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 /* This file uses _Alignof. */
24 #if !_GL_CONFIG_H_INCLUDED
25 #error "Please include config.h first."
26 #endif
27
28 #include <stddef.h>
29
30 /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
31 On older platforms without _Alignof, use a pessimistic bound that is
32 safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
33 On newer platforms, use _Alignof to get a tighter bound. */
34
35 #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
36 # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
37 #else
38 # define FLEXALIGNOF(type) _Alignof (type)
39 #endif
40
41 /* Yield a properly aligned upper bound on the size of a struct of
42 type TYPE with a flexible array member named MEMBER that is
43 followed by N bytes of other data. The result is suitable as an
44 argument to malloc. For example:
45
46 struct s { int a; char d[FLEXIBLE_ARRAY_MEMBER]; };
47 struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
48
49 FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
50 since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms. Nor is
51 it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
52 that causes malloc to yield a pointer that is not properly aligned
53 for TYPE; for example, if sizeof (int) == alignof (int) == 4,
54 malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
55 to malloc (7) and might yield a pointer that is not a multiple of 4
56 (which means the pointer is not properly aligned for struct s),
57 whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
58 equivalent to malloc (8) and must yield a pointer that is a
59 multiple of 4.
60
61 Yield a value less than N if and only if arithmetic overflow occurs. */
62
63 #define FLEXSIZEOF(type, member, n) \
64 ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
65 & ~ (FLEXALIGNOF (type) - 1))
66
67 /* Yield a properly aligned upper bound on the size of a struct of
68 type TYPE with a flexible array member named MEMBER that has N
69 elements. The result is suitable as an argument to malloc.
70 For example:
71
72 struct s { int a; double d[FLEXIBLE_ARRAY_MEMBER]; };
73 struct s *p = malloc (FLEXNSIZEOF (struct s, d, n));
74 */
75 #define FLEXNSIZEOF(type, member, n) \
76 FLEXSIZEOF (type, member, (n) * sizeof (((type *) 0)->member[0]))