Your application may fail with an "Access Violation" error message when you use ODBC or DAO in the InitInstance or DLLMain functions of a DLL (147629)

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.

   // 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();
     ...
   }