use_after_free.c

/* This software was developed at the National Institute of Standards and
 * Technology by employees of the Federal Government in the course of their
 * official duties. Pursuant to title 17 Section 105 of the United States
 * Code this software is not subject to copyright protection and is in the
 * public domain. NIST assumes no responsibility whatsoever for its use by
 * other parties, and makes no guarantees, expressed or implied, about its
 * quality, reliability, or any other characteristic.

 * We would appreciate acknowledgement if the software is used.
 * The SAMATE project website is: http://samate.nist.gov
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct str_t str;
struct str_t {
  union {
    int a;
    char *b;
  } foo;
};
int main(int argc, char *argv[]) {
  str container;

  container.foo.a = 0;
  if ((container.foo.b = (char *)malloc(256 * sizeof(char))) != NULL) {
    strcpy(container.foo.b, "Falut!");
    container.foo.b[0] = 'S';
    printf("%s\n", container.foo.b);
    free(container.foo.b);
    if (container.foo.b)
      container.foo.b[0] = 'S';
  }
  return 0;
}