
<li>Safety checks disabled</li>
<li>Slow compilation speed</li>
<li>Large binary size</li>
<li>Reproducible build</li>
</ul>
{#header_close#}
{#header_open|ReleaseSafe#}
{#shell_samp#}$ zig build-exe example.zig -O ReleaseSafe{#end_shell_samp#}
<ul>
<li>Medium runtime performance</li>
<li>Safety checks enabled</li>
<li>Slow compilation speed</li>
<li>Large binary size</li>
<li>Reproducible build</li>
</ul>
{#header_close#}
{#header_open|ReleaseSmall#}
{#shell_samp#}$ zig build-exe example.zig -O ReleaseSmall{#end_shell_samp#}
<ul>
<li>Medium runtime performance</li>
<li>Safety checks disabled</li>
<li>Slow compilation speed</li>
<li>Small binary size</li>
<li>Reproducible build</li>
</ul>
{#header_close#}
{#see_also|Compile Variables|Zig Build System|Undefined Behavior#}
{#header_close#}
{#header_open|Single Threaded Builds#}
<p>Zig has a compile option <kbd>-fsingle-threaded</kbd> which has the following effects:</p>
<ul>
<li>All {#link|Thread Local Variables#} are treated as regular {#link|Container Level Variables#}.</li>
<li>The overhead of {#link|Async Functions#} becomes equivalent to function call overhead.</li>
<li>The {#syntax#}@import("builtin").single_threaded{#endsyntax#} becomes {#syntax#}true{#endsyntax#}
and therefore various userland APIs which read this variable become more efficient.
For example {#syntax#}std.Mutex{#endsyntax#} becomes
an empty data structure and all of its functions become no-ops.</li>
</ul>
{#header_close#}
{#header_open|Undefined Behavior#}
<p>
Zig has many instances of undefined behavior. If undefined behavior is
detected at compile-time, Zig emits a compile error and refuses to continue.
Most undefined behavior that cannot be detected at compile-time can be detected
at runtime. In these cases, Zig has safety checks. Safety checks can be disabled
on a per-block basis with {#link|@setRuntimeSafety#}. The {#link|ReleaseFast#}
and {#link|ReleaseSmall#} build modes disable all safety checks (except where overridden
by {#link|@setRuntimeSafety#}) in order to facilitate optimizations.
</p>
<p>
When a safety check fails, Zig crashes with a stack trace, like this:
</p>
{#code_begin|test_err|test_undefined_behavior|reached unreachable code#}
test "safety check" {
unreachable;
}
{#code_end#}
{#header_open|Reaching Unreachable Code#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_reaching_unreachable|reached unreachable code#}
comptime {
assert(false);
}
fn assert(ok: bool) void {
if (!ok) unreachable; // assertion failure
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_reaching_unreachable#}
const std = @import("std");
pub fn main() void {
std.debug.assert(false);
}
{#code_end#}
{#header_close#}
{#header_open|Index out of Bounds#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_index_out_of_bounds|index 5 outside array of length 5#}
comptime {
const array: [5]u8 = "hello".*;
const garbage = array[5];
_ = garbage;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_index_out_of_bounds#}
pub fn main() void {
const x = foo("hello");
_ = x;
}
fn foo(x: []const u8) u8 {
return x[5];
}
{#code_end#}
{#header_close#}
{#header_open|Cast Negative Number to Unsigned Integer#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_invalid_cast|type 'u32' cannot represent integer value '-1'#}
comptime {
const value: i32 = -1;
const unsigned: u32 = @intCast(value);
_ = unsigned;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_invalid_cast#}
const std = @import("std");
pub fn main() void {
var value: i32 = -1; // runtime-known
_ = &value;
const unsigned: u32 = @intCast(value);
std.debug.print("value: {}\n", .{unsigned});
}
{#code_end#}
<p>
To obtain the maximum value of an unsigned integer, use {#syntax#}std.math.maxInt{#endsyntax#}.
</p>
{#header_close#}
{#header_open|Cast Truncates Data#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_invalid_cast_truncate|type 'u8' cannot represent integer value '300'#}
comptime {
const spartan_count: u16 = 300;
const byte: u8 = @intCast(spartan_count);
_ = byte;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_invalid_cast_truncate#}
const std = @import("std");
pub fn main() void {
var spartan_count: u16 = 300; // runtime-known
_ = &spartan_count;
const byte: u8 = @intCast(spartan_count);
std.debug.print("value: {}\n", .{byte});
}
{#code_end#}
<p>
To truncate bits, use {#link|@truncate#}.
</p>
{#header_close#}
{#header_open|Integer Overflow#}
{#header_open|Default Operations#}
<p>The following operators can cause integer overflow:</p>
<ul>
<li>{#syntax#}+{#endsyntax#} (addition)</li>
<li>{#syntax#}-{#endsyntax#} (subtraction)</li>
<li>{#syntax#}-{#endsyntax#} (negation)</li>
<li>{#syntax#}*{#endsyntax#} (multiplication)</li>
<li>{#syntax#}/{#endsyntax#} (division)</li>
<li>{#link|@divTrunc#} (division)</li>
<li>{#link|@divFloor#} (division)</li>
<li>{#link|@divExact#} (division)</li>
</ul>
<p>Example with addition at compile-time:</p>
{#code_begin|test_err|test_comptime_overflow|overflow of integer type 'u8' with value '256'#}
comptime {
var byte: u8 = 255;
byte += 1;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_overflow#}
const std = @import("std");
pub fn main() void {
var byte: u8 = 255;
byte += 1;
std.debug.print("value: {}\n", .{byte});
}
{#code_end#}
{#header_close#}
{#header_open|Standard Library Math Functions#}
<p>These functions provided by the standard library return possible errors.</p>
<ul>
<li>{#syntax#}@import("std").math.add{#endsyntax#}</li>
<li>{#syntax#}@import("std").math.sub{#endsyntax#}</li>
<li>{#syntax#}@import("std").math.mul{#endsyntax#}</li>
<li>{#syntax#}@import("std").math.divTrunc{#endsyntax#}</li>
<li>{#syntax#}@import("std").math.divFloor{#endsyntax#}</li>
<li>{#syntax#}@import("std").math.divExact{#endsyntax#}</li>
<li>{#syntax#}@import("std").math.shl{#endsyntax#}</li>
</ul>
<p>Example of catching an overflow for addition:</p>
{#code_begin|exe_err|math_add#}
const math = @import("std").math;
const print = @import("std").debug.print;
pub fn main() !void {
var byte: u8 = 255;
byte = if (math.add(u8, byte, 1)) |result| result else |err| {
print("unable to add one: {s}\n", .{@errorName(err)});
return err;
};
print("result: {}\n", .{byte});
}
{#code_end#}
{#header_close#}
{#header_open|Builtin Overflow Functions#}
<p>
These builtins return a tuple containing whether there was an overflow
(as a {#syntax#}u1{#endsyntax#}) and the possibly overflowed bits of the operation:
</p>
<ul>
<li>{#link|@addWithOverflow#}</li>
<li>{#link|@subWithOverflow#}</li>
<li>{#link|@mulWithOverflow#}</li>
<li>{#link|@shlWithOverflow#}</li>
</ul>
<p>
Example of {#link|@addWithOverflow#}:
</p>
{#code_begin|exe|addWithOverflow_builtin#}
const print = @import("std").debug.print;
pub fn main() void {
const byte: u8 = 255;
const ov = @addWithOverflow(byte, 10);
if (ov[1] != 0) {
print("overflowed result: {}\n", .{ov[0]});
} else {
print("result: {}\n", .{ov[0]});
}
}
{#code_end#}
{#header_close#}
{#header_open|Wrapping Operations#}
<p>
These operations have guaranteed wraparound semantics.
</p>
<ul>
<li>{#syntax#}+%{#endsyntax#} (wraparound addition)</li>
<li>{#syntax#}-%{#endsyntax#} (wraparound subtraction)</li>
<li>{#syntax#}-%{#endsyntax#} (wraparound negation)</li>
<li>{#syntax#}*%{#endsyntax#} (wraparound multiplication)</li>
</ul>
{#code_begin|test|test_wraparound_semantics#}
const std = @import("std");
const expect = std.testing.expect;
const minInt = std.math.minInt;
const maxInt = std.math.maxInt;
test "wraparound addition and subtraction" {
const x: i32 = maxInt(i32);
const min_val = x +% 1;
try expect(min_val == minInt(i32));
const max_val = min_val -% 1;
try expect(max_val == maxInt(i32));
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Exact Left Shift Overflow#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_shlExact_overwlow|operation caused overflow#}
comptime {
const x = @shlExact(@as(u8, 0b01010101), 2);
_ = x;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_shlExact_overflow#}
const std = @import("std");
pub fn main() void {
var x: u8 = 0b01010101; // runtime-known
_ = &x;
const y = @shlExact(x, 2);
std.debug.print("value: {}\n", .{y});
}
{#code_end#}
{#header_close#}
{#header_open|Exact Right Shift Overflow#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_shrExact_overflow|exact shift shifted out 1 bits#}
comptime {
const x = @shrExact(@as(u8, 0b10101010), 2);
_ = x;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_shrExact_overflow#}
const std = @import("std");
pub fn main() void {
var x: u8 = 0b10101010; // runtime-known
_ = &x;
const y = @shrExact(x, 2);
std.debug.print("value: {}\n", .{y});
}
{#code_end#}
{#header_close#}
{#header_open|Division by Zero#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_division_by_zero|division by zero#}
comptime {
const a: i32 = 1;
const b: i32 = 0;
const c = a / b;
_ = c;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_division_by_zero#}
const std = @import("std");
pub fn main() void {
var a: u32 = 1;
var b: u32 = 0;
_ = .{ &a, &b };
const c = a / b;
std.debug.print("value: {}\n", .{c});
}
{#code_end#}
{#header_close#}
{#header_open|Remainder Division by Zero#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_remainder_division_by_zero|division by zero#}
comptime {
const a: i32 = 10;
const b: i32 = 0;
const c = a % b;
_ = c;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_remainder_division_by_zero#}
const std = @import("std");
pub fn main() void {
var a: u32 = 10;
var b: u32 = 0;
_ = .{ &a, &b };
const c = a % b;
std.debug.print("value: {}\n", .{c});
}
{#code_end#}
{#header_close#}
{#header_open|Exact Division Remainder#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_divExact_remainder|exact division produced remainder#}
comptime {
const a: u32 = 10;
const b: u32 = 3;
const c = @divExact(a, b);
_ = c;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_divExact_remainder#}
const std = @import("std");
pub fn main() void {
var a: u32 = 10;
var b: u32 = 3;
_ = .{ &a, &b };
const c = @divExact(a, b);
std.debug.print("value: {}\n", .{c});
}
{#code_end#}
{#header_close#}
{#header_open|Attempt to Unwrap Null#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_unwrap_null|unable to unwrap null#}
comptime {
const optional_number: ?i32 = null;
const number = optional_number.?;
_ = number;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_unwrap_null#}
const std = @import("std");
pub fn main() void {
var optional_number: ?i32 = null;
_ = &optional_number;
const number = optional_number.?;
std.debug.print("value: {}\n", .{number});
}
{#code_end#}
<p>One way to avoid this crash is to test for null instead of assuming non-null, with
the {#syntax#}if{#endsyntax#} expression:</p>
{#code_begin|exe|testing_null_with_if#}
const print = @import("std").debug.print;
pub fn main() void {
const optional_number: ?i32 = null;
if (optional_number) |number| {
print("got number: {}\n", .{number});
} else {
print("it's null\n", .{});
}
}
{#code_end#}
{#see_also|Optionals#}
{#header_close#}
{#header_open|Attempt to Unwrap Error#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_unwrap_error|caught unexpected error 'UnableToReturnNumber'#}
comptime {
const number = getNumberOrFail() catch unreachable;
_ = number;
}
fn getNumberOrFail() !i32 {
return error.UnableToReturnNumber;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_unwrap_error#}
const std = @import("std");
pub fn main() void {
const number = getNumberOrFail() catch unreachable;
std.debug.print("value: {}\n", .{number});
}
fn getNumberOrFail() !i32 {
return error.UnableToReturnNumber;
}
{#code_end#}
<p>One way to avoid this crash is to test for an error instead of assuming a successful result, with
the {#syntax#}if{#endsyntax#} expression:</p>
{#code_begin|exe|testing_error_with_if#}
const print = @import("std").debug.print;
pub fn main() void {
const result = getNumberOrFail();
if (result) |number| {
print("got number: {}\n", .{number});
} else |err| {
print("got error: {s}\n", .{@errorName(err)});
}
}
fn getNumberOrFail() !i32 {
return error.UnableToReturnNumber;
}
{#code_end#}
{#see_also|Errors#}
{#header_close#}
{#header_open|Invalid Error Code#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_invalid_error_code|integer value '11' represents no error#}
comptime {
const err = error.AnError;
const number = @intFromError(err) + 10;
const invalid_err = @errorFromInt(number);
_ = invalid_err;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_invalid_error_code#}
const std = @import("std");
pub fn main() void {
const err = error.AnError;
var number = @intFromError(err) + 500;
_ = &number;
const invalid_err = @errorFromInt(number);
std.debug.print("value: {}\n", .{invalid_err});
}
{#code_end#}
{#header_close#}
{#header_open|Invalid Enum Cast#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_invalid_enum_cast|enum 'test_comptime_invalid_enum_cast.Foo' has no tag with value '3'#}
const Foo = enum {
a,
b,
c,
};
comptime {
const a: u2 = 3;
const b: Foo = @enumFromInt(a);
_ = b;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_invalid_enum_cast#}
const std = @import("std");
const Foo = enum {
a,
b,
c,
};
pub fn main() void {
var a: u2 = 3;
_ = &a;
const b: Foo = @enumFromInt(a);
std.debug.print("value: {s}\n", .{@tagName(b)});
}
{#code_end#}
{#header_close#}
{#header_open|Invalid Error Set Cast#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_invalid_error_set_cast|'error.B' not a member of error set 'error{A,C}'#}
const Set1 = error{
A,
B,
};
const Set2 = error{
A,
C,
};
comptime {
_ = @as(Set2, @errorCast(Set1.B));
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_invalid_error_set_cast#}
const std = @import("std");
const Set1 = error{
A,
B,
};
const Set2 = error{
A,
C,
};
pub fn main() void {
foo(Set1.B);
}
fn foo(set1: Set1) void {
const x: Set2 = @errorCast(set1);
std.debug.print("value: {}\n", .{x});
}
{#code_end#}
{#header_close#}
{#header_open|Incorrect Pointer Alignment#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_incorrect_pointer_alignment|pointer address 0x1 is not aligned to 4 bytes#}
comptime {
const ptr: *align(1) i32 = @ptrFromInt(0x1);
const aligned: *align(4) i32 = @alignCast(ptr);
_ = aligned;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_incorrect_pointer_alignment#}
const mem = @import("std").mem;
pub fn main() !void {
var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
const bytes = mem.sliceAsBytes(array[0..]);
if (foo(bytes) != 0x11111111) return error.Wrong;
}
fn foo(bytes: []u8) u32 {
const slice4 = bytes[1..5];
const int_slice = mem.bytesAsSlice(u32, @as([]align(4) u8, @alignCast(slice4)));
return int_slice[0];
}
{#code_end#}
{#header_close#}
{#header_open|Wrong Union Field Access#}
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_wrong_union_field_access|access of union field 'float' while field 'int' is active#}
comptime {
var f = Foo{ .int = 42 };
f.float = 12.34;
}
const Foo = union {
float: f32,
int: u32,
};
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_wrong_union_field_access#}
const std = @import("std");
const Foo = union {
float: f32,
int: u32,
};
pub fn main() void {
var f = Foo{ .int = 42 };
bar(&f);
}
fn bar(f: *Foo) void {
f.float = 12.34;
std.debug.print("value: {}\n", .{f.float});
}
{#code_end#}
<p>
This safety is not available for {#syntax#}extern{#endsyntax#} or {#syntax#}packed{#endsyntax#} unions.
