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In C, signed integer overflow leads to undefined behavior. However,
many programs and Autoconf tests assume that signed integer overflow after
addition, subtraction, or multiplication silently
wraps around modulo a power of two, using two's complement arithmetic,
so long as you cast the resulting value
to an integer type or store it into an integer variable. Such programs
are portable to the vast majority of modern platforms. However, signed
integer division is not always harmless: for example, on CPUs of the
i386 family, dividing
-1 yields a SIGFPE signal
which by default terminates the program. Worse, taking the remainder
of these two values typically yields the same signal on these CPUs,
even though the C standard requires
INT_MIN % -1 to yield zero
because the expression does not overflow.
GCC users might consider using the -ftrapv option if they are worried about porting their code to the rare platforms where signed integer overflow does not wrap around after addition, subtraction, or multiplication.
Unsigned integer overflow reliably wraps around modulo the word size. This is guaranteed by the C standard and is portable in practice.