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))