Calculate sum of very large numbers

add_large_num.c

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include "stack_list.h"

/* Check if stacks holding numbers are all empty */
int AllEmpty(stack_t **stack, int n)
{
  int i;
  for(i = 0; i < n; i++) {
    if(!Empty(stack[i])) {
      return 0; // at least 1 not empty
    }
  }

  return 1; // all empty
}

stack_t *add_large_num(stack_t **stack, int n)
{
  stack_t *result = Initialize();
  int save, sum, remainder = 0;
  int i;

  while(!(AllEmpty(stack, n) && remainder == 0)) {
    /* Initialize sum of each individual line of num */
    sum = 0;

    /* Pop each stacks and add to sum */
    for(i = 0; i < n; i++) {
      if(!Empty(stack[i])) {
	Pop(&stack[i], &save);
	sum += save;
      }
    }

    /* Add sum with previous remainder */
    sum += remainder;

    /* decide remainder + Fix sum to num */
    remainder = sum / 10;
    sum = sum - remainder * 10;

    /* Push sum to result stack */
    Push(&result, sum);
  }
  
  return result;
}

/* Display to screen also empty the list */
void Print(stack_t **top)
{
  int save;
  
  while(!Empty(*top)) {
    Pop(top, &save);
    printf("%d", save);
  }

  printf("\n");
}

int main(void)
{
  int n, i;
  char ch;
  int num;

  printf("* How many numbers to add: ");
  scanf("%d", &n); getchar();

  stack_t *stack[n];

  /* input numbers */
  for(i = 0; i < n; i++) {
    printf("> Enter number %d: ", i + 1);

    // initialize each stack
    stack[i] = Initialize();

    // then add the number in
    while((ch = getchar()) != '\n') {
      num = ch - '0'; // turn char to digit
      Push(&stack[i], num);
    }
  }

  /* add numbers and print result */
  printf("\n* Sum = ");
  stack_t *sum = add_large_num(stack, n);
  Print(&sum);
  
  return 0;
}

stack_list.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include "stack_list.h"

stack_t *Initialize()
{
  stack_t *top = NULL;
  return top;
}

int Empty(stack_t *top)
{
  return top == NULL;
}

void Push(stack_t **top, data_t data)
{
  // create new stack's node
  stack_t *NewNode = (stack_t*)malloc(sizeof(stack_t));
  assert(NewNode != NULL);
  NewNode->data = data;

  // push into stack and make new top
  NewNode->next = *top;
  *top = NewNode;
}

stack_t *Pop(stack_t **top, data_t *save)
{
  if(Empty(*top)) {
    printf("ERROR: Stack is empty. Can't pop.\n");
    return NULL;
  }

  // save the data
  *save = (**top).data;
  
  stack_t *temp = *top;
  *top = (**top).next;
  free(temp);
  
  return *top;
}

stack_list.h

#ifndef STACK_LIST_H
#define STACK_LIST_H

typedef int data_t;

typedef struct StackNode {
  data_t data;
  struct StackNode *next;
} stack_t;

stack_t *Initialize();
int Empty(stack_t *top);
void Push(stack_t **top, data_t data);
stack_t *Pop(stack_t **top, data_t *save);

#endif