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Date: Sun, 20 Sep 2015 14:33:31 -0400
From: Rich Felker <>
Subject: Re: pthread_join stuck in infinite loop

On Sun, Sep 20, 2015 at 08:07:28PM +0200, Julien Ramseier wrote:
> Hello,
> pthread_join() never returns when calling it on a detached thread.
> I would expect it to return EINVAL instead.

Calling pthread_join on a detached thread or thread id which is no
longer valid results in undefined behavior. You can't do it. See:

  "The behavior is undefined if the value specified by the thread
  argument to pthread_join() does not refer to a joinable thread."


  "The lifetime of a thread ID ends after the thread terminates if it
  was created with the detachstate attribute set to
  PTHREAD_CREATE_DETACHED or if pthread_detach() or pthread_join() has
  been called for that thread. A conforming implementation is free to
  reuse a thread ID after its lifetime has ended. If an application
  attempts to use a thread ID whose lifetime has ended, the behavior
  is undefined.

  If a thread is detached, its thread ID is invalid for use as an
  argument in a call to pthread_detach() or pthread_join()."

Note that it's fundamentally impossible for it to be well-defined for
thread ids whose lifetimes have ended, and it wouldn't really make
sense to have it be defined for still-live detached threads since,
unless you perform additional synchronization to ensure this doesn't
happen, it's possible that a detached thread's exit could race with
your call to pthread_join and cause the id to become invalid.


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