How do you query a pthread to see if it is still running?

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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).

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