Do you need to join a cancelled thread? (pthreads)

Question

I'm a little confused about clean-up order when you're using PThreads with regard to cancellation. Normally, if your thread is detached, it automatically cleans up when it terminates. If it's not detached, you need to join it to reclaim the system resources.

The textbook I'm reading states the following which strangely sounds like joining is optional with regard to cancellation:

"If you need to know when the thread has actually terminated, you must join with it by calling pthread_join after cancelling it."

So, do I need to join a cancelled thread to free its resources - and if not, then why?

Answer 1

TLPI says this:

Upon receiving a cancellation request, a thread whose cancelability is enabled and deferred terminates when it next reaches a cancellation point. If the thread was not detached, then some other thread in the process must join with it, in order to prevent it from becoming a zombie thread.

Also, since canceling a thread isn't usually done immediately (read more about "cancellation points") without joining you can't be sure the thread was actually canceled.

Answer 2

From man pthread_join:

After a canceled thread has terminated, a join with that thread using pthread_join(3) obtains PTHREAD_CANCELED as the thread's exit status. (Joining with a thread is the only way to know that cancellation has completed.)

It seems that joining is not necessary for execution it is necessary if you want know what you did actually succeed.