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diff --git a/lib/flexmember.h b/lib/flexmember.h
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+/* Sizes of structs with flexible array members.
+
+   Copyright 2016 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or
+   modify it under the terms of either:
+
+     * the GNU Lesser General Public License as published by the Free
+       Software Foundation; either version 3 of the License, or (at your
+       option) any later version.
+
+   or
+
+     * the GNU General Public License as published by the Free
+       Software Foundation; either version 2 of the License, or (at your
+       option) any later version.
+
+   or both in parallel, as here.
+   This program 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 this program.  If not, see <http://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
+
+/* 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.
+   This is not simply sizeof (TYPE) + N, since it may require
+   alignment on unusually picky C11 platforms, and
+   FLEXIBLE_ARRAY_MEMBER may be 1 on pre-C11 platforms.
+   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))