lib/alignof.h

   1 /* Determine alignment of types.
   2    Copyright (C) 2003-2004, 2006, 2009-2020 Free Software Foundation, Inc.
   3
   4    This program is free software; you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 2, or (at your option)
   7    any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program; if not, see <https://www.gnu.org/licenses/>.  */
  16
  17 #ifndef _ALIGNOF_H
  18 #define _ALIGNOF_H
  19
  20 #include <stddef.h>
  21
  22 /* alignof_slot (TYPE)
  23    Determine the alignment of a structure slot (field) of a given type,
  24    at compile time.  Note that the result depends on the ABI.
  25    This is the same as alignof (TYPE) and _Alignof (TYPE), defined in
  26    <stdalign.h> if __alignof_is_defined is 1.
  27    Note: The result cannot be used as a value for an 'enum' constant,
  28    due to bugs in HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
  29 #if defined __cplusplus
  30   template <class type> struct alignof_helper { char __slot1; type __slot2; };
  31 # define alignof_slot(type) offsetof (alignof_helper<type>, __slot2)
  32 #else
  33 # define alignof_slot(type) offsetof (struct { char __slot1; type __slot2; }, __slot2)
  34 #endif
  35
  36 /* alignof_type (TYPE)
  37    Determine the good alignment of an object of the given type at compile time.
  38    Note that this is not necessarily the same as alignof_slot(type).
  39    For example, with GNU C on x86 platforms and with clang on Linux/x86:
  40    alignof_type(long long) = 8, but alignof_slot(long long) = 4.
  41    And alignof_type(double) = 8, but
  42    - when -malign-double is not specified:  alignof_slot(double) = 4,
  43    - when -malign-double is specified:      alignof_slot(double) = 8.
  44    Note: The result cannot be used as a value for an 'enum' constant,
  45    due to bugs in HP-UX 10.20 cc and AIX 3.2.5 xlc.  */
  46 #if defined __GNUC__ || defined __clang__ || defined __IBM__ALIGNOF__
  47 # define alignof_type __alignof__
  48 #else
  49 # define alignof_type alignof_slot
  50 #endif
  51
  52 #endif /* _ALIGNOF_H */