Trying to access heap memory after it's been free'd

heap.c

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

// void * return type means that it returns a naked pointer, which is just an address.
void *foo(void) {
  long *bar_ptr = malloc(sizeof(long));
  *bar_ptr = 5;
  printf("The data is held at address: %p, holds %ld bytes\n", bar_ptr, sizeof(long));
  free(bar_ptr);
  return (void *) bar_ptr;
}

int main(void) {
  void *ptr = foo();
  printf("about to print what's at %p\n", ptr);
  long *l_ptr = (long *) ptr;
  printf("%ld\n", *l_ptr);
}