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Documentation/process: Add fallthrough pseudo-keyword

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Describe the fallthrough pseudo-keyword.

Convert the coding-style.rst example to the keyword style.
Add description and links to deprecated.rst.

Miguel Ojeda comments on the eventual [[fallthrough]] syntax:
 "Note that C17/C18 does not have [[fallthrough]].

  C++17 introduced it, as it is mentioned above. I would keep the
  __attribute__((fallthrough)) -> [[fallthrough]] change you did,
  though, since that is indeed the standard syntax (given the paragraph
  references C++17).

  I was told by Aaron Ballman (who is proposing them for C) that it is
  more or less likely that it becomes standardized in C2x. However, it
  is still not added to the draft (other attributes are already,
  though). See N2268 and N2269:

     http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2268.pdf (fallthrough)
     http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2269.pdf (attributes in general)"

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Joe Perches 2019-10-05 09:46:43 -07:00 committed by Linus Torvalds
parent 294f69e662
commit b9918bdcac
2 changed files with 24 additions and 11 deletions
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2
Documentation/process/coding-style.rst
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@ -56,7 +56,7 @@ instead of ``double-indenting`` the ``case`` labels. E.g.:
case 'K':
case 'k':
mem <<= 10;
/* fall through */
fallthrough;
default:
break;
}

33
Documentation/process/deprecated.rst
View File

@ -122,14 +122,27 @@ memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
Implicit switch case fall-through
---------------------------------
The C language allows switch cases to "fall through" when
a "break" statement is missing at the end of a case. This,
however, introduces ambiguity in the code, as it's not always
clear if the missing break is intentional or a bug. As there
have been a long list of flaws `due to missing "break" statements
The C language allows switch cases to "fall-through" when a "break" statement
is missing at the end of a case. This, however, introduces ambiguity in the
code, as it's not always clear if the missing break is intentional or a bug.
As there have been a long list of flaws `due to missing "break" statements
<https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
"implicit fall-through". In order to identify an intentional fall-through
case, we have adopted the marking used by static analyzers: a comment
saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
is more widely handled by C compilers, static analyzers, and IDEs, we can
switch to using that instead.
"implicit fall-through".
In order to identify intentional fall-through cases, we have adopted a
pseudo-keyword macro 'fallthrough' which expands to gcc's extension
__attribute__((__fallthrough__)). `Statement Attributes
<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_
When the C17/C18 [[fallthrough]] syntax is more commonly supported by
C compilers, static analyzers, and IDEs, we can switch to using that syntax
for the macro pseudo-keyword.
All switch/case blocks must end in one of:
break;
fallthrough;
continue;
goto <label>;
return [expression];