Why am I getting “void value not ignored as it ought to be”?

  int a = srand(time(NULL));

The prototype for srand is void srand(unsigned int) (provided you included <stdlib.h>).
This means it returns nothing … but you’re using the value it returns (???) to assign, by initialization, to a.

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Edit: this is what you need to do:

#include <stdlib.h> /* srand(), rand() */
#include <time.h>   /* time() */

#define ARRAY_SIZE 1024

void getdata(int arr[], int n)
{
    for (int i = 0; i < n; i++)
    {
        arr[i] = rand();
    }
}

int main(void)
{
    int arr[ARRAY_SIZE];
    srand(time(0));
    getdata(arr, ARRAY_SIZE);
    /* ... */
}