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csignedness

When does the signedness of an integer really matter?

Due to the way conversions and operations are defined in C, it seems to rarely matter whether you use a signed or an unsigned variable:

uint8_t u; int8_t i;

u = -3;    i = -3;
u *= 2;    i *= 2;
u += 15;   i += 15;
u >>= 2;   i >>= 2;

printf("%u",u); // -> 2
printf("%u",i); // -> 2

So, is there a set of rules to tell under which conditions the signedness of a variable really makes a difference?

Solution

  • It matters in these contexts:

    • division and modulo: -2/2 = 1, -2u/2 = UINT_MAX/2-1, -3%4 = -3, -3u%4 = 1
    • shifts. For negative signed values, the result of >> and << are implementation defined or undefined, resp. For unsigned values, they are always defined.
    • relationals -2 < 0, -2u > 0
    • overflows. x+1 > x may be assumed by the compiler to be always true iff x has signed type.