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+/* Sizes of structs with flexible array members.
+
+ Copyright 2016-2020 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 General Public
+ License as published by the Free Software Foundation; either
+ version 3 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
+ General Public License for more details.
+
+ You should have received a copy of the GNU General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>.
+
+ Written by Paul Eggert. */
+
+#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))