Error in C

Salut,

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

int main()
{
  int *p, *q;

  p = (int *)malloc(sizeof(int));
  q = p;
  *p = 10;
  printf("%d\n", *q);
  *q = 20;
  printf("%d\n", *q);
  return 0;
}

Bonjour tous le monde .. j voulais savoir Pourkoi , mon code affiche l'erreur ?

or que j m'exercerce sur le forum qui donne le meme code ?

#include "stack.h"
#include <stdio.h>

/* Stack Library - This library offers the
minimal stack operations for a stack of integers */

struct stack_rec
{
stack_data data;
struct stack_rec *next;
};

struct stack_rec *top=NULL;

void stack_init()
/* Initializes this library.
Call before calling anything else. */
{
top=NULL;
}

void stack_clear()
/* Clears the stack of all entries. */
{
stack_data x;

while (!stack_empty())
x=stack_pop();
}

int stack_empty()
/* Returns 1 if the stack is empty, 0 otherwise. */
{
if (top==NULL)
return(1);
else
return(0);
}

void stack_push(stack_data d)
/* Pushes the value d onto the stack. */
{
struct stack_rec *temp;
temp=
(struct stack_rec *)malloc(sizeof(struct stack_rec));
temp->data=d;
temp->next=top;
top=temp;
}

stack_data stack_pop()
/* Returns the top element of the stack,
and removes that element.
Returns garbage if the stack is empty. */
{
struct stack_rec *temp;
stack_data d=0;
if (top!=NULL)
{
d=top->data;
temp=top;
top=top->next;
free(temp);
}
return(d);
}

aide moi S.V.P