C语言 乘法 大数相乘

#include<stdio.h>
#include<string.h>
#define N 100

void fun(char *res)         //逆序字符串
{
int len;
len = strlen(res) - 1;

for(int i = 0; i <= len/2; i++)
{
char temp;

temp = res[i];
res[i] = res[len-i];
res[len-i] = temp;
}
}

void mul(char *num1, char *num2, char *res)
{
for(int len1 = strlen(num1)-1; len1 >= 0; len1 --)
{
for(int len2 = strlen(num2)-1; len2 >= 0; len2 --)
{
int i = strlen(num1)-1 - len1 + strlen(num2)-1 - len2;
// i 代表此次相乘结果存放的起始位置
int temp;

if(res[i] == 0)
res[i] = '0';

temp = res[i] -'0' + (num1[len1]-'0') * (num2[len2]-'0');

res[i] = temp % 10 + '0';

temp = temp / 10;

while(temp)     //处理进位
{
if(res[++i] == 0)
res[i] = '0';
temp += res[i] -'0';

res[i] = temp % 10 + '0';

temp = temp / 10;
}
}
}

fun(res);
}

main()
{
char num1
= {0};
char num2
= {0};
char res[2*N] = {0};

puts("please enter the first number:\n");
gets(num1);

puts("\nplease enter the second number:\n");
gets(num2);

mul(num1, num2, res);

puts("\nAfter multiply result is :");
puts(res);
getchar();
}