关于长整数的加 乘，有点问题请教！！！

# include <stdio.h>
# include <string.h>
# include <iostream>
# include <fstream>
using namespace std;
 #define LIMIT 80
 void jia(char *ch1,char *ch2,char *ch3)
 {
     int len1=strlen(ch1),len2=strlen(ch2),len3,i,tmp,num1[LIMIT]={0},num2[LIMIT]={0},num3[LIMIT]={0};
     for(i=len1-1;i>=0;i--)
         num1[len1-1-i]=ch1[i]-48;
     for(i=len2-1;i>=0;i--)
         num2[len2-1-i]=ch2[i]-48;
     i=0,tmp=0,len3=len1>len2?len1:len2;
     while(i<len3)
     {
     num3[i]=num1[i]+num2[i]+tmp;
     tmp=0;
     if(num3[i]>=10){tmp=num3[i]/10;num3[i]%=10;}
     i++;
     }
     if(tmp!=0){num3[i]=tmp;i++;}
     len3=i;
     for(i=len3-1;i>=0;i--) ch3[len3-1-i]=num3[i]+48;
     ch3[len3]='\0';
 
}
     void cheng(char *ch1,char *ch2,char *ch3)
     {
         int i,j,len1=strlen(ch1),len2=strlen(ch2),len3,
                 temp,num1[LIMIT]={0},num2[LIMIT]={0},num3[LIMIT]={0};
         for(i=len1-1;i>=0;i--)
             num1[len1-1-i]=ch1[i]-48;
         for(i=len2-1;i>=0;i--)
             num2[len2-1-i]=ch2[i]-48;
         for(i=0;i<len1;i++)
         {
             temp=0;
         for(j=0;j<len2;j++)
             {
                         num3[i+j]+=num1[i]*num2[j]+temp;
               temp=0;
               if(num3[i+j]>=10)
               {
                   temp=num3[i+j]/10;
                   num3[i+j]%=10;
               }
             }
         while(temp!=0)
             {
                 num3[i+j]+=temp;temp=num3[i+j]/10;j++;
             }
         
         }
         len3=len1+len2;
         while(len3>1&&num3[len3-1]==0)
             len3--;
         for(i=0;i<len3;i++)
             ch3[len3-1-i]=num3[i]+48;
         ch3[len3]='\0';
 
    }
     
     void main()
      {
         cout<<"                        *********主菜单*********\n";
         cout<<"                        *       1 两数相加     *\n";
         cout<<"                        *       2 两数相乘     *\n";
         cout<<"                        *       3 从文件操作   *\n";
         cout<<"                        *       4 退出         *\n";
         cout<<"                        ************************\n";
          char ch1[LIMIT],ch2[LIMIT],ch3[LIMIT];
          void jia(char *ch1,char *ch2,char *ch3);
          void cheng(char *ch1,char *ch2,char *ch3);
          int a=0;
          cin>>a;
          switch(a)
          {
          case 1:
              {
                  printf("请输入第一个数\n");
                  scanf("%s",ch1);
                   printf("请输入第二个数\n");
                   scanf("%s",ch2);
                    printf("\n");
                    jia(ch1,ch2,ch3);
                   printf("两数之和：%s\n",ch3);
                   break;
              }
          case 2:
              {
                   printf("请输入第一个数\n");
                   scanf("%s",ch1);
                   printf("请输入第二个数\n");
                   scanf("%s",ch2);
                   printf("\n");
                   cheng(ch1,ch2,ch3);
                   printf("两数之积为：%s\n",ch3);
                   break;
              }
          case 3:
              {
                  cout<<"从文家中读取数字操作";
                  break;
              }
          case 4:
              {
                  cout<<"退出";
                  break;
              }
          default:
              {
                  cout<<"输入错误";
                  break;
              }
          }
   
 }
由于文件的操作具体在么实现你没有说的太清楚所以就没有写出来 如果需要添加可以再 cout<<"从文家中读取数字操作"; 这里天剑你的代码