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midiOutOpen function

The midiOutOpen function opens a MIDI output device for playback.


MMRESULT midiOutOpen(
   LPHMIDIOUT lphmo,
   UINT       uDeviceID,
   DWORD_PTR  dwCallback,
   DWORD_PTR  dwCallbackInstance,
   DWORD      dwFlags



Pointer to an HMIDIOUT handle. This location is filled with a handle identifying the opened MIDI output device. The handle is used to identify the device in calls to other MIDI output functions.


Identifier of the MIDI output device that is to be opened.


Pointer to a callback function, an event handle, a thread identifier, or a handle of a window or thread called during MIDI playback to process messages related to the progress of the playback. If no callback is desired, specify NULL for this parameter. For more information on the callback function, see MidiOutProc.


User instance data passed to the callback. This parameter is not used with window callbacks or threads.


Callback flag for opening the device. It can be the following values.

Value Meaning
CALLBACK_EVENTThe dwCallback parameter is an event handle. This callback mechanism is for output only.
CALLBACK_FUNCTIONThe dwCallback parameter is a callback function address.
CALLBACK_NULLThere is no callback mechanism. This value is the default setting.
CALLBACK_THREADThe dwCallback parameter is a thread identifier.
CALLBACK_WINDOWThe dwCallback parameter is a window handle.


Return value

Returns MMSYSERR_NOERROR if successful or an error otherwise. Possible error values include the following.

Return codeDescription

No MIDI port was found. This error occurs only when the mapper is opened.


The specified resource is already allocated.


The specified device identifier is out of range.


The specified pointer or structure is invalid.


The system is unable to allocate or lock memory.



To determine the number of MIDI output devices present in the system, use the midiOutGetNumDevs function. The device identifier specified by wDeviceID varies from zero to one less than the number of devices present. MIDI_MAPPER can also be used as the device identifier.

If a window or thread is chosen to receive callback information, the following messages are sent to the window procedure or thread to indicate the progress of MIDI output: MM_MOM_OPEN, MM_MOM_CLOSE, and MM_MOM_DONE.

If a function is chosen to receive callback information, the following messages are sent to the function to indicate the progress of MIDI output: MOM_OPEN, MOM_CLOSE, and MOM_DONE.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]


Mmsystem.h (include Windows.h)





See also

MIDI Functions