posix/flexmember.h

   1 /* Sizes of structs with flexible array members.
   2
   3    Copyright 2016-2023 Free Software Foundation, Inc.
   4
   5    This file is part of the GNU C Library.
   6
   7    The GNU C Library is free software; you can redistribute it and/or
   8    modify it under the terms of the GNU Lesser General Public
   9    License as published by the Free Software Foundation; either
  10    version 2.1 of the License, or (at your option) any later version.
  11
  12    The GNU C Library is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15    Lesser General Public License for more details.
  16
  17    You should have received a copy of the GNU Lesser General Public
  18    License along with the GNU C Library; if not, see
  19    <https://www.gnu.org/licenses/>.  */
  20
  21
  22 #include <stddef.h>
  23
  24 /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
  25    On older platforms without _Alignof, use a pessimistic bound that is
  26    safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
  27    On newer platforms, use _Alignof to get a tighter bound.  */
  28
  29 #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
  30 # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
  31 #else
  32 # define FLEXALIGNOF(type) _Alignof (type)
  33 #endif
  34
  35 /* Yield a properly aligned upper bound on the size of a struct of
  36    type TYPE with a flexible array member named MEMBER that is
  37    followed by N bytes of other data.  The result is suitable as an
  38    argument to malloc.  For example:
  39
  40      struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; };
  41      struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
  42
  43    FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
  44    since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms.  Nor is
  45    it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
  46    that causes malloc to yield a pointer that is not properly aligned
  47    for TYPE; for example, if sizeof (int) == alignof (int) == 4,
  48    malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
  49    to malloc (7) and might yield a pointer that is not a multiple of 4
  50    (which means the pointer is not properly aligned for struct s),
  51    whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
  52    equivalent to malloc (8) and must yield a pointer that is a
  53    multiple of 4.
  54
  55    Yield a value less than N if and only if arithmetic overflow occurs.  */
  56
  57 #define FLEXSIZEOF(type, member, n) \
  58    ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
  59     & ~ (FLEXALIGNOF (type) - 1))