Thread Id differs in main() and start_routine() - Pthread

Question

#include <pthread.h>
#include <stdio.h>

void* printHello (void* threadId)
{
    pthread_t *my_tid = (pthread_t *)threadId;
    printf ("\nIn `printHello ()`: thread id %ld", (long)*my_tid);
    pthread_exit (NULL);
}

int main ()
{
    pthread_t        arrayOfThreadId [5];
    int                  returnValue;
    unsigned int iterate;

    for (iterate = 0; iterate < 5; iterate++)
    {
        if (returnValue = pthread_create (&arrayOfThreadId [iterate],
                                    NULL,
                                    printHello,
                                    (void*) &arrayOfThreadId [iterate]) != 0)
        {
            printf ("\nerror: pthread_create failed with error number %d", returnValue);
        }
        else
        {
            printf ("\nIn `main()`: creating thread %ld", arrayOfThreadId [iterate]);
        }
    }

    printf ("\nBefore `return 0;` in `main()`");
    pthread_exit (NULL);
    return 0;
}

Output:

In 

`main()`: creating thread 139683073914640
In `main()`: creating thread 139683065521936
In `main()`: creating thread 139683057129232
In `main()`: creating thread 139683048736528
In `main()`: creating thread 139683040343824
Before `return 0;` in `main()`
In `printHello ()`: thread id 140735115959408
In `printHello ()`: thread id 4195680
In `printHello ()`: thread id 0
In `printHello ()`: thread id 0
In `printHello ()`: thread id 139683073914640

Why do the threadIDs differ?

I missing some point here. I have done accidentally wrong, it seems.

Answer 1

You're passing the address of your threadId, but you're casting that address to a pthread_t, thus treating the address of an element in your arrayOfThreadId as if it were a pthread_t.

Do this instead:

void* printHello (void* threadId)
{
    pthread_t *my_tid = threadId;
    printf ("\nIn `printHello ()`: thread id %ld\n", (long)*my_tid);

And make sure your main() does not exit until all the threads are finished, else you risk having the array destroyed before the threads pokes into it.