</p>
<p>
To change the active field of a union, assign the entire union, like this:
</p>
{#code_begin|exe|change_active_union_field#}
const std = @import("std");
const Foo = union {
float: f32,
int: u32,
};
pub fn main() void {
var f = Foo{ .int = 42 };
bar(&f);
}
fn bar(f: *Foo) void {
f.* = Foo{ .float = 12.34 };
std.debug.print("value: {}\n", .{f.float});
}
{#code_end#}
<p>
To change the active field of a union when a meaningful value for the field is not known,
use {#link|undefined#}, like this:
</p>
{#code_begin|exe|undefined_active_union_field#}
const std = @import("std");
const Foo = union {
float: f32,
int: u32,
};
pub fn main() void {
var f = Foo{ .int = 42 };
f = Foo{ .float = undefined };
bar(&f);
std.debug.print("value: {}\n", .{f.float});
}
fn bar(f: *Foo) void {
f.float = 12.34;
}
{#code_end#}
{#see_also|union|extern union#}
{#header_close#}
{#header_open|Out of Bounds Float to Integer Cast#}
<p>
This happens when casting a float to an integer where the float has a value outside the
integer type's range.
</p>
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_out_of_bounds_float_to_integer_cast|float value '4294967296' cannot be stored in integer type 'i32'#}
comptime {
const float: f32 = 4294967296;
const int: i32 = @intFromFloat(float);
_ = int;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_out_of_bounds_float_to_integer_cast#}
pub fn main() void {
var float: f32 = 4294967296; // runtime-known
_ = &float;
const int: i32 = @intFromFloat(float);
_ = int;
}
{#code_end#}
{#header_close#}
{#header_open|Pointer Cast Invalid Null#}
<p>
This happens when casting a pointer with the address 0 to a pointer which may not have the address 0.
For example, {#link|C Pointers#}, {#link|Optional Pointers#}, and {#link|allowzero#} pointers
allow address zero, but normal {#link|Pointers#} do not.
</p>
<p>At compile-time:</p>
{#code_begin|test_err|test_comptime_invalid_null_pointer_cast|null pointer casted to type#}
comptime {
const opt_ptr: ?*i32 = null;
const ptr: *i32 = @ptrCast(opt_ptr);
_ = ptr;
}
{#code_end#}
<p>At runtime:</p>
{#code_begin|exe_err|runtime_invalid_null_pointer_cast#}
pub fn main() void {
var opt_ptr: ?*i32 = null;
_ = &opt_ptr;
const ptr: *i32 = @ptrCast(opt_ptr);
_ = ptr;
}
{#code_end#}
{#header_close#}
{#header_close#}
{#header_open|Memory#}
<p>
The Zig language performs no memory management on behalf of the programmer. This is
why Zig has no runtime, and why Zig code works seamlessly in so many environments,
including real-time software, operating system kernels, embedded devices, and
low latency servers. As a consequence, Zig programmers must always be able to answer
the question:
</p>
<p>{#link|Where are the bytes?#}</p>
<p>
Like Zig, the C programming language has manual memory management. However, unlike Zig,
C has a default allocator - <code>malloc</code>, <code>realloc</code>, and <code>free</code>.
When linking against libc, Zig exposes this allocator with {#syntax#}std.heap.c_allocator{#endsyntax#}.
However, by convention, there is no default allocator in Zig. Instead, functions which need to
allocate accept an {#syntax#}Allocator{#endsyntax#} parameter. Likewise, data structures such as
{#syntax#}std.ArrayList{#endsyntax#} accept an {#syntax#}Allocator{#endsyntax#} parameter in
their initialization functions:
</p>
{#code_begin|test|test_allocator#}
const std = @import("std");
const Allocator = std.mem.Allocator;
const expect = std.testing.expect;
test "using an allocator" {
var buffer: [100]u8 = undefined;
var fba = std.heap.FixedBufferAllocator.init(&buffer);
const allocator = fba.allocator();
const result = try concat(allocator, "foo", "bar");
try expect(std.mem.eql(u8, "foobar", result));
}
fn concat(allocator: Allocator, a: []const u8, b: []const u8) ![]u8 {
const result = try allocator.alloc(u8, a.len + b.len);
@memcpy(result[0..a.len], a);
@memcpy(result[a.len..], b);
return result;
}
{#code_end#}
<p>
In the above example, 100 bytes of stack memory are used to initialize a
{#syntax#}FixedBufferAllocator{#endsyntax#}, which is then passed to a function.
As a convenience there is a global {#syntax#}FixedBufferAllocator{#endsyntax#}
available for quick tests at {#syntax#}std.testing.allocator{#endsyntax#},
which will also perform basic leak detection.
</p>
<p>
Zig has a general purpose allocator available to be imported
with {#syntax#}std.heap.GeneralPurposeAllocator{#endsyntax#}. However, it is still recommended to
follow the {#link|Choosing an Allocator#} guide.
</p>
{#header_open|Choosing an Allocator#}
<p>What allocator to use depends on a number of factors. Here is a flow chart to help you decide:
</p>
<ol>
<li>
Are you making a library? In this case, best to accept an {#syntax#}Allocator{#endsyntax#}
as a parameter and allow your library's users to decide what allocator to use.
</li>
<li>Are you linking libc? In this case, {#syntax#}std.heap.c_allocator{#endsyntax#} is likely
the right choice, at least for your main allocator.</li>
<li>
Is the maximum number of bytes that you will need bounded by a number known at
{#link|comptime#}? In this case, use {#syntax#}std.heap.FixedBufferAllocator{#endsyntax#} or
{#syntax#}std.heap.ThreadSafeFixedBufferAllocator{#endsyntax#} depending on whether you need
thread-safety or not.
</li>
<li>
Is your program a command line application which runs from start to end without any fundamental
cyclical pattern (such as a video game main loop, or a web server request handler),
such that it would make sense to free everything at once at the end?
In this case, it is recommended to follow this pattern:
{#code_begin|exe|cli_allocation#}
const std = @import("std");
pub fn main() !void {
var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
defer arena.deinit();
const allocator = arena.allocator();
const ptr = try allocator.create(i32);
std.debug.print("ptr={*}\n", .{ptr});
}
{#code_end#}
When using this kind of allocator, there is no need to free anything manually. Everything
gets freed at once with the call to {#syntax#}arena.deinit(){#endsyntax#}.
</li>
<li>
Are the allocations part of a cyclical pattern such as a video game main loop, or a web
server request handler? If the allocations can all be freed at once, at the end of the cycle,
for example once the video game frame has been fully rendered, or the web server request has
been served, then {#syntax#}std.heap.ArenaAllocator{#endsyntax#} is a great candidate. As
demonstrated in the previous bullet point, this allows you to free entire arenas at once.
Note also that if an upper bound of memory can be established, then
{#syntax#}std.heap.FixedBufferAllocator{#endsyntax#} can be used as a further optimization.
</li>
<li>
Are you writing a test, and you want to make sure {#syntax#}error.OutOfMemory{#endsyntax#}
is handled correctly? In this case, use {#syntax#}std.testing.FailingAllocator{#endsyntax#}.
</li>
<li>
Are you writing a test? In this case, use {#syntax#}std.testing.allocator{#endsyntax#}.
</li>
<li>
Finally, if none of the above apply, you need a general purpose allocator.
Zig's general purpose allocator is available as a function that takes a {#link|comptime#}
{#link|struct#} of configuration options and returns a type.
Generally, you will set up one {#syntax#}std.heap.GeneralPurposeAllocator{#endsyntax#} in
your main function, and then pass it or sub-allocators around to various parts of your
application.
</li>
<li>
You can also consider {#link|Implementing an Allocator#}.
</li>
</ol>
{#header_close#}
{#header_open|Where are the bytes?#}
<p>String literals such as {#syntax#}"hello"{#endsyntax#} are in the global constant data section.
This is why it is an error to pass a string literal to a mutable slice, like this:
</p>
{#code_begin|test_err|test_string_literal_to_slice|expected type '[]u8', found '*const [5:0]u8'#}
fn foo(s: []u8) void {
_ = s;
}
test "string literal to mutable slice" {
foo("hello");
}
{#code_end#}
<p>However if you make the slice constant, then it works:</p>
{#code_begin|test|test_string_literal_to_const_slice#}
fn foo(s: []const u8) void {
_ = s;
}
test "string literal to constant slice" {
foo("hello");
}
{#code_end#}
<p>
Just like string literals, {#syntax#}const{#endsyntax#} declarations, when the value is known at {#link|comptime#},
are stored in the global constant data section. Also {#link|Compile Time Variables#} are stored
in the global constant data section.
</p>
<p>
{#syntax#}var{#endsyntax#} declarations inside functions are stored in the function's stack frame. Once a function returns,
any {#link|Pointers#} to variables in the function's stack frame become invalid references, and
dereferencing them becomes unchecked {#link|Undefined Behavior#}.
</p>
<p>
{#syntax#}var{#endsyntax#} declarations at the top level or in {#link|struct#} declarations are stored in the global
data section.
</p>
<p>
The location of memory allocated with {#syntax#}allocator.alloc{#endsyntax#} or
{#syntax#}allocator.create{#endsyntax#} is determined by the allocator's implementation.
