During my work for libDispatch I had to find a way for implementing TSD de-allocators (which are not available on windows). While doing so I did some extensive debugging on OS X / Linux for understanding what is actually happening when calling dispatch_main(). Weird enough it seemed to me as if the main thread would never execute an item from the main queue but simply go to sleep instead.

To verify this behaviour, I wrote a test to compare the pthread_t handles during execution:

static pthread_t main_thread;

void pass(void* dt){

    pthread_t now = pthread_self();

    assert(pthread_equal(now, main_thread) == 1);

    exit(0);

}

void dispatch_api() {

    dispatch_queue_t q = NULL;

    main_thread = pthread_self();

    q = dispatch_get_main_queue();

	assert(q != NULL);

    dispatch_async_f(q, NULL, pass);

    ...

    dispatch_main();

}

This test is approving my suspicions, as the pthread_equal(...) assertion is constantly failing. So the big question is: What am I missing? I thought the main queue is available as a mean to be sure some blocks execute on the main thread (e.g. when updating the GUI is only possible while being on the main thread).

I welcome any hints or help.

- Marius Zwicker