Re: Multiple instances of CAsyncSocket in same thread

PDB <>
Wed, 15 Apr 2009 13:11:30 -0700 (PDT)
On Apr 14, 2:58 pm, Joseph M. Newcomer <> wrote:

See below...

On Tue, 14 Apr 2009 09:47:42 -0700 (PDT), PDB <paul.buschme...@iseinc-onl=> wrote:

On Apr 2, 8:07 pm, Joseph M. Newcomer <> wrote:

The problem is not CAsyncSockets in the same thread; the problem is th=

at you have deleted

a socket which had pending I/O, so you screwed up in deleting the obje=

ct before you get a

connection failure back. You have to close the socket completely, i=

ncluding a shutdown

call. But if a connection is pending, it is not clear that this doe=

s not result in a race

condition. Do not delete the socket until you know, from its OnConn=

ect notification, that

it has failed to connect.

There's no reason to show us the code. The code is correct. You =

broke the assumptions it

is based on.

Also, please use correct terminology. If the ENSURE macro is causin=

g anything to happen,

it is causing an assertion failure, not an exception. Note that the=

 problem is that the

socket is not in the handle map, because you deleted it. You need t=

o figure out which

callback is being invoked. Do not delete the object until this call=

back has completed.


On Thu, 2 Apr 2009 14:35:26 -0700 (PDT), PDB <paul.buschme...@iseinc-o=> wrote:

I have a problem with multiple sockets in the same CWinThread derived
thread. Here is the issue.
1) Instantiate two CAsyncSocket derived instances which connect
immediately, and all is well.
2) Instantiate one CAsyncSocket derived instance which connects
immediately, and another which does not. I use a timer to monitor =


socket which is not connected, and then close the socket and delete
the instance if it does not connect within the timeout period. Thi=


causes an exception in CAsyncSocket::DoCallBack (I've included a code
scrap). The ENSURE macro is the causing the exception because pSoc=



void PASCAL CAsyncSocket::DoCallBack(WPARAM wParam, LPARAM lParam)
   if (wParam == 0 && lParam == 0)

   // Has the socket be closed - lookup in dead handle list
   CAsyncSocket* pSocket = CAsyncSocket::LookupHandle((SOCKET)=



   // If yes ignore message
   if (pSocket != NULL)

   pSocket = CAsyncSocket::LookupHandle((SOCKET)wParam, FALSE)=


   if (pSocket == NULL)
           // Must be in the middle of an Accept call
           pSocket = CAsyncSocket::LookupHandle(INVALI=


           ENSURE(pSocket != NULL);

           if(pSocket == NULL)

           pSocket->m_hSocket = (SOCKET)wParam;
           CAsyncSocket::DetachHandle(INVALID_SOCKET, FA=



, pSocket, FALSE);


The problem only occurs if one socket connects and the other doesn't
and the time out causes the socket not connected to be closed and
deleted. Note: It is the closing of the socket that causes the
problem, not deleting the pointer to the instance of the socket.

I've also noticed that if I open a listening socket in the thread,
then create an instance of a socket that doesn't connect and the time
out causes it to be closed and delete will result in the same

Lastly, if I turn off the timer and do not overtly close and delete
the socket that is not connected, everything is fine.

While I appreciate that it is not great form to have lots of sockets
on the same thread (for lots of performance reasons), it should work.

Any explanation will be appreciated.

P.S. The article at "A Rewrite of KB192570: An MFC
Asynchronous Socket Example Done Right" has been invaluable, THANK
YOU! We've been using CSocket for years and have always been
concerned about that implementation, we are working on making the app
more robust.

Joseph M. Newcomer [MVP]
MVP Tips:

I did some more testing. The message that comes back is the
FD_CONNECT message. It gets invoked by the calling CAsyncSocket::Clos=


(), which, of course, also deletes the socket object from the map. My
timeout works properly with with only one socket, because the window
providing the notifications is destroyed, hence the FD_CONNECT message
is never sent. For the UI case, we added a boolean variable to
indicate a FD_CONNECT message was pending, and we use it to disable
the DISCONNECT button, so the user cannot cause the assertion
failure. In the thread case, where there is no user interaction, we
just let the WSASocket timeout prevail.

Yes, you can't Close() until the connect failure notification comes back.=



We use sockets in threads, and we may need to terminate the thread
while the FD_CONNECT notification is pending. My tests show that when
the thread cleans up by closing all sockets, connected or not, deletes
the CAsyncSocket derived objects, the assertion failure does not
occur. Our intent in all of this has been to be rid of the blocking
behavior of CSocket. We want to be able to terminate the threads
cleanly, especially(!) when the socket is waiting to connect.

The bottom line is, it appears there is no way to terminate the
connection process before it times out at the WSASocket level using
CAsyncSocket, but I welcome any discussion to the contrary.

When I hit the problem, I had to wait for the connection failure. So y=

ou can't assume

that the thread is shut down until it tells you that it has shut down, wh=

ich may mean your

program exit may be delayed (see my essay on thread termination, part of =

my worker threads


Thanks to all for the insight.

Joseph M. Newcomer [MVP]
MVP Tips:

Yes, I've read your article on thread termination, we use a similar
technique. The problem we are trying to deal with is users who are
starting and stopping "workstations" which are really instantiated
threads, the if they don't get instant feedback, they assume it isn't
working and reboot the computer! Hard to educate them. We were
hoping to use the notification methods of CAsyncSockets to allow us to
terminate a thread in a timely manner even if the socket had not timed
out a the WSA level.

Thanks for your help!

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