“Floating point exception” in code that contains no floats [duplicate]

Answer 1

On the first iteration of the loop, i is 0, so bignum % i is a division by 0, which is what causes the floating point exception. The same happens with j in your inner loop.

Since you're testing for primality, dividing by 0 or by 1 doesn't make sense, so start with 2. Also, you should stop at sqrt(bignum).

uint64_t limit = sqrt(bignum);
for (i = 2; i < limit; i++) {
    ...
    for (j = 2; j < i; j++) {

Answer 2

Divide by 0 is reported as a "floating point exception". Don't ask me why.

You're dividing by 0 in at least two places. Note that the modulo operator % counts as division.