sum.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void sumBaseB(char *a,  
      char *b, int base) 
{ 
    int len_a, len_b; 
  
    len_a = strlen(a);
    len_b = strlen(b);
  
    char sum[1000], sa[1000]={0}, sb[1000]={0};

    int diff; 
    diff = abs(len_a - len_b); 
    int index = 0;
    for (int i = 1; i <= diff; i++) 
      if(len_a < len_b)
        sa[index++] = '0';
      else
        sb[index++] = '0';

    strcat(sa, a);
    strcat(sb, b);
    int curr, carry = 0; 
      
#define max(a, b) (((a) > (b)) ? (a) : (b))
    int mm = 0;
    for (int i = max(len_a, len_b) - 1;  
                           i > -1; i--) { 
          
        // Current Place value for  
        // the resultant sum 
        curr = carry + (sa[i] - '0') +  
                       (sb[i] - '0'); 
  
        // Update carry 
        carry = curr / base; 
  
        // Find current digit 
        curr = curr % base; 
  
        // Update sum result 
        sum[mm++] = (char)(curr + '0'); 
    } 
    if (carry > 0) 
        sum[mm++] = (char)(carry + '0'); 
    for(int i = mm - 1; i >= 0; i--) {
      printf("%c", sum[i]);
    }
    printf("\n");
} 

int main() {
  char s1[1000];
  char s2[1000];
  int n;
  scanf("%s %s %d", s1, s2, &n);
  sumBaseB(s1, s2, n);
  return 0;
}