</p>
<p>TODO: thread local variables</p>
{#header_close#}
{#header_open|Implementing an Allocator#}
<p>Zig programmers can implement their own allocators by fulfilling the Allocator interface.
In order to do this one must read carefully the documentation comments in std/mem.zig and
then supply a {#syntax#}allocFn{#endsyntax#} and a {#syntax#}resizeFn{#endsyntax#}.
</p>
<p>
There are many example allocators to look at for inspiration. Look at std/heap.zig and
{#syntax#}std.heap.GeneralPurposeAllocator{#endsyntax#}.
</p>
{#header_close#}
{#header_open|Heap Allocation Failure#}
<p>
Many programming languages choose to handle the possibility of heap allocation failure by
unconditionally crashing. By convention, Zig programmers do not consider this to be a
satisfactory solution. Instead, {#syntax#}error.OutOfMemory{#endsyntax#} represents
heap allocation failure, and Zig libraries return this error code whenever heap allocation
failure prevented an operation from completing successfully.
</p>
<p>
Some have argued that because some operating systems such as Linux have memory overcommit enabled by
default, it is pointless to handle heap allocation failure. There are many problems with this reasoning:
</p>
<ul>
<li>Only some operating systems have an overcommit feature.
<ul>
<li>Linux has it enabled by default, but it is configurable.</li>
<li>Windows does not overcommit.</li>
<li>Embedded systems do not have overcommit.</li>
<li>Hobby operating systems may or may not have overcommit.</li>
</ul>
</li>
<li>
For real-time systems, not only is there no overcommit, but typically the maximum amount
of memory per application is determined ahead of time.
</li>
<li>
When writing a library, one of the main goals is code reuse. By making code handle
allocation failure correctly, a library becomes eligible to be reused in
more contexts.
</li>
<li>
Although some software has grown to depend on overcommit being enabled, its existence
is the source of countless user experience disasters. When a system with overcommit enabled,
such as Linux on default settings, comes close to memory exhaustion, the system locks up
and becomes unusable. At this point, the OOM Killer selects an application to kill
based on heuristics. This non-deterministic decision often results in an important process
being killed, and often fails to return the system back to working order.
</li>
</ul>
{#header_close#}
{#header_open|Recursion#}
<p>
Recursion is a fundamental tool in modeling software. However it has an often-overlooked problem:
unbounded memory allocation.
</p>
<p>
Recursion is an area of active experimentation in Zig and so the documentation here is not final.
You can read a
<a href="https://ziglang.org/download/0.3.0/release-notes.html#recursion">summary of recursion status in the 0.3.0 release notes</a>.
</p>
<p>
The short summary is that currently recursion works normally as you would expect. Although Zig code
is not yet protected from stack overflow, it is planned that a future version of Zig will provide
such protection, with some degree of cooperation from Zig code required.
</p>
{#header_close#}
{#header_open|Lifetime and Ownership#}
<p>
It is the Zig programmer's responsibility to ensure that a {#link|pointer|Pointers#} is not
accessed when the memory pointed to is no longer available. Note that a {#link|slice|Slices#}
is a form of pointer, in that it references other memory.
</p>
<p>
In order to prevent bugs, there are some helpful conventions to follow when dealing with pointers.
In general, when a function returns a pointer, the documentation for the function should explain
who "owns" the pointer. This concept helps the programmer decide when it is appropriate, if ever,
to free the pointer.
</p>
<p>
For example, the function's documentation may say "caller owns the returned memory", in which case
the code that calls the function must have a plan for when to free that memory. Probably in this situation,
the function will accept an {#syntax#}Allocator{#endsyntax#} parameter.
</p>
<p>
Sometimes the lifetime of a pointer may be more complicated. For example, the
{#syntax#}std.ArrayList(T).items{#endsyntax#} slice has a lifetime that remains
valid until the next time the list is resized, such as by appending new elements.
</p>
<p>
The API documentation for functions and data structures should take great care to explain
the ownership and lifetime semantics of pointers. Ownership determines whose responsibility it
is to free the memory referenced by the pointer, and lifetime determines the point at which
the memory becomes inaccessible (lest {#link|Undefined Behavior#} occur).
</p>
{#header_close#}
{#header_close#}
{#header_open|Compile Variables#}
<p>
Compile variables are accessible by importing the {#syntax#}"builtin"{#endsyntax#} package,
which the compiler makes available to every Zig source file. It contains
compile-time constants such as the current target, endianness, and release mode.
</p>
{#code_begin|syntax|compile_variables#}
const builtin = @import("builtin");
const separator = if (builtin.os.tag == .windows) '\\' else '/';
{#code_end#}
<p>
Example of what is imported with {#syntax#}@import("builtin"){#endsyntax#}:
</p>
{#builtin#}
{#see_also|Build Mode#}
{#header_close#}
{#header_open|Root Source File#}
<p>TODO: explain how root source file finds other files</p>
<p>TODO: pub fn main</p>
<p>TODO: pub fn panic</p>
<p>TODO: if linking with libc you can use export fn main</p>
<p>TODO: order independent top level declarations</p>
<p>TODO: lazy analysis</p>
<p>TODO: using comptime { _ = @import() }</p>
{#header_close#}
{#header_open|Zig Build System#}
<p>
The Zig Build System provides a cross-platform, dependency-free way to declare
the logic required to build a project. With this system, the logic to build
a project is written in a build.zig file, using the Zig Build System API to
declare and configure build artifacts and other tasks.
</p>
<p>
Some examples of tasks the build system can help with:
</p>
<ul>
<li>Performing tasks in parallel and caching the results.</li>
<li>Depending on other projects.</li>
<li>Providing a package for other projects to depend on.</li>
<li>Creating build artifacts by executing the Zig compiler. This includes
building Zig source code as well as C and C++ source code.</li>
<li>Capturing user-configured options and using those options to configure
the build.</li>
<li>Surfacing build configuration as {#link|comptime#} values by providing a
file that can be {#link|imported|@import#} by Zig code.</li>
<li>Caching build artifacts to avoid unnecessarily repeating steps.</li>
<li>Executing build artifacts or system-installed tools.</li>
<li>Running tests and verifying the output of executing a build artifact matches
the expected value.</li>
<li>Running <code>zig fmt</code> on a codebase or a subset of it.</li>
<li>Custom tasks.</li>
</ul>
<p>
To use the build system, run <kbd>zig build --help</kbd>
to see a command-line usage help menu. This will include project-specific
options that were declared in the build.zig script.
</p>
<p>
For the time being, the build system documentation is hosted externally:
<a href="https://ziglang.org/learn/build-system/">Build System Documentation</a>
</p>
{#header_close#}
{#header_open|C#}
<p>
Although Zig is independent of C, and, unlike most other languages, does not depend on libc,
Zig acknowledges the importance of interacting with existing C code.
</p>
<p>
There are a few ways that Zig facilitates C interop.
</p>
{#header_open|C Type Primitives#}
<p>
These have guaranteed C ABI compatibility and can be used like any other type.
</p>
<ul>
<li>{#syntax#}c_char{#endsyntax#}</li>
<li>{#syntax#}c_short{#endsyntax#}</li>
<li>{#syntax#}c_ushort{#endsyntax#}</li>
<li>{#syntax#}c_int{#endsyntax#}</li>
<li>{#syntax#}c_uint{#endsyntax#}</li>
<li>{#syntax#}c_long{#endsyntax#}</li>
<li>{#syntax#}c_ulong{#endsyntax#}</li>
<li>{#syntax#}c_longlong{#endsyntax#}</li>
<li>{#syntax#}c_ulonglong{#endsyntax#}</li>
<li>{#syntax#}c_longdouble{#endsyntax#}</li>
</ul>
<p>
To interop with the C {#syntax#}void{#endsyntax#} type, use {#syntax#}anyopaque{#endsyntax#}.
</p>
{#see_also|Primitive Types#}
{#header_close#}
{#header_open|Import from C Header File#}
<p>
The {#syntax#}@cImport{#endsyntax#} builtin function can be used
to directly import symbols from <code class="file">.h</code> files:
</p>
{#code_begin|exe|cImport_builtin#}
{#link_libc#}
const c = @cImport({
// See https://github.com/ziglang/zig/issues/515
@cDefine("_NO_CRT_STDIO_INLINE", "1");
@cInclude("stdio.h");
});
pub fn main() void {
_ = c.printf("hello\n");
}
{#code_end#}
<p>
The {#syntax#}@cImport{#endsyntax#} function takes an expression as a parameter.
This expression is evaluated at compile-time and is used to control
preprocessor directives and include multiple <code class="file">.h</code> files:
</p>
{#syntax_block|zig|@cImport Expression#}
const builtin = @import("builtin");
const c = @cImport({
@cDefine("NDEBUG", builtin.mode == .ReleaseFast);
if (something) {
@cDefine("_GNU_SOURCE", {});
}
@cInclude("stdlib.h");
if (something) {
@cUndef("_GNU_SOURCE");
}
@cInclude("soundio.h");
});
{#end_syntax_block#}
{#see_also|@cImport|@cInclude|@cDefine|@cUndef|@import#}
{#header_close#}
{#header_open|C Translation CLI#}
<p>
Zig's C translation capability is available as a CLI tool via <kbd>zig translate-c</kbd>.
It requires a single filename as an argument. It may also take a set of optional flags that are
forwarded to clang. It writes the translated file to stdout.
</p>
{#header_open|Command line flags#}
<ul>
<li>
<kbd>-I</kbd>:
Specify a search directory for include files. May be used multiple times. Equivalent to
<a href="https://releases.llvm.org/12.0.0/tools/clang/docs/ClangCommandLineReference.html#cmdoption-clang-i-dir">
clang's <kbd>-I</kbd> flag</a>. The current directory is <em>not</em> included by default;
use <kbd>-I.</kbd> to include it.
</li>
<li>
<kbd>-D</kbd>: Define a preprocessor macro. Equivalent to
<a href="https://releases.llvm.org/12.0.0/tools/clang/docs/ClangCommandLineReference.html#cmdoption-clang-d-macro">
clang's <kbd>-D</kbd> flag</a>.
</li>
<li>
<kbd>-cflags [flags] --</kbd>: Pass arbitrary additional
<a href="https://releases.llvm.org/12.0.0/tools/clang/docs/ClangCommandLineReference.html">command line
flags</a> to clang. Note: the list of flags must end with <kbd>--</kbd>
</li>
<li>
<kbd>-target</kbd>: The {#link|target triple|Targets#} for the translated Zig code.
If no target is specified, the current host target will be used.
</li>
</ul>
{#header_close#}
{#header_open|Using -target and -cflags#}
<p>
<strong>Important!</strong> When translating C code with <kbd>zig translate-c</kbd>,
you <strong>must</strong> use the same <kbd>-target</kbd> triple that you will use when compiling
the translated code. In addition, you <strong>must</strong> ensure that the <kbd>-cflags</kbd> used,
if any, match the cflags used by code on the target system. Using the incorrect <kbd>-target</kbd>
or <kbd>-cflags</kbd> could result in clang or Zig parse failures, or subtle ABI incompatibilities
when linking with C code.
</p>
{#syntax_block|c|varytarget.h#}
long FOO = __LONG_MAX__;
{#end_syntax_block#}
{#shell_samp#}$ zig translate-c -target thumb-freestanding-gnueabihf varytarget.h|grep FOO
pub export var FOO: c_long = 2147483647;
$ zig translate-c -target x86_64-macos-gnu varytarget.h|grep FOO
pub export var FOO: c_long = 9223372036854775807;{#end_shell_samp#}
{#syntax_block|c|varycflags.h#}
enum FOO { BAR };
int do_something(enum FOO foo);
{#end_syntax_block#}
{#shell_samp#}$ zig translate-c varycflags.h|grep -B1 do_something
pub const enum_FOO = c_uint;
pub extern fn do_something(foo: enum_FOO) c_int;
$ zig translate-c -cflags -fshort-enums -- varycflags.h|grep -B1 do_something
pub const enum_FOO = u8;
pub extern fn do_something(foo: enum_FOO) c_int;{#end_shell_samp#}
{#header_close#}
{#header_open|@cImport vs translate-c#}
<p>{#syntax#}@cImport{#endsyntax#} and <kbd>zig translate-c</kbd> use the same underlying
C translation functionality, so on a technical level they are equivalent. In practice,
{#syntax#}@cImport{#endsyntax#} is useful as a way to quickly and easily access numeric constants, typedefs,
and record types without needing any extra setup. If you need to pass {#link|cflags|Using -target and -cflags#}
to clang, or if you would like to edit the translated code, it is recommended to use
<kbd>zig translate-c</kbd> and save the results to a file. Common reasons for editing
the generated code include: changing {#syntax#}anytype{#endsyntax#} parameters in function-like macros to more
specific types; changing {#syntax#}[*c]T{#endsyntax#} pointers to {#syntax#}[*]T{#endsyntax#} or
{#syntax#}*T{#endsyntax#} pointers for improved type safety; and
{#link|enabling or disabling runtime safety|@setRuntimeSafety#} within specific functions.
