Adding Big Numbers

add-big-numbers.c

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

char *add(const char *a, const char *b) {
  size_t pos_1, pos_2, pos_3, len_3;
  unsigned int carry;
    
  pos_1 = strlen(a);
  pos_2 = strlen(b);
  
  len_3 = pos_3 = (pos_1 > pos_2 ? pos_1 : pos_2) + 1;
  
  char *result = malloc(pos_1 + 1);
  strcpy(result, a);
  result = realloc(result, len_3 + 1);
  
  result[pos_3] = '\0';
  
  for(carry = 0; pos_3 > 0; carry /= 10) {
    if (pos_1 > 0) carry += result[--pos_1] - '0';
    if (pos_2 > 0) carry += b[--pos_2] - '0';
    result[--pos_3] = '0' + carry % 10;
  }
  
  while(result[0] == '0' && len_3 > 1) {
    memmove(result, result + 1, len_3--);
  }
  
  return result;
}