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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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6
git-compat-util.h
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@ -31,6 +31,9 @@
#define maximum_signed_value_of_type(a) \
(INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a)))
#define maximum_unsigned_value_of_type(a) \
(UINTMAX_MAX >> (bitsizeof(uintmax_t) - bitsizeof(a)))
/*
* Signed integer overflow is undefined in C, so here's a helper macro
* to detect if the sum of two integers will overflow.
@ -40,6 +43,9 @@
#define signed_add_overflows(a, b) \
((b) > maximum_signed_value_of_type(a) - (a))
#define unsigned_add_overflows(a, b) \
((b) > maximum_unsigned_value_of_type(a) - (a))
#ifdef __GNUC__
#define TYPEOF(x) (__typeof__(x))
#else