This message is sent to a MIDI input driver to allocate a specified device for use by a client application.
DWORD dwParam1
Specifies a far pointer to a MIDIOPENDESC structure. This structure contains additional information for the driver such as instance data from the client and a callback for the client.
DWORD dwParam2
Specifies option flags for opening the device.
CALLBACK_WINDOW
If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be a window handle.
CALLBACK_FUNCTION
If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be the address of a callback function.
CALLBACK_TASK
If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be a handle to a task.
The return value is an error code, or zero (MMSYSERR_NOERROR) if the operation is successful. Possible error codes are:
MMSYSERR_NOTENABLED
The driver failed to load or initialize.
MMSYSERR_ALLOCATED
The device is already allocated by the maximum number of clients the driver supports or the device cannot be opened because of system resource limitations other than memory.
MMSYSERR_NOMEM
The device cannot be opened because of a failure allocating or locking memory.
It is up to the driver to determine the number of clients allowed to use a particular device. Once a device is opened for the maximum number of clients the driver supports, the driver should return MMSYSERR_ALLOCATED for any additional open requests. If the open operation is successful, the driver should use the DriverCallback function to send the client a MIM_OPEN message.