1 /* Sizes of structs with flexible array members.
2
3 Copyright 2016-2023 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 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))