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Subj : Re: pthreads: cancelling thread blocked on berkeley socket accept()
To   : comp.programming.threads
From : Luke Dalessandro
Date : Mon Sep 19 2005 06:57 pm



Sergei Organov wrote:
> David Hopwood <david.nospam.hopwood@blueyonder.co.uk> writes:
> 
> 
>>Luke Dalessandro wrote:
>>
>>>I have a thread running on a listen()->accept()->spawn_job()->listen()
>>>loop on a socket (unix/berkeley). I have the socket set to block on the
>>>accept(). spawn_job() dispatches a thread to handle the incoming request.
>>>
>>>When the application is shut down, I want to cancel the thread and
>>>close() the socket. Is it appropriate to call pthread_cancel() on a
>>>blocked thread? If I call pthread_cancel(listener) and then
>>>pthread_join(listener) from the main (parent) thread, am I guaranteed
>>>that it is safe to close() the socket?
>>
>>You should set a shared 'shouldExit' flag and close() the socket from the
>>main thread. The listening thread will get an error from the accept(), and
>>can then exit when it sees that 'shouldExit' is set.
> 
> 
> I'm afraid async close() is asking for troubles. I'd suggest to call
> shutdown() from main thread, then call close() when accept() returns
> error.
> 
> BTW, this also allows close() call to remain in the destructor.
> 
> -- Sergei.

Sergei,

Ok, so you're saying that a thread blocking on an accept() will return 
with an error if another thread calls shutdown on the socket. So in my 
example I can use the following sequence:

   Thread* t;
   Socket* s;

   s->Shutdown(); // calls os socket shutdown();

   // t gets an accept error: I use error to break my "while (true)"
   // accept loop, and the thread returns.

   t->Join(); // guaranteed to join because thread has returned
   delete s;
   delete t; // if not stack allocated

This seems nicer in the sense that I don't need a shouldExit flag or 
anything.

Thanks guys,
Luke

.

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