RE: [boost] Re: Re: Threads & Exceptions

From: Anthony Williams (anthwil_at_[hidden])
Date: 2002-08-12 04:31:55

From: William Kempf [mailto:williamkempf_at_[hidden]]
Sent: 11 August 2002 23:31
> >From: "Victor A. Wagner, Jr." <vawjr_at_[hidden]>
> >Date: Sun, 11 Aug 2002 11:59:15 -0700
> >
> >At Sunday 2002/08/11 06:48, you wrote:
> >>>From: "Victor A. Wagner, Jr." <vawjr_at_[hidden]>
> >>>At Thursday 2002/08/08 07:49, you wrote:
> >>>>----- Original Message -----
> >>>>From: "Victor A. Wagner, Jr." <vawjr_at_[hidden]>
> >>void my_thread(boost::thread* pthread)
> >>{
> >>   try
> >>   {
> >>      pthread->join();
> >>   }
> >>   catch(...)
> >>   {
> >>      cout << "main() threw an exception" << endl;
> >>   }
> >>   delete pthread;
> >>}
> >>
> >>int main()
> >>{
> >>   boost::thread* main = new thread();
> >>   boost::thread thrd(boost::bind(&my_thread, main));
> >>   thread_exit(); // Yes, this is something that needs added to
> >>                  // Boost.Threads, and will be.
> >>   throw "error!";
> >>}
> >>
> >>The above can't be done portably, AFAIK, and is likely to result in 
> >>termination instead of the expected passage in join().
> >
> >I have no idea what this is supposed to "do" so that it's 
> not portable 
> >seems irrelevant.
> >Although I have no idea what thread_exit() will actually do, 
> having it 
> >execute the next line of code seems counter-intuitive.
> >The following code works as I expected on VC++.NET (not that 
> I'm attempting 
> >to hold it out as either definitive nor conforming) and if 
> the "if (argc > 
> >2)" is removed or changed to, say, >1, the "unexpected 
> exception" occurs, 
> >otherwise this executes to completion without "problem".
> 
> Shoot.  My apologies.  I wrote that way too fast.  The throw 
> should have 
> occured before the call to thread_exit().  The point being 
> that, as per 
> behavior on every platform I'm familiar, thread_exit() (i.e. 
> its platform 
> equivalent) causes the thread to exit, and in the case of the 
> main thread 
> this means the process doesn't terminate until all other 
> threads complete.  
> The throw skips the call to thread_exit(), and thus the 
> terminate() is 
> called because of the uncaught exception.  This means the main thread 
> behaves differently from all other threads with regard to exception 
> handling... unless you 1) can change the language rules or 2) 
> make the 
> exception handling consistent by having threads call 
> terminate() in case 
> there's an uncaught exception as well.

Alternatively, you could just set a terminate_handler which calls
thread_exit, and ensure that uncaught exceptions in all threads terminate
that thread, and only that thread. This is then consistent across all
threads. As I posted before, it works successfully on MSVC/win32threads
MSVC/pthreads, gcc-mingw/win32threads gcc-mingw/pthreads
gcc-sparc-solaris/pthreads. 

If join is called on a thread that terminated through an exception, then you
could notify the caller of join that the thread terminated through an
uncaught exception, but that is another issue.

Anthony