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-rw-r--r--lib/flexmember.h27
1 files changed, 21 insertions, 6 deletions
diff --git a/lib/flexmember.h b/lib/flexmember.h
index fde5bca..1545639 100644
--- a/lib/flexmember.h
+++ b/lib/flexmember.h
@@ -1,6 +1,6 @@
/* Sizes of structs with flexible array members.
- Copyright 2016-2018 Free Software Foundation, Inc.
+ Copyright 2016-2022 Free Software Foundation, Inc.
This file is part of the GNU C Library.
@@ -33,11 +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) \