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/* Sizes of structs with flexible array members.
Copyright 2016-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <stddef.h>
/* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
On older platforms without _Alignof, use a pessimistic bound that is
safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
On newer platforms, use _Alignof to get a tighter bound. */
#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
# define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
#else
# define FLEXALIGNOF(type) _Alignof (type)
#endif
/* Yield a properly aligned upper bound on the size of a struct of
type TYPE with a flexible array member named MEMBER that is
followed by N bytes of other data. The result is suitable as an
argument to malloc. For example:
struct s { int n; char d[FLEXIBLE_ARRAY_MEMBER]; };
struct s *p = malloc (FLEXSIZEOF (struct s, d, n * sizeof (char)));
FLEXSIZEOF (TYPE, MEMBER, N) is not simply (sizeof (TYPE) + N),
since FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms. Nor is
it simply (offsetof (TYPE, MEMBER) + N), as that might yield a size
that causes malloc to yield a pointer that is not properly aligned
for TYPE; for example, if sizeof (int) == alignof (int) == 4,
malloc (offsetof (struct s, d) + 3 * sizeof (char)) is equivalent
to malloc (7) and might yield a pointer that is not a multiple of 4
(which means the pointer is not properly aligned for struct s),
whereas malloc (FLEXSIZEOF (struct s, d, 3 * sizeof (char))) is
equivalent to malloc (8) and must yield a pointer that is a
multiple of 4.
Yield a value less than N if and only if arithmetic overflow occurs. */
#define FLEXSIZEOF(type, member, n) \
((offsetof (type, member) + FLEXALIGNOF (type) - 1 + (n)) \
& ~ (FLEXALIGNOF (type) - 1))
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