Use after free error?

Question

int main(void) {
  int* p = (int*) malloc(sizeof(int));
  int* q = (int*) malloc(sizeof(int));
  *p = 10;
  *q = 20;
  p = q;
  printf(“%d %d”, *p, *q);
  free(p); 
  free(q);
}

Why does the above code contain use-after-free error? There's no more expression after free(p) and free(q). Obviously we are not using them anymore!

Answer 1

You have two problems here.

First, you are deleting the same heap variable twice:

  free(p); 
  free(q);

Second, you have a memory-leak, because the variable created by p is no longer accessible.

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Notice that onebyone's comment is really important. If you change the line:

p = q;

into:

*p = *q;

There would be no problems at all in your code :) Hello Pointers!

Answer 2

You set p to q, so you are free()ing it twice.