Deleting a daemon thread only after it returns

Question

I'm working on a project in which I have a main thread and one daemon thread to perform file outputs. In my main thread I have a field pthread_t * _daemon that I would like to delete, but obviously only after _daemon returns NULL (I understand that using pthread_exit() cause memory leaks).

How can I do it without busy-wait? If I try to use a condition variable I have a problem - When I call signal() form _daemon to wake up my main thread, it deletes _daemon before it _daemon returns NULL.

What I did is to just use a mutex lock that is locked when the program is launched and unlocked before _daemon returns. Is there any better way that would not cause busy wait?

Answer 1

pthread_detach() does what you're looking for. It sounds like it will solve your problem (no leaking) with a lot less complexity!

So you can safely call pthread_detatch(_daemon) when you're done with it inside the other thread, without having to worry about if the thread itself is still running. It does not terminate the thread, instead it causes the thread to be cleaned up when it does terminate.

From the documentation:

The pthread_detach() function shall indicate to the implementation that storage for the thread thread can be reclaimed when that thread terminates. If thread has not terminated, pthread_detach() shall not cause it to terminate.

You can actually create a thread in the detached state to start with by setting attr of:

   int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                      void *(*start_routine) (void *), void *arg);

appropriately at creation time. The pthread_create() manpage says:

When a detached thread terminates, its resources are automatically released back to the system. [snip] Making a thread detached is useful for some types of daemon threads whose exit status the application does not need to care about. By default, a new thread is created in a joinable state, unless attr was set to create the thread in a detached state (using pthread_attr_setdetachstate(3)).

Answer 2

pthread_t is an unsigned long int, so I don't really see a need to make a pointer of pthread_t (you can as well use it's address in pthread functions), but if you insist, then you can delete it after creating a pthread (but then you will have no way to communicate with it, because it's a number of created thread - used for joining, detaching functions etc.).