"Assertion failed - <app name>:File sockcore.cpp" error message when you use a multi-threaded application that uses MFC's socket classes

View products that this article applies to.

This article was previously published under Q140527

SYMPTOMS

A multi-threaded application that uses MFC's socket classes encounters a message box or debug output line that contains an error message similar to the following:

For Visual C++ 2.x:

Assertion failed - <app name>:File sockcore.cpp, Line 837

For Visual C++ 4.0:

Assertion failed - <app name>:File sockcore.cpp, Line 1041

Back to the top

Most frequently, this problem is due to the sharing of CSocket objects between multiple threads.

A CSocket object should be used only in the context of a single thread because the SOCKET handle encapsulated by a CAsyncSocket object is stored in a per-thread handle map. (CSocket is derived from CAsyncSocket.) Other information is stored on a per-thread basis, including a hidden notification window that MFC uses for socket notifications.

The assertion failure line, which can be found in Sockcore.cpp in the \Msvc20\Mfc\Src directory, is:

   ASSERT(pThreadState->m_hSocketWindow != NULL);

This assertion failure occurs because the CSocket object was either created or accepted in the context of another thread. The socket notification window was created in a different thread, and the m_hSocketWindow for the current thread is NULL, thus the assertion failure.

Back to the top

RESOLUTION

As already mentioned, a CAsyncSocket object should be used only in the context of a single thread. If you need to switch the thread that is accessing a SOCKET connection with another thread, then you should use a separate CAsyncSocket object in each thread, and use the Detach and Attach functions to attach the CAsyncSocket object to the SOCKET handle in the thread that is about to use the socket. Use this sequence:

Use Detach() to detach the CAsyncSocket object from the SOCKET handle in the thread that is currently using the CAsyncSocket object.
Use Attach() to attach a different CAsyncSocket object to the SOCKET handle while in the context of the MFC UI thread in which you wish to begin accessing the SOCKET connection.

The code shown in the "Code Sample" section of this article shows how to handle the moment when a listening socket accepts a connection request and then begins a new thread to handle the new connection.

Note One concern often arises that socket notification messages might be lost between the time the call to Detach() is made and the subsequent call to Attach() is made. This is not a concern because of the way socket notifications are handled. The implementation of CAsyncSocket::Attach() makes a call to WSAAsyncSelect to enable notifications. As mentioned in the documentation for WSAAsyncSelect, if any socket notifications were already pending for the SOCKET, they will be re-posted.

Back to the top

MORE INFORMATION

To Accept a socket in the context of one thread and then begin using it in the context of another thread, you need to be sure to detach the CAsyncSocket object in the first thread and attach a different CAsyncSocket object in the second thread. The following code snippet shows how to do it.

Back to the top

Code sample

// ...
class CSockThread : public CWinThread
{
// ... Other function and member declarations
protected:
  CSocket m_sConnected;
};

SOCKET hConnected;

BOOL CSockThread::InitInstance()
{
  // Attach the socket object to the socket handle
  // in the context of this thread.
  // 
  m_sConnected.Attach(hConnected);

  return TRUE;
}

// This listening socket has been constructed
// in the primary thread.
// 
void CListeningSocket::OnAccept(int nErrorCode)
{
  // This CSocket object is used just temporarily
  // to Accept the incoming connection.
  // 
  CSocket sConnected;
  Accept(sConnected);

  // Detach the newly accepted socket and save
  // the SOCKET handle
  hConnected = sConnected.Detach();

  // After Detaching it, it should no longer be
  // used in the context of this thread

  // Start the other thread
  AfxBeginThread(RUNTIME_CLASS(CSockThread));
}

Back to the top

REFERENCES

MFC Technical Note #2 - Although this technical note does not directly address the CAsyncSocket class, it does discuss the mapping of handles to objects. The relationship between a SOCKET handle and a CAsyncSocket object is maintained in much the same way.

Back to the top