sanitisers-guide

Back to attempted double-free

Simple double free

The Code

Here is the code for this example (source):

// Created for COMP2521 UNSW sanitisers guide
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    int *zeroes = calloc(10, sizeof(int));
    free(zeroes);
    free(zeroes);
}

This code allocates an array of zeroes, then frees this array, then attempts to free it again.

The Error

Here is the error message:

• The first stack trace tells us:
  • The error occurs on line 8 inside main()
  • This line is the second free(zeroes);
• The second stack trace tells us:
  • The memory was first freed in line 7 inside main()
  • This line is the first free(zeroes);
• The final stack trace tells us:
  • The freed memory was first allocated on line 6 inside main()
  • This is where we called calloc() to allocate the array

The Problem

From the error message, we can see that the error comes from trying to free the zeroes() array twice - once on line 7 and then again on line 8.

The Fix

Remove one of the free()s.