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

From: William Kempf (williamkempf_at_[hidden])
Date: 2002-08-11 17:30:55

>From: "Victor A. Wagner, Jr." <vawjr_at_[hidden]>
>Reply-To: boost_at_[hidden]
>To: boost_at_[hidden]
>Subject: Re: [boost] Re: Re: Threads & Exceptions
>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.

>#include "stdafx.h"
>#include <iostream>
>#include <string>
>#include <iomanip>
>#include <boost/lexical_cast.hpp>
>
>using namespace std;
>
>int main(int argc, char* argv[])
>{
>         cout << setw(2*argc) << ' ' << "entering main: " << argc << endl;
>         if (++argc < 10)
>         {
>                 cout << setw(2*argc) << ' ' << "calling main: " << argc << 
>endl;
>                 try
>                 {
>                         main(argc, 0);

Results in undefined behavior.  You're not allowed to call main().  Anything 
that happens next means nothing.

>                 }
>                 catch (exception& e)
>                 {
>                         cout << setw(2*argc) << ' ' << "exception: " << 
>argc << " " << e.what() << endl;
>                 }
>                 cout << setw(2*argc) << ' ' << "returned from main: " << 
>argc << endl;
>         }
>         if (argc > 2)
>         {
>                 throw exception(("throwing: " + 
>boost::lexical_cast<string>(argc)).c_str());
>         }
>         cout << setw(2*argc) << ' ' << "returning: " << argc << endl;
>}

I'm not sure what you were trying to illustrate with this code... but the 
use of illegal C++ would seem to make anything you observe irrelavant.

Bill Kempf
williamkempf_at_[hidden]

_________________________________________________________________
Send and receive Hotmail on your mobile device: http://mobile.msn.com