How do you query a pthread to see if it is still running?
In my destructor I want to destroy a thread cleanly.
My goal is to wait for a thread to finish executing and THEN destroy the thread.
The only thing I found about querying the state of a pthread is pthread_attr_setdetachstate but this only tells you if your thread is:
- PTHREAD_CREATE_DETACHED
- PTHREAD_CREATE_JOINABLE
Both of those have nothing to do with whether the thread is still running or not.
How do you query a pthread to see if it is still running?
Answer
It sounds like you have two questions here:
How can I wait until my thread completes?
Answer: This is directly supported by pthreads -- make your thread-to-be-stopped JOINABLE (when it is first started), and use pthread_join() to block your current thread until the thread-to-be-stopped is no longer running.
How can I tell if my thread is still running?
Answer: You can add a "thread_complete" flag to do the trick:
Scenario: Thread A wants to know if Thread B is still alive.
When Thread B is created, it is given a pointer to the "thread_complete" flag address. The "thread_complete" flag should be initialized to NOT_COMPLETED before the thread is created. Thread B's entry point function should immediately call pthread_cleanup_push() to push a "cleanup handler" which sets the "thread_complete" flag to COMPLETED.
See details about cleanup handlers here: pthread cleanup handlers
You'll want to include a corresponding pthread_cleanup_pop(1) call to ensure that the cleanup handler gets called no matter what (i.e. if the thread exits normally OR due to cancellation, etc.).
Then, Thread A can simply check the "thread_complete" flag to see if Thread B has exited yet.
NOTE: Your "thread_complete" flag should be declared "volatile" and should be an atomic type -- the GNU compilers provide the sig_atomic_t for this purpose. This allows the two threads consistent access the same data without the need for synchronization constructs (mutexes/semaphores).