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      1 /* Sizes of structs with flexible array members.
      2 
      3    Copyright 2016-2022 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    Written by Paul Eggert.  */
     22 
     23 #include <stddef.h>
     24 
     25 /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
     26    On older platforms without _Alignof, use a pessimistic bound that is
     27    safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
     28    On newer platforms, use _Alignof to get a tighter bound.  */
     29 
     30 #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
     31 # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
     32 #else
     33 # define FLEXALIGNOF(type) _Alignof (type)
     34 #endif
     35 
     36 /* Yield a properly aligned upper bound on the size of a struct of
     37    type TYPE with a flexible array member named MEMBER that is
     38    followed by N bytes of other data.  The result is suitable as an
     39    argument to malloc.  For example:
     40 
     41      struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; };
     42      struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
     43 
     44    FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
     45    since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms.  Nor is
     46    it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
     47    that causes malloc to yield a pointer that is not properly aligned
     48    for TYPE; for example, if sizeof (int) == alignof (int) == 4,
     49    malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
     50    to malloc (7) and might yield a pointer that is not a multiple of 4
     51    (which means the pointer is not properly aligned for struct s),
     52    whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
     53    equivalent to malloc (8) and must yield a pointer that is a
     54    multiple of 4.
     55 
     56    Yield a value less than N if and only if arithmetic overflow occurs.  */
     57 
     58 #define FLEXSIZEOF(type, member, n) \
     59    ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
     60     & ~ (FLEXALIGNOF (type) - 1))
     61