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 General Public License as published by
   7    the Free Software Foundation; either version 3, 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 General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program; if not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert and Bruno Haible.  */
  19
  20 #ifndef _GL_STDALIGN_H
  21 #define _GL_STDALIGN_H
  22
  23 /* ISO C11 <stdalign.h> for platforms that lack it.
  24
  25    References:
  26    ISO C11 (latest free draft
  27    <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf>)
  28    sections 6.5.3.4, 6.7.5, 7.15.
  29    C++11 (latest free draft
  30    <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf>)
  31    section 18.10. */
  32
  33 /* alignof (TYPE), also known as _Alignof (TYPE), yields the alignment
  34    requirement of a structure member (i.e., slot or field) that is of
  35    type TYPE, as an integer constant expression.
  36
  37    This differs from GCC's __alignof__ operator, which can yield a
  38    better-performing alignment for an object of that type.  For
  39    example, on x86 with GCC, __alignof__ (double) and __alignof__
  40    (long long) are 8, whereas alignof (double) and alignof (long long)
  41    are 4 unless the option '-malign-double' is used.
  42
  43    The result cannot be used as a value for an 'enum' constant, if you
  44    want to be portable to HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
  45 #include <stddef.h>
  46 #if defined __cplusplus
  47    template <class __t> struct __alignof_helper { char __a; __t __b; };
  48 # define _Alignof(type) offsetof (__alignof_helper<type>, __b)
  49 #else
  50 # define _Alignof(type) offsetof (struct { char __a; type __b; }, __b)
  51 #endif
  52 #define alignof _Alignof
  53 #define __alignof_is_defined 1
  54
  55 /* alignas (A), also known as _Alignas (A), aligns a variable or type
  56    to the alignment A, where A is an integer constant expression.  For
  57    example:
  58
  59       int alignas (8) foo;
  60       struct s { int a; int alignas (8) bar; };
  61
  62    aligns the address of FOO and the offset of BAR to be multiples of 8.
  63
  64    A should be a power of two that is at least the type's alignment
  65    and at most the implementation's alignment limit.  This limit is
  66    2**28 on typical GNUish hosts, and 2**13 on MSVC.  To be portable
  67    to MSVC through at least version 10.0, A should be an integer
  68    constant, as MSVC does not support expressions such as 1 << 3.
  69    To be portable to Sun C 5.11, do not align auto variables to
  70    anything stricter than their default alignment.
  71
  72    The following C11 requirements are not supported here:
  73
  74      - If A is zero, alignas has no effect.
  75      - alignas can be used multiple times; the strictest one wins.
  76      - alignas (TYPE) is equivalent to alignas (alignof (TYPE)).
  77
  78    */
  79
  80 #if __GNUC__ || __IBMC__ || __IBMCPP__ || 0x5110 <= __SUNPRO_C
  81 # define _Alignas(a) __attribute__ ((__aligned__ (a)))
  82 #elif 1300 <= _MSC_VER
  83 # define _Alignas(a) __declspec (align (a))
  84 #endif
  85 #ifdef _Alignas
  86 # define alignas _Alignas
  87 # define __alignas_is_defined 1
  88 #endif
  89
  90 #endif /* _GL_STDALIGN_H */