lib/stdalign.in.h

   1 /* A substitute for ISO C11 <stdalign.h>.
   2
   3    Copyright 2011-2012 Free Software Foundation, Inc.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU Lesser General Public License as published by
   7    the Free Software Foundation; either version 2, or (at your option)
   8    any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public License
  16    along with this program; if not, write to the Free Software Foundation,
  17    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  18
  19 /* Written by Paul Eggert and Bruno Haible.  */
  20
  21 #ifndef _GL_STDALIGN_H
  22 #define _GL_STDALIGN_H
  23
  24 /* ISO C11 <stdalign.h> for platforms that lack it.
  25
  26    References:
  27    ISO C11 (latest free draft
  28    <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf>)
  29    sections 6.5.3.4, 6.7.5, 7.15.
  30    C++11 (latest free draft
  31    <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf>)
  32    section 18.10. */
  33
  34 /* alignof (TYPE), also known as _Alignof (TYPE), yields the alignment
  35    requirement of a structure member (i.e., slot or field) that is of
  36    type TYPE, as an integer constant expression.
  37
  38    This differs from GCC's __alignof__ operator, which can yield a
  39    better-performing alignment for an object of that type.  For
  40    example, on x86 with GCC, __alignof__ (double) and __alignof__
  41    (long long) are 8, whereas alignof (double) and alignof (long long)
  42    are 4 unless the option '-malign-double' is used.
  43
  44    The result cannot be used as a value for an 'enum' constant, if you
  45    want to be portable to HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
  46 #include <stddef.h>
  47 #if defined __cplusplus
  48    template <class __t> struct __alignof_helper { char __a; __t __b; };
  49 # define _Alignof(type) offsetof (__alignof_helper<type>, __b)
  50 #else
  51 # define _Alignof(type) offsetof (struct { char __a; type __b; }, __b)
  52 #endif
  53 #define alignof _Alignof
  54 #define __alignof_is_defined 1
  55
  56 /* alignas (A), also known as _Alignas (A), aligns a variable or type
  57    to the alignment A, where A is an integer constant expression.  For
  58    example:
  59
  60       int alignas (8) foo;
  61       struct s { int a; int alignas (8) bar; };
  62
  63    aligns the address of FOO and the offset of BAR to be multiples of 8.
  64
  65    A should be a power of two that is at least the type's alignment
  66    and at most the implementation's alignment limit.  This limit is
  67    2**28 on typical GNUish hosts, and 2**13 on MSVC.  To be portable
  68    to MSVC through at least version 10.0, A should be an integer
  69    constant, as MSVC does not support expressions such as 1 << 3.
  70    To be portable to Sun C 5.11, do not align auto variables to
  71    anything stricter than their default alignment.
  72
  73    The following C11 requirements are not supported here:
  74
  75      - If A is zero, alignas has no effect.
  76      - alignas can be used multiple times; the strictest one wins.
  77      - alignas (TYPE) is equivalent to alignas (alignof (TYPE)).
  78
  79    */
  80
  81 #if __GNUC__ || __IBMC__ || __IBMCPP__ || 0x5110 <= __SUNPRO_C
  82 # define _Alignas(a) __attribute__ ((__aligned__ (a)))
  83 #elif 1300 <= _MSC_VER
  84 # define _Alignas(a) __declspec (align (a))
  85 #endif
  86 #ifdef _Alignas
  87 # define alignas _Alignas
  88 # define __alignas_is_defined 1
  89 #endif
  90
  91 #endif /* _GL_STDALIGN_H */