</p>
{#header_close#}
{#see_also|Targets|C Type Primitives|Pointers|C Pointers|Import from C Header File|@cInclude|@cImport|@setRuntimeSafety#}
{#header_close#}
{#header_open|C Translation Caching#}
<p>
The C translation feature (whether used via <kbd>zig translate-c</kbd> or
{#syntax#}@cImport{#endsyntax#}) integrates with the Zig caching system. Subsequent runs with
the same source file, target, and cflags will use the cache instead of repeatedly translating
the same code.
</p>
<p>
To see where the cached files are stored when compiling code that uses {#syntax#}@cImport{#endsyntax#},
use the <kbd>--verbose-cimport</kbd> flag:
</p>
{#code_begin|exe|verbose_cimport_flag#}
{#link_libc#}
{#code_verbose_cimport#}
const c = @cImport({
@cDefine("_NO_CRT_STDIO_INLINE", "1");
@cInclude("stdio.h");
});
pub fn main() void {
_ = c;
}
{#code_end#}
<p>
<code class="file">cimport.h</code> contains the file to translate (constructed from calls to
{#syntax#}@cInclude{#endsyntax#}, {#syntax#}@cDefine{#endsyntax#}, and {#syntax#}@cUndef{#endsyntax#}),
<code class="file">cimport.h.d</code> is the list of file dependencies, and
<code class="file">cimport.zig</code> contains the translated output.
</p>
{#see_also|Import from C Header File|C Translation CLI|@cInclude|@cImport#}
{#header_close#}
{#header_open|Translation failures#}
<p>
Some C constructs cannot be translated to Zig - for example, <em>goto</em>,
structs with bitfields, and token-pasting macros. Zig employs <em>demotion</em> to allow translation
to continue in the face of non-translatable entities.
</p>
<p>
Demotion comes in three varieties - {#link|opaque#}, <em>extern</em>, and
{#syntax#}@compileError{#endsyntax#}.
C structs and unions that cannot be translated correctly will be translated as {#syntax#}opaque{}{#endsyntax#}.
Functions that contain opaque types or code constructs that cannot be translated will be demoted
to {#syntax#}extern{#endsyntax#} declarations.
Thus, non-translatable types can still be used as pointers, and non-translatable functions
can be called so long as the linker is aware of the compiled function.
</p>
<p>
{#syntax#}@compileError{#endsyntax#} is used when top-level definitions (global variables,
function prototypes, macros) cannot be translated or demoted. Since Zig uses lazy analysis for
top-level declarations, untranslatable entities will not cause a compile error in your code unless
you actually use them.
</p>
{#see_also|opaque|extern|@compileError#}
{#header_close#}
{#header_open|C Macros#}
<p>
C Translation makes a best-effort attempt to translate function-like macros into equivalent
Zig functions. Since C macros operate at the level of lexical tokens, not all C macros
can be translated to Zig. Macros that cannot be translated will be demoted to
{#syntax#}@compileError{#endsyntax#}. Note that C code which <em>uses</em> macros will be
translated without any additional issues (since Zig operates on the pre-processed source
with macros expanded). It is merely the macros themselves which may not be translatable to
Zig.
</p>
<p>Consider the following example:</p>
{#syntax_block|c|macro.c#}
#define MAKELOCAL(NAME, INIT) int NAME = INIT
int foo(void) {
MAKELOCAL(a, 1);
MAKELOCAL(b, 2);
return a + b;
}
{#end_syntax_block#}
{#shell_samp#}$ zig translate-c macro.c > macro.zig{#end_shell_samp#}
{#code_begin|syntax|macro#}
pub export fn foo() c_int {
var a: c_int = 1;
_ = &a;
var b: c_int = 2;
_ = &b;
return a + b;
}
pub const MAKELOCAL = @compileError("unable to translate C expr: unexpected token .Equal"); // macro.c:1:9
{#code_end#}
<p>Note that {#syntax#}foo{#endsyntax#} was translated correctly despite using a non-translatable
macro. {#syntax#}MAKELOCAL{#endsyntax#} was demoted to {#syntax#}@compileError{#endsyntax#} since
it cannot be expressed as a Zig function; this simply means that you cannot directly use
{#syntax#}MAKELOCAL{#endsyntax#} from Zig.
</p>
{#see_also|@compileError#}
{#header_close#}
{#header_open|C Pointers#}
<p>
This type is to be avoided whenever possible. The only valid reason for using a C pointer is in
auto-generated code from translating C code.
</p>
<p>
When importing C header files, it is ambiguous whether pointers should be translated as
single-item pointers ({#syntax#}*T{#endsyntax#}) or many-item pointers ({#syntax#}[*]T{#endsyntax#}).
C pointers are a compromise so that Zig code can utilize translated header files directly.
</p>
<p>{#syntax#}[*c]T{#endsyntax#} - C pointer.</p>
<ul>
<li>Supports all the syntax of the other two pointer types ({#syntax#}*T{#endsyntax#}) and ({#syntax#}[*]T{#endsyntax#}).</li>
<li>Coerces to other pointer types, as well as {#link|Optional Pointers#}.
When a C pointer is coerced to a non-optional pointer, safety-checked
{#link|Undefined Behavior#} occurs if the address is 0.
</li>
<li>Allows address 0. On non-freestanding targets, dereferencing address 0 is safety-checked
{#link|Undefined Behavior#}. Optional C pointers introduce another bit to keep track of
null, just like {#syntax#}?usize{#endsyntax#}. Note that creating an optional C pointer
is unnecessary as one can use normal {#link|Optional Pointers#}.
</li>
<li>Supports {#link|Type Coercion#} to and from integers.</li>
<li>Supports comparison with integers.</li>
<li>Does not support Zig-only pointer attributes such as alignment. Use normal {#link|Pointers#}
please!</li>
</ul>
<p>When a C pointer is pointing to a single struct (not an array), dereference the C pointer to
access the struct's fields or member data. That syntax looks like
this: </p>
<p>{#syntax#}ptr_to_struct.*.struct_member{#endsyntax#}</p>
<p>This is comparable to doing {#syntax#}->{#endsyntax#} in C.</p>
<p> When a C pointer is pointing to an array of structs, the syntax reverts to this:</p>
<p>{#syntax#}ptr_to_struct_array[index].struct_member{#endsyntax#}</p>
{#header_close#}
{#header_open|C Variadic Functions#}
<p>Zig supports extern variadic functions.</p>
{#code_begin|test|test_variadic_function#}
{#link_libc#}
{#code_verbose_cimport#}
const std = @import("std");
const testing = std.testing;
pub extern "c" fn printf(format: [*:0]const u8, ...) c_int;
test "variadic function" {
try testing.expect(printf("Hello, world!\n") == 14);
try testing.expect(@typeInfo(@TypeOf(printf)).Fn.is_var_args);
}
{#code_end#}
<p>
Variadic functions can be implemented using {#link|@cVaStart#}, {#link|@cVaEnd#}, {#link|@cVaArg#} and {#link|@cVaCopy#}.
</p>
{#code_begin|test|test_defining_variadic_function#}
const std = @import("std");
const testing = std.testing;
const builtin = @import("builtin");
fn add(count: c_int, ...) callconv(.C) c_int {
var ap = @cVaStart();
defer @cVaEnd(&ap);
var i: usize = 0;
var sum: c_int = 0;
while (i < count) : (i += 1) {
sum += @cVaArg(&ap, c_int);
}
return sum;
}
test "defining a variadic function" {
if (builtin.cpu.arch == .aarch64 and builtin.os.tag != .macos) {
// https://github.com/ziglang/zig/issues/14096
return error.SkipZigTest;
}
if (builtin.cpu.arch == .x86_64 and builtin.os.tag == .windows) {
// https://github.com/ziglang/zig/issues/16961
return error.SkipZigTest;
}
try std.testing.expectEqual(@as(c_int, 0), add(0));
try std.testing.expectEqual(@as(c_int, 1), add(1, @as(c_int, 1)));
try std.testing.expectEqual(@as(c_int, 3), add(2, @as(c_int, 1), @as(c_int, 2)));
}
{#code_end#}
{#header_close#}
{#header_open|Exporting a C Library#}
<p>
One of the primary use cases for Zig is exporting a library with the C ABI for other programming languages
to call into. The {#syntax#}export{#endsyntax#} keyword in front of functions, variables, and types causes them to
be part of the library API:
</p>
{#code_begin|syntax|mathtest#}
export fn add(a: i32, b: i32) i32 {
return a + b;
}
{#code_end#}
<p>To make a static library:</p>
{#shell_samp#}$ zig build-lib mathtest.zig{#end_shell_samp#}
<p>To make a shared library:</p>
{#shell_samp#}$ zig build-lib mathtest.zig -dynamic{#end_shell_samp#}
<p>Here is an example with the {#link|Zig Build System#}:</p>
{#syntax_block|c|test.c#}
// This header is generated by zig from mathtest.zig
#include "mathtest.h"
#include <stdio.h>
int main(int argc, char **argv) {
int32_t result = add(42, 1337);
printf("%d\n", result);
return 0;
}
{#end_syntax_block#}
{#code_begin|syntax|build_c#}
const std = @import("std");
pub fn build(b: *std.Build) void {
const lib = b.addSharedLibrary(.{
.name = "mathtest",
.root_source_file = .{ .path = "mathtest.zig" },
.version = .{ .major = 1, .minor = 0, .patch = 0 },
});
const exe = b.addExecutable(.{
.name = "test",
});
exe.addCSourceFile(.{ .file = .{ .path = "test.c" }, .flags = &.{"-std=c99"} });
exe.linkLibrary(lib);
exe.linkSystemLibrary("c");
b.default_step.dependOn(&exe.step);
const run_cmd = exe.run();
const test_step = b.step("test", "Test the program");
test_step.dependOn(&run_cmd.step);
}
{#code_end#}
{#shell_samp#}$ zig build test
1379{#end_shell_samp#}
{#see_also|export#}
{#header_close#}
{#header_open|Mixing Object Files#}
<p>
You can mix Zig object files with any other object files that respect the C ABI. Example:
</p>
{#code_begin|syntax|base64#}
const base64 = @import("std").base64;
export fn decode_base_64(
dest_ptr: [*]u8,
dest_len: usize,
source_ptr: [*]const u8,
source_len: usize,
) usize {
const src = source_ptr[0..source_len];
const dest = dest_ptr[0..dest_len];
const base64_decoder = base64.standard.Decoder;
const decoded_size = base64_decoder.calcSizeForSlice(src) catch unreachable;
base64_decoder.decode(dest[0..decoded_size], src) catch unreachable;
return decoded_size;
}
{#code_end#}
{#syntax_block|c|test.c#}
// This header is generated by zig from base64.zig
#include "base64.h"
#include <string.h>
#include <stdio.h>
int main(int argc, char **argv) {
const char *encoded = "YWxsIHlvdXIgYmFzZSBhcmUgYmVsb25nIHRvIHVz";
char buf[200];
size_t len = decode_base_64(buf, 200, encoded, strlen(encoded));
buf[len] = 0;
puts(buf);
return 0;
}
{#end_syntax_block#}
{#code_begin|syntax|build_object#}
const std = @import("std");
pub fn build(b: *std.Build) void {
const obj = b.addObject(.{
.name = "base64",
.root_source_file = .{ .path = "base64.zig" },
});
const exe = b.addExecutable(.{
.name = "test",
});
exe.addCSourceFile(.{ .file = .{ .path = "test.c" }, .flags = &.{"-std=c99",} });
exe.addObject(obj);
exe.linkSystemLibrary("c");
b.installArtifact(exe);
}
{#code_end#}
{#shell_samp#}$ zig build
$ ./zig-out/bin/test
all your base are belong to us{#end_shell_samp#}
{#see_also|Targets|Zig Build System#}
{#header_close#}
{#header_close#}
{#header_open|WebAssembly#}
<p>Zig supports building for WebAssembly out of the box.</p>
{#header_open|Freestanding#}
<p>For host environments like the web browser and nodejs, build as an executable using the freestanding
OS target. Here's an example of running Zig code compiled to WebAssembly with nodejs.</p>
{#code_begin|exe|math#}
{#target_wasm#}
{#additonal_option|-fno-entry#}
{#additonal_option|--export=add#}
extern fn print(i32) void;
export fn add(a: i32, b: i32) void {
print(a + b);
}
{#code_end#}
{#syntax_block|javascript|test.js#}
const fs = require('fs');
const source = fs.readFileSync("./math.wasm");
const typedArray = new Uint8Array(source);
WebAssembly.instantiate(typedArray, {
env: {
print: (result) => { console.log(`The result is ${result}`); }
}}).then(result => {
const add = result.instance.exports.add;
add(1, 2);
});
{#end_syntax_block#}
{#shell_samp#}$ node test.js
The result is 3{#end_shell_samp#}
{#header_close#}
{#header_open|WASI#}
<p>Zig's support for WebAssembly System Interface (WASI) is under active development.
