Question

edit:
I made the wrong assumption that threads started running on pthread_join when they actually start running on pthread_create.

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I’m learning to use Posix threads, and I’ve read that:
pthread_join() - wait for thread termination

So, in the code sample, main’s exit(0) is not reached until both started threads end.
But after the first call to pthread_join(), main continues executing, because the second call to pthread_join() actually runs, and the message in between is printed.
So how’s this? does main continue executing while both threads aren’t finished yet? or doesn’t it?
I know this isn’t a reliable way of testing, but the second test message always gets printed after both threads are finished, no matter how long the loop is. (at least on my machine when I tried it)

void *print_message_function( void *ptr )
{
    char *message = (char *) ptr;
    for( int a = 0; a < 1000; ++a )
        printf( "%s - %i\n", message, a );
    return NULL;
}
//
int main( int argc, char *argv[] )
{
    pthread_t thread1, thread2;
    char message1[] = "Thread 1";
    char message2[] = "Thread 2";
    int  iret1, iret2;
    //
    iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);
    iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
    //
    pthread_join( thread1, NULL);
    printf( "Let's see when is this printed...\n" );
    pthread_join( thread2, NULL); 
    printf( "And this one?...\n" );
    //
    printf("Thread 1 returns: %d\n",iret1);
    printf("Thread 2 returns: %d\n",iret2);
    exit(0);
}

Answer 1

The function pthread_join waits for the thread to finish or returns immediately if the thread is already done.

So in your case

pthread_join( thread1, NULL); /* Start waiting for thread1. */
printf( "Let's see when is this printed...\n" ); /* Done waiting for thread1. */

pthread_join( thread2, NULL); /* Start waiting for thread2. */
printf( "And this one?...\n" ); /* Done waiting for thread2. */

But after the first call to pthread_join(), main continues executing,
because the second call to pthread_join() actually runs, and the
message in between is printed.

False. pthread_join waits unless thread1 is already done.