Multiplying without the use of the * operator.

mul.c

#include <stdio.h>

int mul(int a, int b)
{
    if (b == 1)
        return a;
    else if (b == 0)
        return 0;
    return (a << 1) + mul(a, b - 2);
}

int main()
{
    int a, b;
    scanf("%d%d", &a, &b);
    printf("%i\n", mul(a, b));
    return 0;
}