Re: pthread_join stuck in infinite loop

[Rich Felker <dalias@libc.org>]

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Calling pthread_join on a detached thread or thread id which is no
longer valid results in undefined behavior.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:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_join.html

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

and:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_02

  "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.

Rich