MIDM_OPEN

See also MIDIOPENDESC, MIDM_CLOSE

Sent to a MIDI input driver to allocate a specified device for use by a client application.

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.

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_EVENT	If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be an event handle.
CALLBACK_FUNCTION	If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be the address of a callback function.
CALLBACK_THREAD	If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be a handle to a thread.
CALLBACK_WINDOW	If this flag is specified, dwCallback in the MIDIOPENDESC structure is assumed to be a window handle.

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.