When DriverProc receives an MCI message, its double-word return value must be assigned as follows:
·If no errors occur, return zero.
·If an error occurs, return one of the MCI error return values in the low word. (If the error is device-specific, winmm.dll places the device ID in the high word.)
The MCI error return values are defined in mmsystem.h, and are prefixed with "MCIERR_". For a definition of each MCI error return value, see the Win32 SDK. Note that an additional set of error return values is defined for mciSendString. These additional values are returned by winmm.dll, not by MCI drivers.
When DriverProc receives one of the standard driver messages, its return value must be zero if an error occurs. Note that this is opposite to the situation for MCI messages.