How to calculate modulus of the form (a*b)%c?

Question

How to calculate modulus of the form (a*b)%c?

i want to calculate modulus of multiplication of two int numbers where they are almost in the stage of overflow...

here c is also int

Answer 1

(a * b) % c == ((a % c) * (b % c)) % c

Answer 2

What about ((a % c) * (b % c)) % c? Depending on your architecture this could be faster or slower than casting to a bigger type.

Answer 3

You may cast a and c to long long, so the multiplication won't overflow.

((long long)a * (long long)b) % c