Example of using the standard library and reading command line arguments:</p>
{#code_begin|exe|wasi_args#}
{#target_wasi#}
const std = @import("std");
pub fn main() !void {
var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
const gpa = general_purpose_allocator.allocator();
const args = try std.process.argsAlloc(gpa);
defer std.process.argsFree(gpa, args);
for (args, 0..) |arg, i| {
std.debug.print("{}: {s}\n", .{ i, arg });
}
}
{#code_end#}
{#shell_samp#}$ wasmtime wasi_args.wasm 123 hello
0: wasi_args.wasm
1: 123
2: hello{#end_shell_samp#}
<p>A more interesting example would be extracting the list of preopens from the runtime.
This is now supported in the standard library via {#syntax#}std.fs.wasi.Preopens{#endsyntax#}:</p>
{#code_begin|exe|wasi_preopens#}
{#target_wasi#}
const std = @import("std");
const fs = std.fs;
pub fn main() !void {
var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
const gpa = general_purpose_allocator.allocator();
var arena_instance = std.heap.ArenaAllocator.init(gpa);
defer arena_instance.deinit();
const arena = arena_instance.allocator();
const preopens = try fs.wasi.preopensAlloc(arena);
for (preopens.names, 0..) |preopen, i| {
std.debug.print("{}: {s}\n", .{ i, preopen });
}
}
{#code_end#}
{#shell_samp#}$ wasmtime --dir=. wasi_preopens.wasm
0: stdin
1: stdout
2: stderr
3: .
{#end_shell_samp#}
{#header_close#}
{#header_close#}
{#header_open|Targets#}
<p>
<strong>Target</strong> refers to the computer that will be used to run an executable.
It is composed of the CPU architecture, the set of enabled CPU features, operating system,
minimum and maximum operating system version, ABI, and ABI version.
</p>
<p>
Zig is a general-purpose programming language which means that it is designed to
generate optimal code for a large set of targets. The command <code>zig targets</code>
provides information about all of the targets the compiler is aware of.</p>
<p>When no target option is provided to the compiler, the default choice
is to target the <strong>host computer</strong>, meaning that the
resulting executable will be <em>unsuitable for copying to a different
computer</em>. In order to copy an executable to another computer, the compiler
needs to know about the target requirements via the <code>-target</code> option.
</p>
<p>
The Zig Standard Library ({#syntax#}@import("std"){#endsyntax#}) has
cross-platform abstractions, making the same source code viable on many targets.
Some code is more portable than other code. In general, Zig code is extremely
portable compared to other programming languages.
</p>
<p>
Each platform requires its own implementations to make Zig's
cross-platform abstractions work. These implementations are at various
degrees of completion. Each tagged release of the compiler comes with
release notes that provide the full support table for each target.
</p>
{#header_close#}
{#header_open|Style Guide#}
<p>
These coding conventions are not enforced by the compiler, but they are shipped in
this documentation along with the compiler in order to provide a point of
reference, should anyone wish to point to an authority on agreed upon Zig
coding style.
</p>
{#header_open|Avoid Redundancy in Names#}
<p>Avoid these words in type names:</p>
<ul>
<li>Value</li>
<li>Data</li>
<li>Context</li>
<li>Manager</li>
<li>utils, misc, or somebody's initials</li>
</ul>
<p>Everything is a value, all types are data, everything is context, all logic manages state.
Nothing is communicated by using a word that applies to all types.</p>
<p>Temptation to use "utilities", "miscellaneous", or somebody's initials
is a failure to categorize, or more commonly, overcategorization. Such
declarations can live at the root of a module that needs them with no
namespace needed.</p>
{#header_close#}
{#header_open|Avoid Redundant Names in Fully-Qualified Namespaces#}
<p>Every declaration is assigned a <strong>fully qualified
namespace</strong> by the compiler, creating a tree structure. Choose names based
on the fully-qualified namespace, and avoid redundant name segments.</p>
{#code_begin|exe|redundant_fqn#}
const std = @import("std");
pub const json = struct {
pub const JsonValue = union(enum) {
number: f64,
boolean: bool,
// ...
};
};
pub fn main() void {
std.debug.print("{s}\n", .{@typeName(json.JsonValue)});
}
{#code_end#}
<p>In this example, "json" is repeated in the fully-qualified namespace. The solution
is to delete <code>Json</code> from <code>JsonValue</code>. In this example we have
an empty struct named <code>json</code> but remember that files also act
as part of the fully-qualified namespace.</p>
<p>This example is an exception to the rule specified in {#link|Avoid Redundancy in Names#}.
The meaning of the type has been reduced to its core: it is a json value. The name
cannot be any more specific without being incorrect.</p>
{#header_close#}
{#header_open|Whitespace#}
<ul>
<li>
4 space indentation
</li>
<li>
Open braces on same line, unless you need to wrap.
</li>
<li>If a list of things is longer than 2, put each item on its own line and
exercise the ability to put an extra comma at the end.
</li>
<li>
Line length: aim for 100; use common sense.
</li>
</ul>
{#header_close#}
{#header_open|Names#}
<p>
Roughly speaking: {#syntax#}camelCaseFunctionName{#endsyntax#}, {#syntax#}TitleCaseTypeName{#endsyntax#},
{#syntax#}snake_case_variable_name{#endsyntax#}. More precisely:
</p>
<ul>
<li>
If {#syntax#}x{#endsyntax#} is a {#syntax#}type{#endsyntax#}
then {#syntax#}x{#endsyntax#} should be {#syntax#}TitleCase{#endsyntax#}, unless it
is a {#syntax#}struct{#endsyntax#} with 0 fields and is never meant to be instantiated,
in which case it is considered to be a "namespace" and uses {#syntax#}snake_case{#endsyntax#}.
</li>
<li>
If {#syntax#}x{#endsyntax#} is callable, and {#syntax#}x{#endsyntax#}'s return type is
{#syntax#}type{#endsyntax#}, then {#syntax#}x{#endsyntax#} should be {#syntax#}TitleCase{#endsyntax#}.
</li>
<li>
If {#syntax#}x{#endsyntax#} is otherwise callable, then {#syntax#}x{#endsyntax#} should
be {#syntax#}camelCase{#endsyntax#}.
</li>
<li>
Otherwise, {#syntax#}x{#endsyntax#} should be {#syntax#}snake_case{#endsyntax#}.
</li>
</ul>
<p>
Acronyms, initialisms, proper nouns, or any other word that has capitalization
rules in written English are subject to naming conventions just like any other
word. Even acronyms that are only 2 letters long are subject to these
conventions.
</p>
<p>
File names fall into two categories: types and namespaces. If the file
(implicitly a struct) has top level fields, it should be named like any
other struct with fields using <code class="file">TitleCase</code>. Otherwise,
it should use <code class="file">snake_case</code>. Directory names should be
<code class="file">snake_case</code>.
</p>
<p>
These are general rules of thumb; if it makes sense to do something different,
do what makes sense. For example, if there is an established convention such as
{#syntax#}ENOENT{#endsyntax#}, follow the established convention.
</p>
{#header_close#}
{#header_open|Examples#}
{#syntax_block|zig|style_example.zig#}
const namespace_name = @import("dir_name/file_name.zig");
const TypeName = @import("dir_name/TypeName.zig");
var global_var: i32 = undefined;
const const_name = 42;
const primitive_type_alias = f32;
const string_alias = []u8;
const StructName = struct {
field: i32,
};
const StructAlias = StructName;
fn functionName(param_name: TypeName) void {
var functionPointer = functionName;
functionPointer();
functionPointer = otherFunction;
functionPointer();
}
const functionAlias = functionName;
fn ListTemplateFunction(comptime ChildType: type, comptime fixed_size: usize) type {
return List(ChildType, fixed_size);
}
fn ShortList(comptime T: type, comptime n: usize) type {
return struct {
field_name: [n]T,
fn methodName() void {}
};
}
// The word XML loses its casing when used in Zig identifiers.
const xml_document =
\\<?xml version="1.0" encoding="UTF-8"?>
\\<document>
\\</document>
;
const XmlParser = struct {
field: i32,
};
// The initials BE (Big Endian) are just another word in Zig identifier names.
fn readU32Be() u32 {}
{#end_syntax_block#}
<p>
See the {#link|Zig Standard Library#} for more examples.
</p>
{#header_close#}
{#header_open|Doc Comment Guidance#}
<ul>
<li>Omit any information that is redundant based on the name of the thing being documented.</li>
<li>Duplicating information onto multiple similar functions is encouraged because it helps IDEs and other tools provide better help text.</li>
<li>Use the word <strong>assume</strong> to indicate invariants that cause {#link|Undefined Behavior#} when violated.</li>
<li>Use the word <strong>assert</strong> to indicate invariants that cause <em>safety-checked</em> {#link|Undefined Behavior#} when violated.</li>
</ul>
{#header_close#}
{#header_close#}
{#header_open|Source Encoding#}
<p>Zig source code is encoded in UTF-8. An invalid UTF-8 byte sequence results in a compile error.</p>
<p>Throughout all zig source code (including in comments), some code points are never allowed:</p>
<ul>
<li>Ascii control characters, except for U+000a (LF), U+000d (CR), and U+0009 (HT): U+0000 - U+0008, U+000b - U+000c, U+000e - U+0001f, U+007f.</li>
<li>Non-Ascii Unicode line endings: U+0085 (NEL), U+2028 (LS), U+2029 (PS).</li>
</ul>
<p>
LF (byte value 0x0a, code point U+000a, {#syntax#}'\n'{#endsyntax#}) is the line terminator in Zig source code.
This byte value terminates every line of zig source code except the last line of the file.
It is recommended that non-empty source files end with an empty line, which means the last byte would be 0x0a (LF).
</p>
<p>
Each LF may be immediately preceded by a single CR (byte value 0x0d, code point U+000d, {#syntax#}'\r'{#endsyntax#})
to form a Windows style line ending, but this is discouraged. Note that in multiline strings, CRLF sequences will
be encoded as LF when compiled into a zig program.
A CR in any other context is not allowed.
</p>
<p>
HT hard tabs (byte value 0x09, code point U+0009, {#syntax#}'\t'{#endsyntax#}) are interchangeable with
SP spaces (byte value 0x20, code point U+0020, {#syntax#}' '{#endsyntax#}) as a token separator,
but use of hard tabs is discouraged. See {#link|Grammar#}.
</p>
<p>
For compatibility with other tools, the compiler ignores a UTF-8-encoded byte order mark (U+FEFF)
if it is the first Unicode code point in the source text. A byte order mark is not allowed anywhere else in the source.
</p>
<p>
Note that running <kbd>zig fmt</kbd> on a source file will implement all recommendations mentioned here.
</p>
<p>
Note that a tool reading Zig source code can make assumptions if the source code is assumed to be correct Zig code.
For example, when identifying the ends of lines, a tool can use a naive search such as <code>/\n/</code>,
or an <a href="https://msdn.microsoft.com/en-us/library/dd409797.aspx">advanced</a>
search such as <code>/\r\n?|[\n\u0085\u2028\u2029]/</code>, and in either case line endings will be correctly identified.
For another example, when identifying the whitespace before the first token on a line,
a tool can either use a naive search such as <code>/[ \t]/</code>,
or an <a href="https://tc39.es/ecma262/#sec-characterclassescape">advanced</a> search such as <code>/\s/</code>,
and in either case whitespace will be correctly identified.
</p>
{#header_close#}
{#header_open|Keyword Reference#}
<div class="table-wrapper">
<table>
<thead>
<tr>
<th scope="col">Keyword</th>
<th scope="col">Description</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">
<pre>{#syntax#}addrspace{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}addrspace{#endsyntax#} keyword.
<ul>
<li>TODO add documentation for addrspace</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}align{#endsyntax#}</pre>
</th>
<td>
{#syntax#}align{#endsyntax#} can be used to specify the alignment of a pointer.
It can also be used after a variable or function declaration to specify the alignment of pointers to that variable or function.
