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

#define char_to_int(a) (int) (a - '0')
#define int_to_char(a) (char)(a + '0')


int main()
{
    char a[1000]={"1844674407"};
    char b[1000]={"3709551615"};
    unsigned long long c[1000]={0};
    int i = 0, j = 0, k = 0;

    memset(c,0,sizeof(c));

    /*完成图片中的1，2步，结果从数组开头开始存放*/
    for (i = 0; i<strlen(a); i++)
    {
        k = i;
        for (j = 0; j<strlen(b); j++){
            c[k] += char_to_int(a[i]) * char_to_int(b[j]);
            k++;
        }
    }
    /*完成图片中的3步,从后面往前面*/
    for (i = k-1; i>0; i--)
    {
        if (c[i] >= 10) {
            c[i-1] = c[i]/10 + c[i-1];
            c[i] = c[i]%10;
        }
    }

    /*输出*/
    for(i=0;i<k;i++)
        printf("%llu",c[i]);putchar('\n');


    return 0;
}

$gcc -o main *.c
$main
6842914925636017305