long-numbers-02.c

#include <stdio.h>
#include <error.h>

#define N 4

void add_numbers (char *a, char *b, char *s)
{
  int carry = 0;
  for (int i = N - 1; i >= 0; i--)
    {
      int A = a[i] - '0';
      int B = b[i] - '0';
      int S = A + B + carry;
      s[i] = S + '0';
    }
  if (carry)
    error (1, 1, "overflow");
}

int main (void)
{
  char *a = "1234";
  char *b = "3456";
  char s[N] = "0000";
  add_numbers (a, b, s); 
  return 0;
}