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[PATCH] CodingStyle: add typedefs chapter
Add a chapter on typedefs, copied from an email from Linus to lkml on Feb. 3, 2006. (Subject: Re: [RFC][PATCH 1/5] Virtualization/containers: startup) Signed-off-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Randy Dunlap
Linus Torvalds
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3439dd86e3
commit
226a6b84aa
@ -155,7 +155,83 @@ problem, which is called the function-growth-hormone-imbalance syndrome.
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See next chapter.
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Chapter 5: Functions
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Chapter 5: Typedefs
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Please don't use things like "vps_t".
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It's a _mistake_ to use typedef for structures and pointers. When you see a
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vps_t a;
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in the source, what does it mean?
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In contrast, if it says
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struct virtual_container *a;
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you can actually tell what "a" is.
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Lots of people think that typedefs "help readability". Not so. They are
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useful only for:
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(a) totally opaque objects (where the typedef is actively used to _hide_
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what the object is).
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Example: "pte_t" etc. opaque objects that you can only access using
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the proper accessor functions.
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NOTE! Opaqueness and "accessor functions" are not good in themselves.
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The reason we have them for things like pte_t etc. is that there
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really is absolutely _zero_ portably accessible information there.
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(b) Clear integer types, where the abstraction _helps_ avoid confusion
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whether it is "int" or "long".
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u8/u16/u32 are perfectly fine typedefs, although they fit into
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category (d) better than here.
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NOTE! Again - there needs to be a _reason_ for this. If something is
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"unsigned long", then there's no reason to do
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typedef unsigned long myflags_t;
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but if there is a clear reason for why it under certain circumstances
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might be an "unsigned int" and under other configurations might be
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"unsigned long", then by all means go ahead and use a typedef.
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(c) when you use sparse to literally create a _new_ type for
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type-checking.
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(d) New types which are identical to standard C99 types, in certain
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exceptional circumstances.
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Although it would only take a short amount of time for the eyes and
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brain to become accustomed to the standard types like 'uint32_t',
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some people object to their use anyway.
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Therefore, the Linux-specific 'u8/u16/u32/u64' types and their
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signed equivalents which are identical to standard types are
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permitted -- although they are not mandatory in new code of your
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own.
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When editing existing code which already uses one or the other set
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of types, you should conform to the existing choices in that code.
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(e) Types safe for use in userspace.
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In certain structures which are visible to userspace, we cannot
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require C99 types and cannot use the 'u32' form above. Thus, we
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use __u32 and similar types in all structures which are shared
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with userspace.
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Maybe there are other cases too, but the rule should basically be to NEVER
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EVER use a typedef unless you can clearly match one of those rules.
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In general, a pointer, or a struct that has elements that can reasonably
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be directly accessed should _never_ be a typedef.
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Chapter 6: Functions
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Functions should be short and sweet, and do just one thing. They should
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fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
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@ -183,7 +259,7 @@ and it gets confused. You know you're brilliant, but maybe you'd like
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to understand what you did 2 weeks from now.
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Chapter 6: Centralized exiting of functions
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Chapter 7: Centralized exiting of functions
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Albeit deprecated by some people, the equivalent of the goto statement is
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used frequently by compilers in form of the unconditional jump instruction.
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@ -220,7 +296,7 @@ out:
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return result;
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}
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Chapter 7: Commenting
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Chapter 8: Commenting
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Comments are good, but there is also a danger of over-commenting. NEVER
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try to explain HOW your code works in a comment: it's much better to
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@ -240,7 +316,7 @@ When commenting the kernel API functions, please use the kerneldoc format.
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See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
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for details.
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Chapter 8: You've made a mess of it
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Chapter 9: You've made a mess of it
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That's OK, we all do. You've probably been told by your long-time Unix
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user helper that "GNU emacs" automatically formats the C sources for
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@ -288,7 +364,7 @@ re-formatting you may want to take a look at the man page. But
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remember: "indent" is not a fix for bad programming.
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Chapter 9: Configuration-files
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Chapter 10: Configuration-files
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For configuration options (arch/xxx/Kconfig, and all the Kconfig files),
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somewhat different indentation is used.
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@ -313,7 +389,7 @@ support for file-systems, for instance) should be denoted (DANGEROUS), other
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experimental options should be denoted (EXPERIMENTAL).
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Chapter 10: Data structures
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Chapter 11: Data structures
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Data structures that have visibility outside the single-threaded
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environment they are created and destroyed in should always have
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@ -344,7 +420,7 @@ Remember: if another thread can find your data structure, and you don't
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have a reference count on it, you almost certainly have a bug.
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Chapter 11: Macros, Enums and RTL
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Chapter 12: Macros, Enums and RTL
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Names of macros defining constants and labels in enums are capitalized.
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@ -399,7 +475,7 @@ The cpp manual deals with macros exhaustively. The gcc internals manual also
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covers RTL which is used frequently with assembly language in the kernel.
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Chapter 12: Printing kernel messages
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Chapter 13: Printing kernel messages
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Kernel developers like to be seen as literate. Do mind the spelling
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of kernel messages to make a good impression. Do not use crippled
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@ -410,7 +486,7 @@ Kernel messages do not have to be terminated with a period.
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Printing numbers in parentheses (%d) adds no value and should be avoided.
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Chapter 13: Allocating memory
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Chapter 14: Allocating memory
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The kernel provides the following general purpose memory allocators:
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kmalloc(), kzalloc(), kcalloc(), and vmalloc(). Please refer to the API
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@ -429,7 +505,7 @@ from void pointer to any other pointer type is guaranteed by the C programming
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language.
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Chapter 14: The inline disease
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Chapter 15: The inline disease
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There appears to be a common misperception that gcc has a magic "make me
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faster" speedup option called "inline". While the use of inlines can be
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@ -457,7 +533,7 @@ something it would have done anyway.
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Chapter 15: References
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Appendix I: References
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The C Programming Language, Second Edition
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by Brian W. Kernighan and Dennis M. Ritchie.
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@ -481,4 +557,4 @@ Kernel CodingStyle, by greg@kroah.com at OLS 2002:
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http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/
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--
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Last updated on 30 December 2005 by a community effort on LKML.
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Last updated on 30 April 2006.
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