Stack Implementation In C

My code, compiles fine in GCC.. your advices are very welcomed since i'm a beginner

/*
 *	  Stack.c
 *
 *	  Program to demonstrate the stack concept
 *
 *	  by Ahmed M. Sha'alan, disturbed@live.com, www.thedb.com/m
 */

#include <stdio.h>
#include <stdlib.h>										//for the system() and exit() functions

#define STACKSIZE 100									//the stack size

struct STACK {											//stack structure
	short int top;
	int items[STACKSIZE];
};

typedef struct STACK stk;								//a short-hand for the stack (stk badal stack y3ny :D)

int empty(stk *ptr){									//checks if the stack is empty
	return(ptr->top<0?1:0);								//shorter and makes more sense than the IF statment
}

int full(stk *ptr) {									//checks if the stack is full
	return(ptr->top>=STACKSIZE?1:0);					//^^ foo2 :D
}

void push(stk *ptr, int *x) {							//pushes a value to the top of the stack
		ptr->items[++ptr->top]=*x;
}

int pop(stk *ptr) {										//displays the top value of the stack
		return (ptr->items[ptr->top--]);
}

void print_pop(stk *ptr){								//the pop() function but with empty() check and messages
	if(empty(ptr))	
		puts(">>The stack is empty!");		
	else
		printf(">>The last value in the stack is: %d.\n", pop(ptr));
}

void print_push(stk *ptr){								//the push() function with the checking and messages
	int x;
	if(full(ptr))
		puts(">>The stack is full!");
	else{
		printf(">>Enter the value you want to push: ");
		fflush(stdin);									//a clever way to clear the standard input (keyboard input) to prevent weird behavoir, try to remove it and see what happens
		scanf("%d", &x);								
		push(ptr, &x);
		puts(">>done!");
	}
}

void print_all(stk *ptr) {								//prints all the elements in the stack
	unsigned short int i=0;
	system("cls");
	puts("********************");
	if (empty(ptr))
		puts(">>The stack is empty!");
	else{
		for (;i<=ptr->top;i++)
		printf("|%d- %d\n",i+1, ptr->items);
	}
	puts("********************");
}


void interface(stk *ptr) {								//the "interface" lol, i KNOW it's a console aplication :D
	unsigned short int choice;							//we only need numbers from 1 to 4
	system("cls");
	puts("\n=====================================");
	puts("Stack Implementation in C...");
	puts("=====================================");
BEGINING:
	puts("\n=======================================================");
	puts("1- Press 1 for putting a value in the stack.");
	puts("2- Press 2 for displaying the last value in the stack.");
	puts("3- Press 3 for displaying ALL the values in the stack.");
	puts("4- Press 4 for exiting the program.");
	puts("=======================================================");
	printf("What do you want to do? ");
	fflush(stdin);										//read foo2 ^ :D
	scanf("%d", &choice);
	puts("");											//new line
	switch (choice) {
		case 1:
			print_push(ptr);
			break;
		case 2:
			print_pop(ptr);	
			break;
		case 3:
			print_all(ptr);
			break;
		case 4:
			exit(1); 
		default:
			puts(">>Please make sure you've entered a valid option..");
			break;		
	}
	goto BEGINING;										//repeat again and again til the user exits
}

int main() {											//finaly the main :D
	stk stack;
	stack.top=-1;										//sets the top to -1 for the stack to act right
	interface(&stack);
	return 0;
}