| [3611] | 1 | /* Sizes of structs with flexible array members.
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| 2 |
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| 3 | Copyright 2016-2022 Free Software Foundation, Inc.
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| 4 |
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| 5 | This file is part of the GNU C Library.
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| 6 |
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| 7 | The GNU C Library is free software; you can redistribute it and/or
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| 8 | modify it under the terms of the GNU Lesser General Public
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| 9 | License as published by the Free Software Foundation; either
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| 10 | version 2.1 of the License, or (at your option) any later version.
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| 11 |
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| 12 | The GNU C Library is distributed in the hope that it will be useful,
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| 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
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| 15 | Lesser General Public License for more details.
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| 16 |
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| 17 | You should have received a copy of the GNU Lesser General Public
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| 18 | License along with the GNU C Library; if not, see
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| 19 | <https://www.gnu.org/licenses/>.
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| 20 |
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| 21 | Written by Paul Eggert. */
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| 22 |
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| 23 | #include <stddef.h>
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| 24 |
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| 25 | /* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
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| 26 | On older platforms without _Alignof, use a pessimistic bound that is
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| 27 | safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
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| 28 | On newer platforms, use _Alignof to get a tighter bound. */
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| 29 |
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| 30 | #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
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| 31 | # define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
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| 32 | #else
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| 33 | # define FLEXALIGNOF(type) _Alignof (type)
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| 34 | #endif
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| 35 |
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| 36 | /* Yield a properly aligned upper bound on the size of a struct of
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| 37 | type TYPE with a flexible array member named MEMBER that is
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| 38 | followed by N bytes of other data. The result is suitable as an
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| 39 | argument to malloc. For example:
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| 40 |
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| 41 | struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; };
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| 42 | struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
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| 43 |
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| 44 | FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
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| 45 | since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms. Nor is
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| 46 | it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
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| 47 | that causes malloc to yield a pointer that is not properly aligned
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| 48 | for TYPE; for example, if sizeof (int) == alignof (int) == 4,
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| 49 | malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
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| 50 | to malloc (7) and might yield a pointer that is not a multiple of 4
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| 51 | (which means the pointer is not properly aligned for struct s),
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| 52 | whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
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| 53 | equivalent to malloc (8) and must yield a pointer that is a
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| 54 | multiple of 4.
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| 55 |
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| 56 | Yield a value less than N if and only if arithmetic overflow occurs. */
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| 57 |
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| 58 | #define FLEXSIZEOF(type, member, n) \
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| 59 | ((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
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| 60 | & ~ (FLEXALIGNOF (type) - 1))
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