Like all other types of installable drivers, MCI drivers must define a DriverProc entry point. This function must handle the standard driver messages. Additionally, it must handle MCI messages. Within winmm.dll, application calls to mciSendCommand and mciSendString become calls to DrvSendMessage, which is described in the Win32 SDK and is the standard method for calling a driver's DriverProc function.
Like the standard messages, MCI messages are defined as constant values that can be used in a C-language case statement. These constants are defined in mmsystem.h.
This section also provides information on DriverProc parameters and DriverProc return values for MCI drivers.