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-rw-r--r--lib/flexmember.h48
1 files changed, 33 insertions, 15 deletions
diff --git a/lib/flexmember.h b/lib/flexmember.h
index 3ef4f9802..1b19a2bfd 100644
--- a/lib/flexmember.h
+++ b/lib/flexmember.h
@@ -1,19 +1,22 @@
/* Sizes of structs with flexible array members.
- Copyright 2016-2017 Free Software Foundation, Inc.
+ Copyright 2016-2021 Free Software Foundation, Inc.
- This program 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 3 of the License, or
- (at your option) any later version.
+ This file is part of the GNU C Library.
- This program is distributed in the hope that it will be useful,
+ 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.
+ 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 this program. If not, see <http://www.gnu.org/licenses/>.
+ 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/>.
Written by Paul Eggert. */
@@ -30,11 +33,26 @@
# 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 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) \