kc3-lang/gnulib/doc/posix-headers/stdalign.texi

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• Hash : 0804812c
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  Date : 2022-09-13T17:29:36
  

  stdalign: alignas and alignof are C23 keywords
  
  * m4/stdalign.m4 (gl_STDALIGN_H): Check also whether
  alignas and alignof are keywords and so don’t need stdalign.h,
  and define HAVE_C_ALIGNASOF if so.
  If not, arrange for config.h to #define alignas and alignof
  by including <stdalign.h>.
  * modules/stdalign: No need to include <stdalign.h>.
  * tests/test-stdalign.c: Do not include stdalign.h.
  

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• doc/posix-headers/stdalign.texi

• @node stdalign.h
  @section @file{stdalign.h}
  
  POSIX specification:@* Not in POSIX yet, but we expect it will be.
  ISO C11 (latest free draft
  @url{http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf})
  sections 6.5.3.4, 6.7.5, 7.15.
  C++11 (latest free draft
  @url{http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf})
  section 18.10.
  
  Gnulib module: stdalign
  
  Portability problems fixed by Gnulib:
  @itemize
  @item
  On older C platforms @code{<stdalign.h>} must be included before using
  @code{alignas} or @code{alignof}.  For example, GCC versions before 13 do not
  support these keywords, which were standardized by C23.
  @item
  This header file is missing on many platforms:
  FreeBSD 6.4, NetBSD 7.1, OpenBSD 6.7, Minix 3.3.0, AIX 7.1, HP-UX 11.31, IRIX 6.5, Solaris 11.3, mingw, MSVC 14, Android 9.0.
  @item
  Clang 3.0's @code{<stdalign.h>} does not define @code{alignof}.
  @item
  The @code{alignof} macro returns too large values for
  the types @code{double} and @code{long long} in GCC 4.7.0.
  @item
  Older C platforms might not support the obsolescent
  @code{_Alignas} and @code{_Alignof} keywords or macros.
  This portability problem should not matter with code using this module,
  as such code should use @code{alignas} and @code{alignof} instead.
  @end itemize
  
  Portability problems not fixed by Gnulib:
  @itemize
  @item
  In C11 and later, the operand of @code{alignof} must be a
  parenthesized type.  Recent versions of GCC support an extension in
  which the operand can also be a unary expression, as with
  @code{sizeof}.  The Gnulib substitute does not support this extension.
  @item
  On most pre-C11 platforms, the operand of
  @code{alignof} cannot be a structure type containing a
  flexible array member.
  @item
  The @code{alignas} keyword or macro is not always supported.
  Supported compilers include any compiler supporting C11 or later,
  which includes GCC, IBM C, Sun C 5.9 and later,
  and MSVC 7.0 and later.
  @item
  Some compilers do not support alignment via
  @code{alignas} of @code{auto} variables (i.e.,
  variables on the stack).  They diagnose and ignore the alignment: Sun
  C 5.11.
  @item
  Some linkers do not support operands of @code{alignas}
  that are greater than 8: mingw.
  @item
  Some compilers require the operand of @code{alignas}
  to be a single integer constant, not an expression: MSVC 7.0 through
  at least 10.0.
  @item
  The Sun C 5.13 (2014) compiler sometimes mishandles the alignment of multiple
  external variables that are declared close together with
  @code{alignas}.  The bug is fixed in Sun C 5.15, also known as Oracle
  Developer Studio 12.6 (2017).
  @item
  @code{<stdalign.h>} must be #included before @code{_Alignas} and
  @code{_Alignof} can be used.
  @item
  In C11 and C17, @code{<stdalign.h>} defines the macros
  @code{__alignas_is_defined} and
  @code{__alignof_is_defined} to 1.
  In C23, these macros are not defined.
  @item
  You cannot assume that @code{alignas} and @code{alignof} are reserved words;
  they might be macros.
  @end itemize
  

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