Calling Windows Functions from a Device Driver

• Microsoft Windows Device Driver Kit (DDK) for Windows versions 3.0 and 3.1

SUMMARY

Some device drivers need to call functions provided by the user dynamic- link library (DLL) of Microsoft Windows. The MessageBox() function is an example of one of these functions. However, a driver cannot implicitly link to the user DLL. If a driver attempts to implicitly link to user, it will fail to load and Windows will not run. The load fails because the drivers are loaded before user (user requires the drivers). An implicit link to user forces the kernel to load the user module when the driver is loaded; which Windows cannot do. Therefore, the driver must establish the link at a later time. This article explains how this is accomplished.

MORE INFORMATION

An example of a driver calling a function in user is the communications driver (COMM.DRV) which displays a message box to resolve contention on the serial and parallel ports.

The source code for COMM.DRV is included in the Windows Device Development Kit (DDK). The driver's Contention_Dlg function displays dynamically links to user and displays the message box. For Windows 3.1, the Contention_Dlg function is in the IBMSETUP.ASM file; for Windows 3.0 this function is in the IBMCOM.ASM file.

There are two steps required:

1. Use GetModuleHandle("user") to retrieve the module handle to user.

2. Call GetProcAddress(hUserModule, "MESSAGEBOX") to retrieve the FAR procedure address of the MessageBox function.