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1 | /* Sizes of structs with flexible array members. |
2 | ||
5df4cba6 | 3 | Copyright 2016-2020 Free Software Foundation, Inc. |
49e4877c | 4 | |
c0c3707f | 5 | This file is part of the GNU C Library. |
49e4877c | 6 | |
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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, | |
49e4877c | 13 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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14 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
15 | General Public License for more details. | |
49e4877c | 16 | |
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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/>. | |
49e4877c PA |
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 | ||
c0c3707f CB |
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 | ||
49e4877c PA |
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)) |