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Uninitialised Index

The Code

Here is the code for this example (source):

// Created for COMP2521 sanitiser guide

#include <stdio.h>
#include <stdlib.h>

#define N 10

int main(void) {
    int array[N] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    int *printOrder = malloc(N * sizeof(int));
    for (int i = 0; i < N; i++) {
        int printIdx = printOrder[i];
        printf("%d\n", array[printIdx]);
    }
    free(printOrder);
}

This code creates array containing 0..9, then mallocs an array printOrder of indexes that stores the order to print out array.

The Error

Here is the error message:

• The error occurs on line 13 - printf("%d\n", array[printIdx]);
• The uninitialised value was stored into a variable on line 12 - int printIdx = printOrder[i];
• The uninitialised value was originally created on line 10 - int *printOrder = malloc(N * sizeof(int));

The Problem

When we allocated memory for the printOrder array we didn’t initialise it, so each element of this array contains an uninitialised value.

We stored one of these values into printIdx in line 12 which we then use in line 13 when we try to print, and so get an error.

The Fix

Make sure that the printOrder array is initialised. If we don’t know what to put in there yet, we can initialise it to a default like 0 or invalid value like -1.