<ul>
<li>See also {#link|Alignment#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}allowzero{#endsyntax#}</pre>
</th>
<td>
The pointer attribute {#syntax#}allowzero{#endsyntax#} allows a pointer to have address zero.
<ul>
<li>See also {#link|allowzero#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}and{#endsyntax#}</pre>
</th>
<td>
The boolean operator {#syntax#}and{#endsyntax#}.
<ul>
<li>See also {#link|Operators#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}anyframe{#endsyntax#}</pre>
</th>
<td>
{#syntax#}anyframe{#endsyntax#} can be used as a type for variables which hold pointers to function frames.
<ul>
<li>See also {#link|Async Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}anytype{#endsyntax#}</pre>
</th>
<td>
Function parameters can be declared with {#syntax#}anytype{#endsyntax#} in place of the type.
The type will be inferred where the function is called.
<ul>
<li>See also {#link|Function Parameter Type Inference#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}asm{#endsyntax#}</pre>
</th>
<td>
{#syntax#}asm{#endsyntax#} begins an inline assembly expression. This allows for directly controlling the machine code generated on compilation.
<ul>
<li>See also {#link|Assembly#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}async{#endsyntax#}</pre>
</th>
<td>
{#syntax#}async{#endsyntax#} can be used before a function call to get a pointer to the function's frame when it suspends.
<ul>
<li>See also {#link|Async Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}await{#endsyntax#}</pre>
</th>
<td>
{#syntax#}await{#endsyntax#} can be used to suspend the current function until the frame provided after the {#syntax#}await{#endsyntax#} completes.
{#syntax#}await{#endsyntax#} copies the value returned from the target function's frame to the caller.
<ul>
<li>See also {#link|Async Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}break{#endsyntax#}</pre>
</th>
<td>
{#syntax#}break{#endsyntax#} can be used with a block label to return a value from the block.
It can also be used to exit a loop before iteration completes naturally.
<ul>
<li>See also {#link|Blocks#}, {#link|while#}, {#link|for#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}callconv{#endsyntax#}</pre>
</th>
<td>
{#syntax#}callconv{#endsyntax#} can be used to specify the calling convention in a function type.
<ul>
<li>See also {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}catch{#endsyntax#}</pre>
</th>
<td>
{#syntax#}catch{#endsyntax#} can be used to evaluate an expression if the expression before it evaluates to an error.
The expression after the {#syntax#}catch{#endsyntax#} can optionally capture the error value.
<ul>
<li>See also {#link|catch#}, {#link|Operators#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}comptime{#endsyntax#}</pre>
</th>
<td>
{#syntax#}comptime{#endsyntax#} before a declaration can be used to label variables or function parameters as known at compile time.
It can also be used to guarantee an expression is run at compile time.
<ul>
<li>See also {#link|comptime#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}const{#endsyntax#}</pre>
</th>
<td>
{#syntax#}const{#endsyntax#} declares a variable that can not be modified.
Used as a pointer attribute, it denotes the value referenced by the pointer cannot be modified.
<ul>
<li>See also {#link|Variables#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}continue{#endsyntax#}</pre>
</th>
<td>
{#syntax#}continue{#endsyntax#} can be used in a loop to jump back to the beginning of the loop.
<ul>
<li>See also {#link|while#}, {#link|for#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}defer{#endsyntax#}</pre>
</th>
<td>
{#syntax#}defer{#endsyntax#} will execute an expression when control flow leaves the current block.
<ul>
<li>See also {#link|defer#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}else{#endsyntax#}</pre>
</th>
<td>
{#syntax#}else{#endsyntax#} can be used to provide an alternate branch for {#syntax#}if{#endsyntax#}, {#syntax#}switch{#endsyntax#},
{#syntax#}while{#endsyntax#}, and {#syntax#}for{#endsyntax#} expressions.
<ul>
<li>If used after an if expression, the else branch will be executed if the test value returns false, null, or an error.</li>
<li>If used within a switch expression, the else branch will be executed if the test value matches no other cases.</li>
<li>If used after a loop expression, the else branch will be executed if the loop finishes without breaking.</li>
<li>See also {#link|if#}, {#link|switch#}, {#link|while#}, {#link|for#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}enum{#endsyntax#}</pre>
</th>
<td>
{#syntax#}enum{#endsyntax#} defines an enum type.
<ul>
<li>See also {#link|enum#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}errdefer{#endsyntax#}</pre>
</th>
<td>
{#syntax#}errdefer{#endsyntax#} will execute an expression when control flow leaves the current block if the function returns an error, the errdefer expression can capture the unwrapped value.
<ul>
<li>See also {#link|errdefer#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}error{#endsyntax#}</pre>
</th>
<td>
{#syntax#}error{#endsyntax#} defines an error type.
<ul>
<li>See also {#link|Errors#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}export{#endsyntax#}</pre>
</th>
<td>
{#syntax#}export{#endsyntax#} makes a function or variable externally visible in the generated object file.
Exported functions default to the C calling convention.
<ul>
<li>See also {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}extern{#endsyntax#}</pre>
</th>
<td>
{#syntax#}extern{#endsyntax#} can be used to declare a function or variable that will be resolved at link time, when linking statically
or at runtime, when linking dynamically.
<ul>
<li>See also {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}fn{#endsyntax#}</pre>
</th>
<td>
{#syntax#}fn{#endsyntax#} declares a function.
<ul>
<li>See also {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}for{#endsyntax#}</pre>
</th>
<td>
A {#syntax#}for{#endsyntax#} expression can be used to iterate over the elements of a slice, array, or tuple.
<ul>
<li>See also {#link|for#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}if{#endsyntax#}</pre>
</th>
<td>
An {#syntax#}if{#endsyntax#} expression can test boolean expressions, optional values, or error unions.
For optional values or error unions, the if expression can capture the unwrapped value.
<ul>
<li>See also {#link|if#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}inline{#endsyntax#}</pre>
</th>
<td>
{#syntax#}inline{#endsyntax#} can be used to label a loop expression such that it will be unrolled at compile time.
It can also be used to force a function to be inlined at all call sites.
<ul>
<li>See also {#link|inline while#}, {#link|inline for#}, {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}linksection{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}linksection{#endsyntax#} keyword can be used to specify what section the function or global variable will be put into (e.g. <code>.text</code>).
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}noalias{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}noalias{#endsyntax#} keyword.
<ul>
<li>TODO add documentation for noalias</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}noinline{#endsyntax#}</pre>
</th>
<td>
{#syntax#}noinline{#endsyntax#} disallows function to be inlined in all call sites.
<ul>
<li>See also {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}nosuspend{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}nosuspend{#endsyntax#} keyword can be used in front of a block, statement or expression, to mark a scope where no suspension points are reached.
In particular, inside a {#syntax#}nosuspend{#endsyntax#} scope:
<ul>
<li>Using the {#syntax#}suspend{#endsyntax#} keyword results in a compile error.</li>
<li>Using {#syntax#}await{#endsyntax#} on a function frame which hasn't completed yet results in safety-checked {#link|Undefined Behavior#}.</li>
<li>Calling an async function may result in safety-checked {#link|Undefined Behavior#}, because it's equivalent to <code>await async some_async_fn()</code>, which contains an {#syntax#}await{#endsyntax#}.</li>
</ul>
Code inside a {#syntax#}nosuspend{#endsyntax#} scope does not cause the enclosing function to become an {#link|async function|Async Functions#}.
<ul>
<li>See also {#link|Async Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}opaque{#endsyntax#}</pre>
</th>
<td>
{#syntax#}opaque{#endsyntax#} defines an opaque type.
<ul>
<li>See also {#link|opaque#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}or{#endsyntax#}</pre>
</th>
<td>
The boolean operator {#syntax#}or{#endsyntax#}.
<ul>
<li>See also {#link|Operators#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}orelse{#endsyntax#}</pre>
</th>
<td>
{#syntax#}orelse{#endsyntax#} can be used to evaluate an expression if the expression before it evaluates to null.
<ul>
<li>See also {#link|Optionals#}, {#link|Operators#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}packed{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}packed{#endsyntax#} keyword before a struct definition changes the struct's in-memory layout
to the guaranteed {#syntax#}packed{#endsyntax#} layout.
<ul>
<li>See also {#link|packed struct#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}pub{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}pub{#endsyntax#} in front of a top level declaration makes the declaration available
to reference from a different file than the one it is declared in.
<ul>
<li>See also {#link|import#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}resume{#endsyntax#}</pre>
</th>
<td>
{#syntax#}resume{#endsyntax#} will continue execution of a function frame after the point the function was suspended.
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}return{#endsyntax#}</pre>
</th>
<td>
{#syntax#}return{#endsyntax#} exits a function with a value.
<ul>
<li>See also {#link|Functions#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}struct{#endsyntax#}</pre>
</th>
<td>
{#syntax#}struct{#endsyntax#} defines a struct.
<ul>
<li>See also {#link|struct#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}suspend{#endsyntax#}</pre>
</th>
<td>
{#syntax#}suspend{#endsyntax#} will cause control flow to return to the call site or resumer of the function.
{#syntax#}suspend{#endsyntax#} can also be used before a block within a function,
to allow the function access to its frame before control flow returns to the call site.
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}switch{#endsyntax#}</pre>
</th>
<td>
A {#syntax#}switch{#endsyntax#} expression can be used to test values of a common type.
{#syntax#}switch{#endsyntax#} cases can capture field values of a {#link|Tagged union#}.
<ul>
<li>See also {#link|switch#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}test{#endsyntax#}</pre>
</th>
<td>
The {#syntax#}test{#endsyntax#} keyword can be used to denote a top-level block of code
used to make sure behavior meets expectations.
<ul>
<li>See also {#link|Zig Test#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}threadlocal{#endsyntax#}</pre>
</th>
<td>
{#syntax#}threadlocal{#endsyntax#} can be used to specify a variable as thread-local.
<ul>
<li>See also {#link|Thread Local Variables#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}try{#endsyntax#}</pre>
</th>
<td>
{#syntax#}try{#endsyntax#} evaluates an error union expression.
If it is an error, it returns from the current function with the same error.
Otherwise, the expression results in the unwrapped value.
<ul>
<li>See also {#link|try#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}union{#endsyntax#}</pre>
</th>
<td>
{#syntax#}union{#endsyntax#} defines a union.
<ul>
<li>See also {#link|union#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}unreachable{#endsyntax#}</pre>
</th>
<td>
{#syntax#}unreachable{#endsyntax#} can be used to assert that control flow will never happen upon a particular location.
Depending on the build mode, {#syntax#}unreachable{#endsyntax#} may emit a panic.
<ul>
<li>Emits a panic in {#syntax#}Debug{#endsyntax#} and {#syntax#}ReleaseSafe{#endsyntax#} mode, or when using <kbd>zig test</kbd>.</li>
<li>Does not emit a panic in {#syntax#}ReleaseFast{#endsyntax#} and {#syntax#}ReleaseSmall{#endsyntax#} mode.</li>
<li>See also {#link|unreachable#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}usingnamespace{#endsyntax#}</pre>
</th>
<td>
{#syntax#}usingnamespace{#endsyntax#} is a top-level declaration that imports all the public declarations of the operand,
which must be a struct, union, or enum, into the current scope.
<ul>
<li>See also {#link|usingnamespace#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}var{#endsyntax#}</pre>
</th>
<td>
{#syntax#}var{#endsyntax#} declares a variable that may be modified.
<ul>
<li>See also {#link|Variables#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}volatile{#endsyntax#}</pre>
</th>
<td>
{#syntax#}volatile{#endsyntax#} can be used to denote loads or stores of a pointer have side effects.
It can also modify an inline assembly expression to denote it has side effects.
<ul>
<li>See also {#link|volatile#}, {#link|Assembly#}</li>
</ul>
</td>
</tr>
<tr>
<th scope="row">
<pre>{#syntax#}while{#endsyntax#}</pre>
</th>
<td>
A {#syntax#}while{#endsyntax#} expression can be used to repeatedly test a boolean, optional, or error union expression,
and cease looping when that expression evaluates to false, null, or an error, respectively.
<ul>
<li>See also {#link|while#}</li>
</ul>
</td>
</tr>
</tbody>
</table>
</div>
{#header_close#}
{#header_open|Appendix#}
{#header_open|Containers#}
<p>
A <em>container</em> in Zig is any syntactical construct that acts as a namespace to hold {#link|variable|Container Level Variables#} and {#link|function|Functions#} declarations.
Containers are also type definitions which can be instantiated.
{#link|Structs|struct#}, {#link|enums|enum#}, {#link|unions|union#}, {#link|opaques|opaque#}, and even Zig source files themselves are containers.
