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 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)) 60