1 /* Sizes of structs with flexible array members. 2 3 Copyright 2016-2020 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 General Public 9 License as published by the Free Software Foundation; either 10 version 3 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 General Public License for more details. 16 17 You should have received a copy of the GNU 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