Stack / Linked List implementation.

stack.c

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

/**
   Very simple stack / linked list data structure for
   holding integers.
 **/


typedef struct Cell {
  struct Cell* next;
  int value;
} llCell;

typedef struct {
  llCell* head;
} Stack;

void push(Stack* stack, int item)
{
  llCell* newCell;
  newCell = malloc (sizeof (llCell));
  newCell->value = item;
  newCell->next = stack->head;

  // make the new cell the head of the stack
  stack->head = newCell;
}

int pop(Stack* stack)
{
  if(stack->head == NULL) {
    // stack is empty
    return -1;
  } else {
    llCell* oldHead = stack->head;
    stack->head = oldHead->next;
    int output = oldHead->value;
    free(oldHead);
    return output;
  }
}

Stack* allocate_stack()
{
  Stack* stack;
  stack = (Stack*) malloc (sizeof (Stack));
  stack->head = NULL;
  return stack;
}

int main(void)
{
  Stack* stack = allocate_stack();

  push(stack, 1);
  push(stack, 2);
  push(stack, 3);
  push(stack, 4);

  printf("%d\n", pop(stack)); // 4
  printf("%d\n", pop(stack)); // 3
  printf("%d\n", pop(stack)); // 2
  printf("%d\n", pop(stack)); // 1
  printf("%d\n", pop(stack)); // -1
  
  return 0;
}