</p>
<p>
Although containers (except Zig source files) use curly braces to surround their definition, they should not be confused with {#link|blocks|Blocks#} or functions.
Containers do not contain statements.
</p>
{#header_close#}
{#header_open|Grammar#}
{#syntax_block|peg|grammar.y#}
Root <- skip container_doc_comment? ContainerMembers eof
# *** Top level ***
ContainerMembers <- ContainerDeclaration* (ContainerField COMMA)* (ContainerField / ContainerDeclaration*)
ContainerDeclaration <- TestDecl / ComptimeDecl / doc_comment? KEYWORD_pub? Decl
TestDecl <- KEYWORD_test (STRINGLITERALSINGLE / IDENTIFIER)? Block
ComptimeDecl <- KEYWORD_comptime Block
Decl
<- (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE? / KEYWORD_inline / KEYWORD_noinline)? FnProto (SEMICOLON / Block)
/ (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE?)? KEYWORD_threadlocal? GlobalVarDecl
/ KEYWORD_usingnamespace Expr SEMICOLON
FnProto <- KEYWORD_fn IDENTIFIER? LPAREN ParamDeclList RPAREN ByteAlign? AddrSpace? LinkSection? CallConv? EXCLAMATIONMARK? TypeExpr
VarDeclProto <- (KEYWORD_const / KEYWORD_var) IDENTIFIER (COLON TypeExpr)? ByteAlign? AddrSpace? LinkSection?
GlobalVarDecl <- VarDeclProto (EQUAL Expr)? SEMICOLON
ContainerField <- doc_comment? KEYWORD_comptime? !KEYWORD_fn (IDENTIFIER COLON)? TypeExpr ByteAlign? (EQUAL Expr)?
# *** Block Level ***
Statement
<- KEYWORD_comptime ComptimeStatement
/ KEYWORD_nosuspend BlockExprStatement
/ KEYWORD_suspend BlockExprStatement
/ KEYWORD_defer BlockExprStatement
/ KEYWORD_errdefer Payload? BlockExprStatement
/ IfStatement
/ LabeledStatement
/ SwitchExpr
/ VarDeclExprStatement
ComptimeStatement
<- BlockExpr
/ VarDeclExprStatement
IfStatement
<- IfPrefix BlockExpr ( KEYWORD_else Payload? Statement )?
/ IfPrefix AssignExpr ( SEMICOLON / KEYWORD_else Payload? Statement )
LabeledStatement <- BlockLabel? (Block / LoopStatement)
LoopStatement <- KEYWORD_inline? (ForStatement / WhileStatement)
ForStatement
<- ForPrefix BlockExpr ( KEYWORD_else Statement )?
/ ForPrefix AssignExpr ( SEMICOLON / KEYWORD_else Statement )
WhileStatement
<- WhilePrefix BlockExpr ( KEYWORD_else Payload? Statement )?
/ WhilePrefix AssignExpr ( SEMICOLON / KEYWORD_else Payload? Statement )
BlockExprStatement
<- BlockExpr
/ AssignExpr SEMICOLON
BlockExpr <- BlockLabel? Block
# An expression, assignment, or any destructure, as a statement.
VarDeclExprStatement
<- VarDeclProto (COMMA (VarDeclProto / Expr))* EQUAL Expr SEMICOLON
/ Expr (AssignOp Expr / (COMMA (VarDeclProto / Expr))+ EQUAL Expr)? SEMICOLON
# *** Expression Level ***
# An assignment or a destructure whose LHS are all lvalue expressions.
AssignExpr <- Expr (AssignOp Expr / (COMMA Expr)+ EQUAL Expr)?
SingleAssignExpr <- Expr (AssignOp Expr)?
Expr <- BoolOrExpr
BoolOrExpr <- BoolAndExpr (KEYWORD_or BoolAndExpr)*
BoolAndExpr <- CompareExpr (KEYWORD_and CompareExpr)*
CompareExpr <- BitwiseExpr (CompareOp BitwiseExpr)?
BitwiseExpr <- BitShiftExpr (BitwiseOp BitShiftExpr)*
BitShiftExpr <- AdditionExpr (BitShiftOp AdditionExpr)*
AdditionExpr <- MultiplyExpr (AdditionOp MultiplyExpr)*
MultiplyExpr <- PrefixExpr (MultiplyOp PrefixExpr)*
PrefixExpr <- PrefixOp* PrimaryExpr
PrimaryExpr
<- AsmExpr
/ IfExpr
/ KEYWORD_break BreakLabel? Expr?
/ KEYWORD_comptime Expr
/ KEYWORD_nosuspend Expr
/ KEYWORD_continue BreakLabel?
/ KEYWORD_resume Expr
/ KEYWORD_return Expr?
/ BlockLabel? LoopExpr
/ Block
/ CurlySuffixExpr
IfExpr <- IfPrefix Expr (KEYWORD_else Payload? Expr)?
Block <- LBRACE Statement* RBRACE
LoopExpr <- KEYWORD_inline? (ForExpr / WhileExpr)
ForExpr <- ForPrefix Expr (KEYWORD_else Expr)?
WhileExpr <- WhilePrefix Expr (KEYWORD_else Payload? Expr)?
CurlySuffixExpr <- TypeExpr InitList?
InitList
<- LBRACE FieldInit (COMMA FieldInit)* COMMA? RBRACE
/ LBRACE Expr (COMMA Expr)* COMMA? RBRACE
/ LBRACE RBRACE
TypeExpr <- PrefixTypeOp* ErrorUnionExpr
ErrorUnionExpr <- SuffixExpr (EXCLAMATIONMARK TypeExpr)?
SuffixExpr
<- KEYWORD_async PrimaryTypeExpr SuffixOp* FnCallArguments
/ PrimaryTypeExpr (SuffixOp / FnCallArguments)*
PrimaryTypeExpr
<- BUILTINIDENTIFIER FnCallArguments
/ CHAR_LITERAL
/ ContainerDecl
/ DOT IDENTIFIER
/ DOT InitList
/ ErrorSetDecl
/ FLOAT
/ FnProto
/ GroupedExpr
/ LabeledTypeExpr
/ IDENTIFIER
/ IfTypeExpr
/ INTEGER
/ KEYWORD_comptime TypeExpr
/ KEYWORD_error DOT IDENTIFIER
/ KEYWORD_anyframe
/ KEYWORD_unreachable
/ STRINGLITERAL
/ SwitchExpr
ContainerDecl <- (KEYWORD_extern / KEYWORD_packed)? ContainerDeclAuto
ErrorSetDecl <- KEYWORD_error LBRACE IdentifierList RBRACE
GroupedExpr <- LPAREN Expr RPAREN
IfTypeExpr <- IfPrefix TypeExpr (KEYWORD_else Payload? TypeExpr)?
LabeledTypeExpr
<- BlockLabel Block
/ BlockLabel? LoopTypeExpr
LoopTypeExpr <- KEYWORD_inline? (ForTypeExpr / WhileTypeExpr)
ForTypeExpr <- ForPrefix TypeExpr (KEYWORD_else TypeExpr)?
WhileTypeExpr <- WhilePrefix TypeExpr (KEYWORD_else Payload? TypeExpr)?
SwitchExpr <- KEYWORD_switch LPAREN Expr RPAREN LBRACE SwitchProngList RBRACE
# *** Assembly ***
AsmExpr <- KEYWORD_asm KEYWORD_volatile? LPAREN Expr AsmOutput? RPAREN
AsmOutput <- COLON AsmOutputList AsmInput?
AsmOutputItem <- LBRACKET IDENTIFIER RBRACKET STRINGLITERAL LPAREN (MINUSRARROW TypeExpr / IDENTIFIER) RPAREN
AsmInput <- COLON AsmInputList AsmClobbers?
AsmInputItem <- LBRACKET IDENTIFIER RBRACKET STRINGLITERAL LPAREN Expr RPAREN
AsmClobbers <- COLON StringList
# *** Helper grammar ***
BreakLabel <- COLON IDENTIFIER
BlockLabel <- IDENTIFIER COLON
FieldInit <- DOT IDENTIFIER EQUAL Expr
WhileContinueExpr <- COLON LPAREN AssignExpr RPAREN
LinkSection <- KEYWORD_linksection LPAREN Expr RPAREN
AddrSpace <- KEYWORD_addrspace LPAREN Expr RPAREN
# Fn specific
CallConv <- KEYWORD_callconv LPAREN Expr RPAREN
ParamDecl
<- doc_comment? (KEYWORD_noalias / KEYWORD_comptime)? (IDENTIFIER COLON)? ParamType
/ DOT3
ParamType
<- KEYWORD_anytype
/ TypeExpr
# Control flow prefixes
IfPrefix <- KEYWORD_if LPAREN Expr RPAREN PtrPayload?
WhilePrefix <- KEYWORD_while LPAREN Expr RPAREN PtrPayload? WhileContinueExpr?
ForPrefix <- KEYWORD_for LPAREN ForArgumentsList RPAREN PtrListPayload
# Payloads
Payload <- PIPE IDENTIFIER PIPE
PtrPayload <- PIPE ASTERISK? IDENTIFIER PIPE
PtrIndexPayload <- PIPE ASTERISK? IDENTIFIER (COMMA IDENTIFIER)? PIPE
PtrListPayload <- PIPE ASTERISK? IDENTIFIER (COMMA ASTERISK? IDENTIFIER)* COMMA? PIPE
# Switch specific
SwitchProng <- KEYWORD_inline? SwitchCase EQUALRARROW PtrIndexPayload? SingleAssignExpr
SwitchCase
<- SwitchItem (COMMA SwitchItem)* COMMA?
/ KEYWORD_else
SwitchItem <- Expr (DOT3 Expr)?
# For specific
ForArgumentsList <- ForItem (COMMA ForItem)* COMMA?
ForItem <- Expr (DOT2 Expr?)?
# Operators
AssignOp
<- ASTERISKEQUAL
/ ASTERISKPIPEEQUAL
/ SLASHEQUAL
/ PERCENTEQUAL
/ PLUSEQUAL
/ PLUSPIPEEQUAL
/ MINUSEQUAL
/ MINUSPIPEEQUAL
/ LARROW2EQUAL
/ LARROW2PIPEEQUAL
/ RARROW2EQUAL
/ AMPERSANDEQUAL
/ CARETEQUAL
/ PIPEEQUAL
/ ASTERISKPERCENTEQUAL
/ PLUSPERCENTEQUAL
/ MINUSPERCENTEQUAL
/ EQUAL
CompareOp
<- EQUALEQUAL
/ EXCLAMATIONMARKEQUAL
/ LARROW
/ RARROW
/ LARROWEQUAL
/ RARROWEQUAL
BitwiseOp
<- AMPERSAND
/ CARET
/ PIPE
/ KEYWORD_orelse
/ KEYWORD_catch Payload?
BitShiftOp
<- LARROW2
/ RARROW2
/ LARROW2PIPE
AdditionOp
<- PLUS
/ MINUS
/ PLUS2
/ PLUSPERCENT
/ MINUSPERCENT
/ PLUSPIPE
/ MINUSPIPE
MultiplyOp
<- PIPE2
/ ASTERISK
/ SLASH
/ PERCENT
/ ASTERISK2
/ ASTERISKPERCENT
/ ASTERISKPIPE
PrefixOp
<- EXCLAMATIONMARK
/ MINUS
/ TILDE
/ MINUSPERCENT
/ AMPERSAND
/ KEYWORD_try
/ KEYWORD_await
PrefixTypeOp
<- QUESTIONMARK
/ KEYWORD_anyframe MINUSRARROW
/ SliceTypeStart (ByteAlign / AddrSpace / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*
/ PtrTypeStart (AddrSpace / KEYWORD_align LPAREN Expr (COLON Expr COLON Expr)? RPAREN / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*
/ ArrayTypeStart
SuffixOp
<- LBRACKET Expr (DOT2 (Expr? (COLON Expr)?)?)? RBRACKET
/ DOT IDENTIFIER
/ DOTASTERISK
/ DOTQUESTIONMARK
FnCallArguments <- LPAREN ExprList RPAREN
# Ptr specific
SliceTypeStart <- LBRACKET (COLON Expr)? RBRACKET
PtrTypeStart
<- ASTERISK
/ ASTERISK2
/ LBRACKET ASTERISK (LETTERC / COLON Expr)? RBRACKET
ArrayTypeStart <- LBRACKET Expr (COLON Expr)? RBRACKET
# ContainerDecl specific
ContainerDeclAuto <- ContainerDeclType LBRACE container_doc_comment? ContainerMembers RBRACE
ContainerDeclType
<- KEYWORD_struct (LPAREN Expr RPAREN)?
