Some Windows Functions Cannot Be Called from LibMain

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

In the Microsoft Windows graphical environment, when a dynamic-link library (DLL) is implicitly linked to an application, the LibMain function in the DLL should not call any dialog box, messaging, or window management functions. Making such a call causes a FatalExit error under the debugging version of Windows because the application has not created a message queue when the LibMain function for an implicitly loaded DLL executes.

MORE INFORMATION

Every Windows DLL must include an automatic initialization function, which is typically called LibMain. Windows calls this function when the DLL is initially loaded, giving the DLL an opportunity to perform one-time startup processing.

This startup processing typically includes registering window classes, initializing the DLL's local heap, and setting initial values for the DLL's global variables. However, a DLL's LibMain function cannot safely make many Windows calls.

When Windows loads an application that is implicitly linked to a DLL, it also loads the DLL on behalf of the application. Windows loads the DLL and calls the DLL's LibMain function before calling the application's WinMain function and before creating the application's message queue.

While the DLL is executing, it uses certain system resources that are owned by the application. One of these resources is the application's message queue. If the application's message queue has not been created, the DLL cannot do any messaging or tasking (because these operations require a message queue). In other words, the DLL cannot call any Windows functions that would send a message or yield control to another application. Attempting to do so causes a FatalExit error.

Because the DLL's LibMain function may be called before the application's message queue is created, the LibMain function cannot do any messaging or tasking. Such calls include the dialog box, messaging and window management functions as well as other functions that could result in a message being sent.