sanitisers-guide

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Uninitialised Pointer

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 = malloc(sizeof(*n));
    n->value = 0;
    n->next->value = 1;
}

This code tries to create a linked list containing the values 0 and 1.

The Error

Here is the error message:

• The error occurs on line 13 - n->next->value = 1;
• The uninitialised value was originally created on line 11 - struct node *n = malloc(sizeof(*n));

The Problem

On line 13, we try to set the value field of n->next. However, when we allocated memory for n we never initialised n->next, so trying to write to this pointer causes an error.

The Fix

We need to initialise n->next by allocating memory to it (i.e. n->next = malloc(...)).