Re: [boost] Boost.Threads, N2178, N2184, et al

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2007-03-27 12:33:13

On Mar 27, 2007, at 11:47 AM, Peter Dimov wrote:

>
>> join()
>> {
>>     // Don't want to pretend to join with non-existent
>>     //   (or detached) thread, make noise
>>     if (handle_ == 0)
>>         throw ?;
>>     while (!tl->done)
>>         tl->cv1.wait();
>>     detach();  // point of contention with N2178, makes join non- 
>> const,
>>                //   and disables multi-join.  Without it, resources  
>> not
>>                //   released until explicit detach() or ~thread()
>> }
>>
> ... we can still remove this detach() call from join() and the user  
> is free
> to get the old semantics back by just calling detach() explicitly  
> after
> join() (whereas the reverse is not true).

Ok, I'm still having pain from realloc doing too much. :-)  Maybe this  
is a safe spot to tweak the pthreads semantics since ~thread() *must*  
catch any leaks and C doesn't have that option.  I've tweaked the  
pseudo code as below and am continuing to look for problems with it.   
I believe this comes closer to Anthony's earlier post.  And note the  
const qualifier I put on join!!! :-)  (reserving the right to waffle  
back...)

pthread_t handle_;
thread_local_data* tl;

detach()
{
     // If has already been detached, that means tl may already be  
released
     //   therefore not safe to go through twice.
     // But maybe ok to ignore subsequent detach?
     if (handle_ != 0)
     {
         tl->wait_on_owner = false;
         tl->cv2.notify_all();
         handle_ = 0;
     }
}

cancel()
{
     // Canceling a non-existent thread seems harmless
     if (handle_ != 0)
         tl->cancel_pending = true;
}

join() const
{
     // Don't want to pretend to join with non-existent
     //   (or detached) thread, make noise
     if (handle_ == 0)
         throw ?;
     while (!tl->done)
         tl->cv1.wait();
}

thread::id get_id() const {return thread::id(handle_);}

~thread()
{
     cancel();  // harmless if already canceled
     detach();  // harmless if already detached
}

start()
{
     allocate_all_resources();  // tl != 0
     tl->done = false;
     tl->wait_on_owner = true;
     tl->cancel_pending = false;
     try
     {
         f();
     }
     catch (thread_canceled&)
     {
     }
     catch (...)
     {
         terminate();
     }
     tl->done = true;
     tl->cv1.notify_all();
     while (tl->wait_on_owner)
         tl->cv2.wait();
     release_all_resources(); // tl = 0
}

-Howard