/ KEYWORD_opaque
/ KEYWORD_enum (LPAREN Expr RPAREN)?
/ KEYWORD_union (LPAREN (KEYWORD_enum (LPAREN Expr RPAREN)? / Expr) RPAREN)?
# Alignment
ByteAlign <- KEYWORD_align LPAREN Expr RPAREN
# Lists
IdentifierList <- (doc_comment? IDENTIFIER COMMA)* (doc_comment? IDENTIFIER)?
SwitchProngList <- (SwitchProng COMMA)* SwitchProng?
AsmOutputList <- (AsmOutputItem COMMA)* AsmOutputItem?
AsmInputList <- (AsmInputItem COMMA)* AsmInputItem?
StringList <- (STRINGLITERAL COMMA)* STRINGLITERAL?
ParamDeclList <- (ParamDecl COMMA)* ParamDecl?
ExprList <- (Expr COMMA)* Expr?
# *** Tokens ***
eof <- !.
bin <- [01]
bin_ <- '_'? bin
oct <- [0-7]
oct_ <- '_'? oct
hex <- [0-9a-fA-F]
hex_ <- '_'? hex
dec <- [0-9]
dec_ <- '_'? dec
bin_int <- bin bin_*
oct_int <- oct oct_*
dec_int <- dec dec_*
hex_int <- hex hex_*
ox80_oxBF <- [\200-\277]
oxF4 <- '\364'
ox80_ox8F <- [\200-\217]
oxF1_oxF3 <- [\361-\363]
oxF0 <- '\360'
ox90_0xBF <- [\220-\277]
oxEE_oxEF <- [\356-\357]
oxED <- '\355'
ox80_ox9F <- [\200-\237]
oxE1_oxEC <- [\341-\354]
oxE0 <- '\340'
oxA0_oxBF <- [\240-\277]
oxC2_oxDF <- [\302-\337]
# From https://lemire.me/blog/2018/05/09/how-quickly-can-you-check-that-a-string-is-valid-unicode-utf-8/
# First Byte Second Byte Third Byte Fourth Byte
# [0x00,0x7F]
# [0xC2,0xDF] [0x80,0xBF]
# 0xE0 [0xA0,0xBF] [0x80,0xBF]
# [0xE1,0xEC] [0x80,0xBF] [0x80,0xBF]
# 0xED [0x80,0x9F] [0x80,0xBF]
# [0xEE,0xEF] [0x80,0xBF] [0x80,0xBF]
# 0xF0 [0x90,0xBF] [0x80,0xBF] [0x80,0xBF]
# [0xF1,0xF3] [0x80,0xBF] [0x80,0xBF] [0x80,0xBF]
# 0xF4 [0x80,0x8F] [0x80,0xBF] [0x80,0xBF]
mb_utf8_literal <-
oxF4 ox80_ox8F ox80_oxBF ox80_oxBF
/ oxF1_oxF3 ox80_oxBF ox80_oxBF ox80_oxBF
/ oxF0 ox90_0xBF ox80_oxBF ox80_oxBF
/ oxEE_oxEF ox80_oxBF ox80_oxBF
/ oxED ox80_ox9F ox80_oxBF
/ oxE1_oxEC ox80_oxBF ox80_oxBF
/ oxE0 oxA0_oxBF ox80_oxBF
/ oxC2_oxDF ox80_oxBF
ascii_char_not_nl_slash_squote <- [\000-\011\013-\046-\050-\133\135-\177]
char_escape
<- "\\x" hex hex
/ "\\u{" hex+ "}"
/ "\\" [nr\\t'"]
char_char
<- mb_utf8_literal
/ char_escape
/ ascii_char_not_nl_slash_squote
string_char
<- char_escape
/ [^\\"\n]
container_doc_comment <- ('//!' [^\n]* [ \n]* skip)+
doc_comment <- ('///' [^\n]* [ \n]* skip)+
line_comment <- '//' ![!/][^\n]* / '////' [^\n]*
line_string <- ("\\\\" [^\n]* [ \n]*)+
skip <- ([ \n] / line_comment)*
CHAR_LITERAL <- "'" char_char "'" skip
FLOAT
<- "0x" hex_int "." hex_int ([pP] [-+]? dec_int)? skip
/ dec_int "." dec_int ([eE] [-+]? dec_int)? skip
/ "0x" hex_int [pP] [-+]? dec_int skip
/ dec_int [eE] [-+]? dec_int skip
INTEGER
<- "0b" bin_int skip
/ "0o" oct_int skip
/ "0x" hex_int skip
/ dec_int skip
STRINGLITERALSINGLE <- "\"" string_char* "\"" skip
STRINGLITERAL
<- STRINGLITERALSINGLE
/ (line_string skip)+
IDENTIFIER
<- !keyword [A-Za-z_] [A-Za-z0-9_]* skip
/ "@" STRINGLITERALSINGLE
BUILTINIDENTIFIER <- "@"[A-Za-z_][A-Za-z0-9_]* skip
AMPERSAND <- '&' ![=] skip
AMPERSANDEQUAL <- '&=' skip
ASTERISK <- '*' ![*%=|] skip
ASTERISK2 <- '**' skip
ASTERISKEQUAL <- '*=' skip
ASTERISKPERCENT <- '*%' ![=] skip
ASTERISKPERCENTEQUAL <- '*%=' skip
ASTERISKPIPE <- '*|' ![=] skip
ASTERISKPIPEEQUAL <- '*|=' skip
CARET <- '^' ![=] skip
CARETEQUAL <- '^=' skip
COLON <- ':' skip
COMMA <- ',' skip
DOT <- '.' ![*.?] skip
DOT2 <- '..' ![.] skip
DOT3 <- '...' skip
DOTASTERISK <- '.*' skip
DOTQUESTIONMARK <- '.?' skip
EQUAL <- '=' ![>=] skip
EQUALEQUAL <- '==' skip
EQUALRARROW <- '=>' skip
EXCLAMATIONMARK <- '!' ![=] skip
EXCLAMATIONMARKEQUAL <- '!=' skip
LARROW <- '<' ![<=] skip
LARROW2 <- '<<' ![=|] skip
LARROW2EQUAL <- '<<=' skip
LARROW2PIPE <- '<<|' ![=] skip
LARROW2PIPEEQUAL <- '<<|=' skip
LARROWEQUAL <- '<=' skip
LBRACE <- '{' skip
LBRACKET <- '[' skip
LPAREN <- '(' skip
MINUS <- '-' ![%=>|] skip
MINUSEQUAL <- '-=' skip
MINUSPERCENT <- '-%' ![=] skip
MINUSPERCENTEQUAL <- '-%=' skip
MINUSPIPE <- '-|' ![=] skip
MINUSPIPEEQUAL <- '-|=' skip
MINUSRARROW <- '->' skip
PERCENT <- '%' ![=] skip
PERCENTEQUAL <- '%=' skip
PIPE <- '|' ![|=] skip
PIPE2 <- '||' skip
PIPEEQUAL <- '|=' skip
PLUS <- '+' ![%+=|] skip
PLUS2 <- '++' skip
PLUSEQUAL <- '+=' skip
PLUSPERCENT <- '+%' ![=] skip
PLUSPERCENTEQUAL <- '+%=' skip
PLUSPIPE <- '+|' ![=] skip
PLUSPIPEEQUAL <- '+|=' skip
LETTERC <- 'c' skip
QUESTIONMARK <- '?' skip
RARROW <- '>' ![>=] skip
RARROW2 <- '>>' ![=] skip
RARROW2EQUAL <- '>>=' skip
RARROWEQUAL <- '>=' skip
RBRACE <- '}' skip
RBRACKET <- ']' skip
RPAREN <- ')' skip
SEMICOLON <- ';' skip
SLASH <- '/' ![=] skip
SLASHEQUAL <- '/=' skip
TILDE <- '~' skip
end_of_word <- ![a-zA-Z0-9_] skip
KEYWORD_addrspace <- 'addrspace' end_of_word
KEYWORD_align <- 'align' end_of_word
KEYWORD_allowzero <- 'allowzero' end_of_word
KEYWORD_and <- 'and' end_of_word
KEYWORD_anyframe <- 'anyframe' end_of_word
KEYWORD_anytype <- 'anytype' end_of_word
KEYWORD_asm <- 'asm' end_of_word
KEYWORD_async <- 'async' end_of_word
KEYWORD_await <- 'await' end_of_word
KEYWORD_break <- 'break' end_of_word
KEYWORD_callconv <- 'callconv' end_of_word
KEYWORD_catch <- 'catch' end_of_word
KEYWORD_comptime <- 'comptime' end_of_word
KEYWORD_const <- 'const' end_of_word
KEYWORD_continue <- 'continue' end_of_word
KEYWORD_defer <- 'defer' end_of_word
KEYWORD_else <- 'else' end_of_word
KEYWORD_enum <- 'enum' end_of_word
KEYWORD_errdefer <- 'errdefer' end_of_word
KEYWORD_error <- 'error' end_of_word
KEYWORD_export <- 'export' end_of_word
KEYWORD_extern <- 'extern' end_of_word
KEYWORD_fn <- 'fn' end_of_word
KEYWORD_for <- 'for' end_of_word
KEYWORD_if <- 'if' end_of_word
KEYWORD_inline <- 'inline' end_of_word
KEYWORD_noalias <- 'noalias' end_of_word
KEYWORD_nosuspend <- 'nosuspend' end_of_word
KEYWORD_noinline <- 'noinline' end_of_word
KEYWORD_opaque <- 'opaque' end_of_word
KEYWORD_or <- 'or' end_of_word
KEYWORD_orelse <- 'orelse' end_of_word
KEYWORD_packed <- 'packed' end_of_word
KEYWORD_pub <- 'pub' end_of_word
KEYWORD_resume <- 'resume' end_of_word
KEYWORD_return <- 'return' end_of_word
KEYWORD_linksection <- 'linksection' end_of_word
KEYWORD_struct <- 'struct' end_of_word
KEYWORD_suspend <- 'suspend' end_of_word
KEYWORD_switch <- 'switch' end_of_word
KEYWORD_test <- 'test' end_of_word
KEYWORD_threadlocal <- 'threadlocal' end_of_word
KEYWORD_try <- 'try' end_of_word
KEYWORD_union <- 'union' end_of_word
KEYWORD_unreachable <- 'unreachable' end_of_word
KEYWORD_usingnamespace <- 'usingnamespace' end_of_word
KEYWORD_var <- 'var' end_of_word
KEYWORD_volatile <- 'volatile' end_of_word
KEYWORD_while <- 'while' end_of_word
keyword <- KEYWORD_addrspace / KEYWORD_align / KEYWORD_allowzero / KEYWORD_and
/ KEYWORD_anyframe / KEYWORD_anytype / KEYWORD_asm / KEYWORD_async
/ KEYWORD_await / KEYWORD_break / KEYWORD_callconv / KEYWORD_catch
/ KEYWORD_comptime / KEYWORD_const / KEYWORD_continue / KEYWORD_defer
/ KEYWORD_else / KEYWORD_enum / KEYWORD_errdefer / KEYWORD_error / KEYWORD_export
/ KEYWORD_extern / KEYWORD_fn / KEYWORD_for / KEYWORD_if
/ KEYWORD_inline / KEYWORD_noalias / KEYWORD_nosuspend / KEYWORD_noinline
/ KEYWORD_opaque / KEYWORD_or / KEYWORD_orelse / KEYWORD_packed
/ KEYWORD_pub / KEYWORD_resume / KEYWORD_return / KEYWORD_linksection
/ KEYWORD_struct / KEYWORD_suspend / KEYWORD_switch / KEYWORD_test
/ KEYWORD_threadlocal / KEYWORD_try / KEYWORD_union / KEYWORD_unreachable
/ KEYWORD_usingnamespace / KEYWORD_var / KEYWORD_volatile / KEYWORD_while
{#end_syntax_block#}
{#header_close#}
{#header_open|Zen#}
<ul>
<li>Communicate intent precisely.</li>
<li>Edge cases matter.</li>
<li>Favor reading code over writing code.</li>
<li>Only one obvious way to do things.</li>
<li>Runtime crashes are better than bugs.</li>
<li>Compile errors are better than runtime crashes.</li>
<li>Incremental improvements.</li>
<li>Avoid local maximums.</li>
<li>Reduce the amount one must remember.</li>
<li>Focus on code rather than style.</li>
<li>Resource allocation may fail; resource deallocation must succeed.</li>
<li>Memory is a resource.</li>
<li>Together we serve the users.</li>
</ul>
{#header_close#}
{#header_close#}
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