INF: ODBC Desktop Drivers and OLE in a Multithreaded App

MORE INFORMATION

When SQLConnect or SQLDriverConnect is called, ODBC DeskTop Drivers initialize OLE using threads that they create internally. As a result of OLE initialization, three hidden windows--DDEServerWindow, OLEChannelWnd, and OLEMainthreadwndname--are created.

If the thread that made the database connection is idle, then the hidden windows will not respond to any broadcast DDE messages sent by other applications. For example, if an application broadcasts a message such as WM_DDE_INITIATE to all windows, the SendMessage hangs until all the top- level windows have received the message. Because the thread that created the hidden windows is idle and does not respond, the application hangs. This is a known OLE issue in mutithreaded applications.

For more detailed information on OLE threads and sample code, please refer to the following article in the Microsoft Knowledge Base: To prevent the DDE app from continuing to hang after SQLConnect or SQLDriverConnect has returned, all threads that call SQLConnect or SQLDriverConnect need to handle the DDE messages sent by the DDE app. The following example gives an overview of how to accomplish this:

void doit()
{
RETCODE rc;             /* Return code for ODBC functions  */ 
HENV henv;              /* Environment handle              */ 
HDBC hdbc;              /* Connection handle               */ 
HSTMT hstmt;            /* Statement handle                */ 

//variables
unsigned char connStrOut[256];
MSG msg;
HANDLE myhandle;

myhandle = GetCurrentThread();

//Allocate ODBC handles and make the connection to the database
SQLAllocEnv(&henv);
rc = SQLAllocConnect(henv, &hdbc);
rc = SQLDriverConnect(hdbc, 0,(unsigned char *)"DSN=hello", SQL_NTS,
connStrOut, 256, NULL,
        SQL_DRIVER_NOPROMPT);

//Handle messages to prevent other OLE applications from hanging
MsgWaitForMultipleObjects( 1, &myhandle, FALSE, INFINITE,QS_ALLINPUT);
AfxMessageBox("type in something",MB_OK);

//Disconnect and free the ODBC handles
SQLDisconnect(hdbc);
SQLFreeConnect(hdbc);
SQLFreeEnv(henv);
ExitThread(0);
}


void CTestMultiThread::OnTest()
{
HANDLE hTestThread;
DWORD  dwThreadID ;
hTestThread = CreateThread((LPSECURITY_ATTRIBUTES)NULL,0,
          (LPTHREAD_START_ROUTINE)doit,this,0,&dwThreadID);
return;
}
				

NOTE: With the ODBC Desktop Database Drivers 2.0 running on Windows NT 3.5, only one thread can make a SQLConnect or SQLDriverConnect call. Other threads in the same application cannot make the connection calls at any time while the application is running. Once a connection is made on a thread, the connection (hdbc) can be shared by other threads. This is because OLE did not support multithreading in Windows NT 3.5.

In Windows NT 3.51, the above does not apply as OLE supports multithreading. However, you need to ensure that the first thread that calls SQLConnect or SQLDriverConnect is the last thread to call SQLDisconnect.