Program does "big multiplication" - Should be able to handle numbers of _ANY_ length

bugmulti.c

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

/* Program does "big multiplication"
   Should be able to handle numbers of _ANY_ length
   
   Input via stdin:- 
   N
   X1 Y1
   X2 Y2
   
   Where N == number of lines to follow
   Xn Yn == Pair of number to multiply up to 10,000 decimal digits per number
   
   Example input:
   1
   101 6667
   
   Example output:
   673367
*/

typedef struct {
  int size;
  short *digits;
} IntArray;

void atoIntArray (char *input, IntArray *rtv) {
  int c;
  rtv->size = strlen(input);
  rtv->digits = (short *) malloc(rtv->size * sizeof(short));
  for(c = 0; c < rtv->size; c++) {
    rtv->digits[c] = input[c] - 48;
  }
}

void print_IntArray (IntArray *rtv) {
  int c;
  int print = 0;
  for(c = 0; c < rtv->size; c++) {
    if (print > 0 || rtv->digits[c] > 0) {
      print++;
      printf("%d", rtv->digits[c]);
    }
  }
  if (print == 0) {
    printf("0");
  }
  printf("\n");
}

void do_add(IntArray *rtv, short amount, int pos) {
  int tmpres = rtv->digits[pos] + amount;
  if (tmpres > 9) {
    do_add(rtv, floor(tmpres / 10), (pos - 1));
    rtv->digits[pos] = tmpres % 10;
  } else {
    rtv->digits[pos] = tmpres;
  }
}

void do_multi(char *inputline) {
  char *pch;
  IntArray left, right, output;
  int r, l;
  pch = strtok (inputline, " ");
  atoIntArray(pch, &left);
  pch = strtok (NULL, "\n"); 
  atoIntArray(pch, &right);
  output.size = right.size + left.size;
  output.digits = (short *) malloc((output.size) * sizeof(short));
  for(r = 0; r < output.size; r++) {
    output.digits[r] = 0;
  }
  for(r = right.size - 1; r >= 0; r--) {
    for(l = left.size - 1; l >= 0; l--) {
      do_add(&output, (right.digits[r] * left.digits[l]), (l + r + 1));
    }
  }
  free(right.digits);
  free(left.digits);
  print_IntArray(&output);
  free(output.digits);
}
 
int main() {
  int bytes_read;
  int max_line = 102400;
  int num_lines;
  int curr_line = 0;
  char *stringline;
  stringline = (char *) malloc(max_line + 1);
  bytes_read = getline (&stringline, &max_line, stdin);
  if (bytes_read == -1) {
      puts ("ERROR!");
  } else {
    num_lines = atoi(stringline);
    while (curr_line < num_lines) {
      bytes_read = getline (&stringline, &max_line, stdin);
      do_multi(stringline);
      curr_line++;
    }
  }
  return 0;
}