detecting multiplication of uint64_t integers overflow with C

Question

Is there any efficient and portable way to check when multiplication operations with int64_t or uint64_t operands overflow in C?

For instance, for addition of uint64_t I

Answer 1

It might not detect exact overflows, but in general you can test the result of your multiplication on a logarithmic scale:

if (log(UINT64_MAX-1) - log(a) - log(b) < 0) overflow_detected(); // subtracting 1 to allow some tolerance when the numbers are converted to double
else prod = a * b;

It depends if you really need to do multiplication up to exact UINT64_MAX, otherwise this a very portable and convenient way to check multiplications of large numbers.