All of the Stack parameters for the functions in lab4c.c are pointers to the Stack structure.

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

/* typical C boolean set-up */
#define TRUE 1
#define FALSE 0

typedef struct StackStruct
{
 int*    darr;  /* pointer to dynamic array  */
 int     size;  /* amount of space allocated */
 int     inUse; /* top of stack indicator 
                   - counts how many values are on the stack   */
} Stack;

void init (Stack* s)
{
 s->size = 2;
 s->darr = (int*) malloc ( sizeof (int) * s->size );
 s->inUse  = 0;
}

void push (Stack* s, int val)
{
  /* QUESTION 7 */
  /* check if enough space currently on stack and grow if needed */
  if(s->inUse+1 >= s->size){ //If whole array is inn use i.e inUse +1 = size
      s->size=s->size*2; //Increase the size to 2 times the current
       //Using realloc to resize the size of array, it automatically retains previous elemnts
      s->darr = (int *)realloc(s->darr,(s->size)*sizeof(int));
  }
  
  /* add val onto stack */
  s->darr[s->inUse] = val;
  s->inUse = s->inUse + 1;
}

int isEmpty (Stack* s)
{
  if ( s->inUse == 0)
    return TRUE;
  else
    return FALSE;
}


int top (Stack* s)
{
  return (  s->darr[s->inUse-1] );
}

/* QUESTION 9 */
void pop (Stack* s)
{
  s->inUse = s->inUse - 1;
}

void reset (Stack* s)
{
    s->size=2;
    free(s->darr);
    s->darr = (int*) malloc ( sizeof (int) * s->size );
    s->inUse  = 0;

}

int main (int argc, char** argv)
{
 Stack st1;

 init (&st1);

 push (&st1, 20);
 push (&st1, 30);
 push (&st1, 40);
 push (&st1, 50);
  
 while ( isEmpty(&st1) == FALSE)
   {
    int value = top (&st1);
    printf ("The top value on the stack is %d\n", value);
    
    /* QUESTION 10 */
    pop (&st1);
   }

 printf ("The stack is now empty\n");
}