/* Sizes of structs with flexible array members. Copyright 2016-2020 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with the GNU C Library; if not, see <https://www.gnu.org/licenses/>. Written by Paul Eggert. */ #include <stddef.h> /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below. On older platforms without _Alignof, use a pessimistic bound that is safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1. On newer platforms, use _Alignof to get a tighter bound. */ #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112 # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1)) #else # define FLEXALIGNOF(type) _Alignof (type) #endif /* Yield a properly aligned upper bound on the size of a struct of type TYPE with a flexible array member named MEMBER that is followed by N bytes of other data. The result is suitable as an argument to malloc. For example: struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; }; struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char))); FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N), since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms. Nor is it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size that causes malloc to yield a pointer that is not properly aligned for TYPE; for example, if sizeof (int) == alignof (int) == 4, malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent to malloc (7) and might yield a pointer that is not a multiple of 4 (which means the pointer is not properly aligned for struct s), whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is equivalent to malloc (8) and must yield a pointer that is a multiple of 4. Yield a value less than N if and only if arithmetic overflow occurs. */ #define FLEXSIZEOF(type, member, n) \ ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \ & ~ (FLEXALIGNOF (type) - 1))