Memory error in program that fills array with random numbers

Question

I have a program with creates an array and fills its with random numbers. It then checks for any duplicates and prints out how many times each duplicate has occurred. All works fine except Memcheck is telling me there are 39 errors and I cannot figure out what is causing them (I think the issue is coming from the repeated() method)?

Cheers, Harry

Code -

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

int main(void) {
    int array_size = 0;
    int *my_array;
    int i = 0;

    printf("Enter the size of the array:\n");
    scanf("%d", &array_size);

    my_array = malloc(array_size * sizeof my_array[0]);
    if (NULL == my_array) {
        fprintf(stderr, "memory allocation failed!\n");
        return EXIT_FAILURE;
    }

    for (i = 0; i < array_size; i++) {
        my_array[i] = rand() % array_size;
    }
    printf("What's in the array:\n");
    for (i = 0; i < array_size; i++) {
        printf("%d ", my_array[i]);
    }
    printf("\n");

    repeated(my_array, array_size);

    free(my_array);

    return EXIT_SUCCESS;
}

void repeated(int *my_array, int array_size) {
    int *array_tracker;
    int i;

    array_tracker = malloc(array_size * sizeof array_tracker[0]);

    for (i = 0; i < array_size; i++) {
        array_tracker[my_array[i]]++;
    }

    for (i = 0; i < array_size; i++) {
        if (array_tracker[i] > 1) {
            printf("%d occurs %d times\n", i, array_tracker[i]);
        }
    }

    free(array_tracker);
}

Memcheck output -

==23999== Memcheck, a memory error detector
==23999== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==23999== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==23999== Command: ./lab20c-prog
==23999== 
==23999== Conditional jump or move depends on uninitialised value(s)
==23999==    at 0x40091F: repeated (random.c:15)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== Conditional jump or move depends on uninitialised value(s)
==23999==    at 0x519148A: vfprintf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5199718: printf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x40092E: repeated (random.c:16)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== Use of uninitialised value of size 8
==23999==    at 0x518DD8B: _itoa_word (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5192552: vfprintf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5199718: printf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x40092E: repeated (random.c:16)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== Conditional jump or move depends on uninitialised value(s)
==23999==    at 0x518DD95: _itoa_word (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5192552: vfprintf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5199718: printf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x40092E: repeated (random.c:16)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== Conditional jump or move depends on uninitialised value(s)
==23999==    at 0x5192669: vfprintf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5199718: printf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x40092E: repeated (random.c:16)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== Conditional jump or move depends on uninitialised value(s)
==23999==    at 0x5191536: vfprintf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5199718: printf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x40092E: repeated (random.c:16)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== Conditional jump or move depends on uninitialised value(s)
==23999==    at 0x51915B9: vfprintf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x5199718: printf (in /usr/lib64/libc-2.24.so)
==23999==    by 0x40092E: repeated (random.c:16)
==23999==    by 0x400799: main (random.c:42)
==23999== 
==23999== 
==23999== HEAP SUMMARY:
==23999==     in use at exit: 0 bytes in 0 blocks
==23999==   total heap usage: 4 allocs, 4 frees, 1,052,792 bytes allocated
==23999== 
==23999== All heap blocks were freed -- no leaks are possible
==23999== 
==23999== For counts of detected and suppressed errors, rerun with: -v
==23999== Use --track-origins=yes to see where uninitialised values come from
==23999== ERROR SUMMARY: 39 errors from 7 contexts (suppressed: 0 from 0)

Barmar · Accepted Answer · 2017-09-21 00:19:13Z

1

You never initialized the contents of array_tracker. So when you do:

array_tracker[my_array[i]]++;

you're incrementing an uninitialized value. That's why you get lots of complaints about using uninitialized values.

You could use calloc() instead of malloc() to allocate space and initialize all the elements to 0.

array_tracker = calloc(array_size, sizeof array_tracker[0]);

answered Sep 21, 2017 at 0:19

Barmar

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3 Comments

Alejandro Iván Over a year ago

May I ask... what's sizeof array_tracker[0]? I thought array_tracker was actually NULL until it's initialized?

bipll Over a year ago

@AlejandroIván First of all, it can be NULL, yet it is its element that is measured. Next, sizeof is unevaluated context.

Barmar Over a year ago

@AlejandroIván sizeof just looks at the type, not the value.

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