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61 lines
2.6 KiB
C
61 lines
2.6 KiB
C
/* Sizes of structs with flexible array members.
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Copyright 2016-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>.
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Written by Paul Eggert. */
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#include <stddef.h>
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/* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
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On older platforms without _Alignof, use a pessimistic bound that is
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safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
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On newer platforms, use _Alignof to get a tighter bound. */
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#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
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# define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
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#else
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# define FLEXALIGNOF(type) _Alignof (type)
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#endif
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/* Yield a properly aligned upper bound on the size of a struct of
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type TYPE with a flexible array member named MEMBER that is
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followed by N bytes of other data. The result is suitable as an
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argument to malloc. For example:
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struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; };
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struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
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FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
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since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms. Nor is
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it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
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that causes malloc to yield a pointer that is not properly aligned
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for TYPE; for example, if sizeof (int) == alignof (int) == 4,
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malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
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to malloc (7) and might yield a pointer that is not a multiple of 4
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(which means the pointer is not properly aligned for struct s),
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whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
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equivalent to malloc (8) and must yield a pointer that is a
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multiple of 4.
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Yield a value less than N if and only if arithmetic overflow occurs. */
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#define FLEXSIZEOF(type, member, n) \
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((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
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& ~ (FLEXALIGNOF (type) - 1))
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