Why does my program print out weird array values?

Question

In my attempt to learn a bit about pointers, I tried to create a program that allocates memory for an array of given size (where the size is defined by the value of the constant ARR_SIZE) and prints out elements of that array to the console.

I cannot understand the results I am getting... For example, when I run the code shown below, I get {0, 4206628, 4206628, 3, 4, 5, 6, 2686696, 8, 9}, which makes absolutely no sense to me. I would like to know what is the reason of such behaviour and what to modify in order to obtain the anticipated results.

#define ARR_SIZE 10

void print_array(int loc);
int memory_alloc();

int main()
{
    int array_loc = memory_alloc();

    print_array(array_loc);

    return 0;
}

void print_array(int loc)
{
    int i = 0;
    int *arr_ptr;

    arr_ptr = loc;

    while(i < ARR_SIZE)
    {
        printf("%d\n", *arr_ptr);
        *(arr_ptr++);
        i++;
    }
}

int memory_alloc()
{
    int array[ARR_SIZE];
    int i = 0;

    for(i; i < ARR_SIZE; i++)
    {
        array[i] = i;
    }

    return &array;
}

Have you looked at your compiler output? If it is half-decent, it should tell you what is wrong. — Tom Fenech
– Tom Fenech, Commented Sep 11, 2014 at 10:28

E_net4 · Accepted Answer · 2014-09-11 14:17:29Z

2

What you are observing is Undefined Behavior, because the array that was created in memory_alloc only exists in this function's scope. Once the program leaves it, that array will no longer exist.

An alternative is to allocate the array in the heap:

int memory_alloc()
{
    int *array = malloc(ARR_SIZE * sizeof(int));
    int i = 0;

    for(i; i < ARR_SIZE; i++)
    {
        array[i] = i;
    }

    return array;
}

After you're done using the array, it should be freed.

int main()
{
    int* array_pointer = memory_alloc();

    // use array here

    free(array_pointer);
    return 0;
}

edited Sep 11, 2014 at 14:17

answered Sep 11, 2014 at 10:28

E_net4

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Comments

Jayesh Bhoi · Accepted Answer · 2014-09-11 10:31:00Z

1

int array[ARR_SIZE];

here integer array is local to function and it will not longer available after function end.

To make it work you can do like

static int array[ARR_SIZE];

And

return array; //no need &

edited Sep 11, 2014 at 10:31

answered Sep 11, 2014 at 10:22

Jayesh Bhoi

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Comments

Haris · Accepted Answer · 2014-09-11 10:26:12Z

1

the array that you are declaring in memory_alloc() function

int array[ARR_SIZE];

this array is a local variable. its scope is within the function memory_alloc() and as soon as this function returns, all the local variables are destroyed (so is your array[])

instead, declare the array in main(), and pass the array to memory_alloc().

answered Sep 11, 2014 at 10:26

Haris

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