[c-prog] about an adder

[Haohao]

hello!
I'm new to C programming.And I've got to make an adder with numbers beyond the 
integer storage limits.My program can excute successfully on VC.But it gets 
wrong on cygnus or cygwin.I don't kown why.Could you please give me some 
help?Thanks very much.
 
 
PS: #include<stdio.h>
#include<stdlib.h>
void sum(int a[30],int b[30]);
int i = 0,j = 0,k2;
int s[30]={0};
int main()
{
char c1,c2;
int x;
int num1[30],num2[30];

printf("enter the first number:");
 while( ( c1 = getchar() ) != '\n'){
 
      i++;
      num1[i] = atoi(&c1);
  
 }

printf("enter the second number:");
 while( ( c2 = getchar() ) != '\n'){
      j++;
      num2[j] = atoi(&c2);
 }
 
 if(i >= j)
      k2 = i + 1;
 else
      k2 = j + 1;
 
 sum(num1,num2);
 
 for(x = 1;x <= k2;x++){
    printf("%d",s[x]);
 }
return 0;
}

void sum(int a[30],int b[30])
{
     int y,k; 
 
     if(i >= j){
          k = i + 1;
   
          while(i != 0 && j != 0){
           s[k--] = a[i--] + b[j--];
          }
  
          while(k != 0 && i != 0){
           s[k--] = a[i--];
         }
     }/*end if*/
 
    else{
          k = j + 1;
  
          while(i != 0 && j != 0){
               s[k--] = a[i--] + b[j--];
             }
          while(k != 0 && j != 0){
               s[k--] = b[j--];
          }
  
        }/*end else*/
 
     for(y = k2;y >= 1;y-- ){
          if(s[y] > 10){
               s[y] = s[y] - 10;
               s[y - 1] = s[y - 1] + 1;
             }/*end if*/
     }/*end for*/
}/*end sum*/


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