| 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)) |