sanitisers-guide

Back to SEGV on unknown address

NULL Pointer Dereference

The Code

Here is the code for this example (source):

// Created for COMP2521 sanitiser guide

#include <stdlib.h>

struct node {
    int value;
    struct node *next;
};

int main(void) {
    struct node *n = NULL;
    n->value = 69;
    n->next = NULL;
}

The Error

Here is the error message:

• We get runtime errors telling us that we tried to access a struct node NULL pointer, and that we tried to write (store) an int
• The hint tell us this error comes from accessing a NULL pointer
• The error occurs on line 7 - n->value = 69;

The Problem

The pointer accessed on line 7 is n, since we try to write to one of its fields using ->. Since the issue is accessing a NULL pointer, this tells us that n is NULL (which we knew already).

The Fix

Avoid this error by adding an if statement checking whether n is NULL before trying to access it, or try figure out why n is NULL and fix that.