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

Subject: Re: [Boost-users] Unable to catch exception using boost::asio
From: Etienne Philip Pretorius (icewolfhunter_at_[hidden])
Date: 2009-07-15 10:04:38

Alex Black wrote:
> (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;
> 	}
> }
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Um, broken pipe = closed file desciptor.

Check if the socket is actually valid in the shard_ptr...

BTW, are you using Async operations.... if so then the exception will 
not originate from your code but from io_service.run().