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compat: helper for detecting unsigned overflow

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The idiom (a + b < a) works fine for detecting that an unsigned
integer has overflowed, but a more explicit

	unsigned_add_overflows(a, b)

might be easier to read.

Define such a macro, expanding roughly to ((a) < UINT_MAX - (b)).
Because the expansion uses each argument only once outside of sizeof()
expressions, it is safe to use with arguments that have side effects.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jonathan Nieder
2010-10-10 21:59:26 -05:00
committed by Junio C Hamano
parent a8e4a5943a
commit 1368f65002
4 changed files with 11 additions and 4 deletions
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5
strbuf.c
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@ -63,7 +63,8 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
void strbuf_grow(struct strbuf *sb, size_t extra)
{
if (sb->len + extra + 1 <= sb->len)
if (unsigned_add_overflows(extra, 1) ||
unsigned_add_overflows(sb->len, extra + 1))
die("you want to use way too much memory");
if (!sb->alloc)
sb->buf = NULL;
@ -152,7 +153,7 @@ int strbuf_cmp(const struct strbuf *a, const struct strbuf *b)
void strbuf_splice(struct strbuf *sb, size_t pos, size_t len,
const void *data, size_t dlen)
{
if (pos + len < pos)
if (unsigned_add_overflows(pos, len))
die("you want to use way too much memory");
if (pos > sb->len)
die("`pos' is too far after the end of the buffer");