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     Written by Paul Eggert.  */
      22  
      23  #ifndef GAWK
      24  /* This file uses _Alignof.  */
      25  #if !_GL_CONFIG_H_INCLUDED
      26   #error "Please include config.h first."
      27  #endif
      28  #endif /* GAWK */
      29  
      30  #include <stddef.h>
      31  
      32  /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
      33     On older platforms without _Alignof, use a pessimistic bound that is
      34     safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
      35     On newer platforms, use _Alignof to get a tighter bound.  */
      36  
      37  #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
      38  # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
      39  #else
      40  # define FLEXALIGNOF(type) _Alignof (type)
      41  #endif
      42  
      43  /* Yield a properly aligned upper bound on the size of a struct of
      44     type TYPE with a flexible array member named MEMBER that is
      45     followed by N bytes of other data.  The result is suitable as an
      46     argument to malloc.  For example:
      47  
      48       struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; };
      49       struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
      50  
      51     FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
      52     since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms.  Nor is
      53     it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
      54     that causes malloc to yield a pointer that is not properly aligned
      55     for TYPE; for example, if sizeof (int) == alignof (int) == 4,
      56     malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
      57     to malloc (7) and might yield a pointer that is not a multiple of 4
      58     (which means the pointer is not properly aligned for struct s),
      59     whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
      60     equivalent to malloc (8) and must yield a pointer that is a
      61     multiple of 4.
      62  
      63     Yield a value less than N if and only if arithmetic overflow occurs.  */
      64  
      65  #define FLEXSIZEOF(type, member, n) \
      66     ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
      67      & ~ (FLEXALIGNOF (type) - 1))