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1 | /* Sizes of structs with flexible array members. |
2 | ||
dff8da6b | 3 | Copyright 2016-2024 Free Software Foundation, Inc. |
c66c9082 AZ |
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 | |
30891f35 | 19 | <https://www.gnu.org/licenses/>. */ |
c66c9082 | 20 | |
c66c9082 AZ |
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 | ||
9c774ad7 AZ |
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 | ||
c66c9082 AZ |
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)) |