/*
  CSE 374 Winter 2014
  Lecture 10, Slide 7 Example
*/

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

int main() {

  /* malloc returns NULL if the request cannot be satisfied */

  /* free on a NULL pointer does nothing */

  /* only use free on a pointer that points to memory 
     allocated by malloc or calloc */

  int *p = (int *) malloc(sizeof(int));
  p = NULL;
  /* LEAK - forgot to free the memory pointed at by p first */
  
  int *q = (int *) malloc(sizeof(int));
  free(q);
  /*  free(q); */
  /* this memory has already been freed, may cause a crash */
  /* comment out the second "free(q)" if this program crashes */

  int *r = (int *) malloc(sizeof(int));
  free(r);
  /* allocated and then freed memory */
  
  /* allocate some memory, have s point to it, and store the value 19 */
  int *s = (int *) malloc(sizeof(int));
  *s = 19;
  printf("*s is %d\n", *s);

  /* it is an error to use a pointer after it has been freed */
  /* (we already freed r and attempt to use it again below) */

  /* the memory pointed to by s was allocated immediately after 
     the memory pointed to by r was freed.  It could be possible 
     that malloc allocates the same block of memory again. */

  /* could crash, but what if s == r ? */
  *r = 17;
  printf("*s is %d\n", *s);

  /* experiment with this in gdb */
  /* compile with gcc -g -Wall options */
  /* set a breakpoint at main */
  /* display p, q, r, and s (separate display commands) to see the addresses assigned */
  /* step through the program a line at a time */

  return 0;
}