PRB: Problems If ODBC or DAO in DLL's InitInstance or DLLMain

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2, 5.0

SYMPTOMS

If you use ODBC or DAO in the InitInstance or DLLMain functions of a DLL, your application may fail with an "Access Violation" or display some other unanticipated behavior. This can occur when you use the MFC Database Classes (ODBC-based or DAO-based), the ODBC API, or the dbDao classes provided with the DAO SDK.

CAUSE

The reason for this behavior is that ODBC drivers, DAO, or any of their underlying components may spawn multiple threads. Creating threads during initialization of a Win32 DLL can leave Windows in an undefined state.

When a Win32 DLL is loaded, Windows calls the DLL's DllMain function with a reason for calling of DLL_PROCESS_ATTACH.

When a new thread is created in Win32, each DLL in the process space will have its DllMain function called by Windows. Windows signals that a new thread has been created by passing a reason for calling of DLL_THREAD_ATTACH.

For a regular DLL that uses MFC, MFC supplies a DllMain that calls InitInstance. If you call a function in InitInstance that directly or indirectly creates one or more threads, you may experience unexpected behavior. This is because your application is within the context of DllMain (in InitInstance) with a DLL_PROCESS_ATTACH call, and creating a thread will result in Windows needing to call your DllMain again with a DLL_THREAD_ATTACH. The behavior resulting from this action is undefined.

If you are writing an extension DLL, you must supply your own implementation of the DllMain function. If you directly or indirectly create threads within this DllMain function, you will also experience this problem.

RESOLUTION

Do not create threads explicitly or implicitly within the context of DllMain.

Provide a function in your DLL for complex initialization that you can call outside of the scope of DllMain.

STATUS

This behavior is by design.

MORE INFORMATION

Currently the behavior described in this article is exhibited when using the Microsoft Access version 3 ODBC driver, using the ODBC SDK, or using MFC ODBC-based database classes. It can also occur when instantiating instances of MFC DAO-based database classes or the dbDao classes.

This behavior is not limited to a specific ODBC driver, DAO, or ODBC in general. You may experience this type of behavior with any API that creates threads.

If you suspect that this might be the cause of failure in your application, you can verify it by putting a breakpoint in the DllMain function and examining how many times it is called and by examining the dwReason parameter that is passed to it. If this is a regular DLL that uses MFC, set the breakpoint in MFC's DllMain function.

Sample Code

The following code is for MFC ODBC-based database classes but applies equally well to MFC DAO-based database classes, or the dbDao classes.

   // This code should reproduce the problem for a regular DLL that uses
   // MFC.

   BOOL CMyDllApp::InitInstance()
   {
     CDatabase db;
     if(db.Open("MyDataSource")) //Open Microsoft Access 3 ODBC datasource
       db.Close();
     ...
   }

   // To fix the problem, put the code in your own initialize function
   // that you call explicitly from your application.

   BOOL CMyDllApp::MyInitFunction()
   {
     CDatabase db;
     if(db.Open("MyDataSource")) //Open an Access 3 ODBC datasource
       db.Close();
     ...
   }

REFERENCES

"Advanced Windows" (Chapter 11) by Jeffery Richter, ISBN 1-55615-677-4.

For more information, please see the following related articles in the Microsoft Knowledge Base: