Re: [Boost-users] Unable to catch exception using boost::asio

Subject: Re: [Boost-users] Unable to catch exception using boost::asio
From: Alex Black (alex_at_[hidden])
Date: 2009-07-15 09:56:44

(Response is below)

 
> On Tue, Jul 14, 2009 at 7:49 PM, Alex Black<alex_at_[hidden]> wrote:
> >>> I'm using boost::asio for sockets, writing a response out to a
> > client.
> >>>
> >>> If the client stops part way through getting the 
> response, then my 
> >>> server process dies:
> >>>
> >>> terminate called after throwing an instance of
> >>>
> > 
> 'boost::exception_detail::clone_impl<boost::exception_detail::error_in
> >>> fo_injector<boost::system::system_error>
> >>>>'
> >>> ? what():? Broken pipe
> >>>
> >>> The code though is surrounded by try catch like this:
> >>>
> >>> try
> >>> {
> >>> ??? ...
> >>> }
> >>> catch ( std::exception& e )
> >>> {
> >>> ??? cout << e.what() << endl;
> >>> }
> >>> catch ( ... )
> >>> {
> >>> ???? cout << "unexpected exception" << endl; }
> >>>
> >>> How do I prevent my process from aborting/terminating like this?
> > v
> >
> >>Have you verified that the code inside the try block executes on the
> > same thread that the exception is occuring in?
> >
> > Not explicitly, how would I do that?
> >
> > thx
> >
> 
> First put a breakpoint inside the try block.  When it gets 
> hit use whatever mechanism your debugger provides to see what 
> thread you're in.  In windows using MSVC, for example, you'd 
> just add a watch for a variable called $TID which a special 
> pseudo-variable that shows you the thread id.  For other 
> debuggers you'd have to consult the manual.
>  Next register an unhandled exception handler.
> 
> #include <exception>
> 
> void unhandled_exception_handler()
> {
>    std::cout << "Got an unhandled exception!" << std::endl; }
> 
> int main(int argc, char** argv)
> {
>    std::set_terminate(unhandled_exception_handler);
> 
>    //rest of your program.
> }
> 
> Then put a breakpoint inside unhandled_exception_handler and 
> when it gets hit walk up the callstack in the debugger to see 
> where the exception is occuring.  In addition you can 
> determine the thread id again using the same method you used 
> in the first step.  My guess is that they're not in the same 
> thread.  I'm not sure if it applies to your scenario, but 
> remember that the operation's completion handler is invoked 
> on is not necessarily the same thread that the request is 
> issued on depending on how you're using the io_service 
> object.  So while you have a try {} catch(...) {} in the 
> completion handler, if the exception is on the other thread 
> that does you no good.
> 

Thanks for the tip, I didn't know about set_terminate, I made that
change.

I put a breakpoint in my terminate handler, and the callstack looks
like:

32 unhandled_exception_handler() /home/noirs/workspace/myfile.cpp:35
0x0000000000438764	
31 <symbol is not available> 0x00007f9ceb0d04b6	
30 <symbol is not available> 0x00007f9ceb0cf6ab	
29 __gxx_personality_v0()  0x00007f9ceb0d0278	
28 <symbol is not available> 0x00007f9ceab7cff3	
27 _Unwind_Resume()  0x00007f9ceab7d0b8	
26 ~ProtoBufStreamAdaptor() /home/noirs/workspace/myfile2.cpp:10
0x000000000042c6e5	
25 CWebServer::HandleGetNeighbors()
/home/noirs/workspace/myfile3.cpp:215 0x000000000041d2d8	
24 CWebServer::OnHttpRequest() /home/noirs/workspace/myfile3.cpp:89
0x000000000041e790	
23 CBaseWebServer::HandleHTTPRequest()
/home/noirs/workspace/myfile4.cpp:92 0x00000000004092ef	
22 CBaseWebServer::HandleAccept() /home/noirs/workspace/myfile4.cpp:61
0x0000000000409a9e	

HandleAccept looks like:

void CBaseWebServer::HandleAccept(shared_ptr<tcp::socket> pSocket, const
boost::system::error_code& error)
{	if ( !error )
        {
                try
                {
                        HandleHTTPRequest(pSocket);

                        StartAccept();
                }
                catch ( ... )
                {
                        cout << "Unhandled exception in HandleAccept";
                }
        }
        else
        {
                cout << "HandleAccept received error: " <<
error.message().c_str() << endl;
        }
}