Unexpected behavior from unsigned_int64;

Question

unsigned__int64 difference;
difference=(64*33554432);
printf ("size %I64u \n", difference);
difference=(63*33554432);
printf ("size %I64u \n", difference);

the first # is ridiculously large. The second number is the correct answer. How does changing it from 62 to 63 cause such a change?

First value is 18446744071562067968 Second value is 2113929216

Sorry the values were 64 and 63, not 63 and 62.

Answer 1

Unless qualified otherwise, integer literals are of type int. I would assume that on the platform you're on, an int is 32-bit. So the calculation (64*33554432) overflows and becomes negative. You then cast this to a unsigned __int64, so this now gets flipped back to a very very large positive integer.

Voila:

int main()
{
    int a1 = (64*33554432);
    int a2 = (63*33554432);

    printf("%08x\n", a1);    // 80000000  (negative)
    printf("%08x\n", a2);    // 7e000000  (positive)

    unsigned __int64 b1 = a1;
    unsigned __int64 b2 = a2;

    printf("%016llx\n", b1); // ffffffff80000000
    printf("%016llx\n", b2); // 000000007e000000
}