#include "stdio.h"
#include "stdlib.h"
#include "string.h"

char *BigNumMultiply(const char *n1, const char *n2)
{
    // quit if n1 or n2 is invalid
    if (!n1 || !n2) {
        return NULL;
    }

    // get length
    int Len1 = strlen(n1);
    int Len2 = strlen(n2);
    int Len = Len1 + Len2;

    // allocate result buffer
    char *ret = (char *)malloc(Len + 1);
    if (!ret) {
        return NULL;
    }
    memset(ret, 0, Len + 1);

    // multiply
    for (int i = Len1 - 1; i >= 0; --i) {
        for (int j = Len2 - 1; j >= 0; --j) {
            int k = i + j + 1;
            // multiply digit by digit
            ret[k] += (n1[i] - ‘0‘) * (n2[j] - ‘0‘);
            
            // add to upper position
            if (ret[k] >= 10){
                ret[k - 1] += ret[k] / 10;
                ret[k] = ret[k] % 10;
            }
        }
    }

    // handle first 0
    int d = ret[0] == 0 ? 1 : 0;
    for (int i = 0; i < Len - d; ++i) {
        ret[i] = ret[i + d] + ‘0‘;
    }
    ret[Len - d] = ‘\0‘;

    return ret;
}

int main(int argc, char* argv[])
{
    char n1[] = "1234567890";
    char n2[] = "987654321";
    char *ret = BigNumMultiply(n1, n2);
    printf("%s * %s = %s\n", n1, n2, ret);
    free(ret);
    ret = NULL;

    getchar();
    return 0;
}