CODEWITHSUNDEEP
c++thread-safetypthreads
nitz
nitz

Reputation: 462

Is it acceptable and safe to pthread_join myself?

I've got a setup something a bit like this:

void* work(void*) { while (true) {/*do work*/} return 0;}

class WorkDoer
{
private:
    pthread_t id;
public:
    WorkDoer() { pthread_create(&id, NULL, work, (void*)this); }
    void Shutdown() { pthread_join(id, NULL); /*other cleanup*/ }
}

There's some cases where Shutdown() is called from the main thread, and some other cases where I want to call shutdown from within the thread itself (returning from that thread right after).

The documentation for pthread_join() says that it will return a EDEADLK if the calling thread is the same as the one passed.

My question is: Is that an okay thing to do, and if so is it safe? (thus ignoring the join fail, because I'll be nicely ending the thread in a moment anyways?) Or, is it something that should be avoided?

Upvotes: 2

Views: 4005

Answers (3)

user405725
user405725

Reputation:

You can certainly call pthread_join() from the running thread itself, and as you have found out the call itself will handle it properly giving you an error code. However, there are a few problems:

  1. It doesn't make any sense because the join won't join anything. It will merely tell you that you are doing something wrong.
  2. The thread itself won't exit upon calling pthread_join() on itself.
  3. Even if the thread exists, its state won't be cleaned up properly. Some other thread (i.e. your application's main thread) should call pthread_join() unless the thread was created as “detached”.

So in other words this approach is as acceptable as a bug in your program.

As a solution I would recommend to revisit the design and make sure that Shutdown() is called from the right place and at the right time. After all, the name “shutdown” doesn't make a lot of sense here because it doesn't shutdown a thing. What it does is merely waiting for a thread to finish and cleans up its state after that happens.

When you want to end the worker thread, either return from the thread routine or call pthread_exit(). Then make sure that whoever started a thread cleans things up by calling pthread_join().

If you want to force the thread to stop, consider using pthread_kill() to signal a thread or, alternatively, implement some sort of message passing that you can use to "tell" thread to stop doing whatever it is doing.

Upvotes: 5

alk
alk

Reputation: 70941

Why not just let the thread call pthread_detach(pthread_self()); and then exit. No need to call pthread_join() then anymore and risking to have it fail.

Upvotes: 1

lcs
lcs

Reputation: 4245

   The pthread_join() function may fail if:

   EDEADLK
          A deadlock was detected or the value  of  thread  specifies  the
          calling thread.

I would say use at your own risk.

Upvotes: 1

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