midiInOpen function (mmeapi.h)

The midiInOpen function opens a specified MIDI input device.


MMRESULT midiInOpen(
  UINT      uDeviceID,
  DWORD_PTR dwCallback,
  DWORD_PTR dwInstance,
  DWORD     fdwOpen



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


Identifier of the MIDI input device to be opened.


Pointer to a callback function, a thread identifier, or a handle of a window called with information about incoming MIDI messages. For more information on the callback function, see MidiInProc.


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


Callback flag for opening the device and, optionally, a status flag that helps regulate rapid data transfers. It can be the following values.

Value Meaning
CALLBACK_FUNCTION The dwCallback parameter is a callback procedure address.
CALLBACK_NULL There is no callback mechanism. This value is the default setting.
CALLBACK_THREAD The dwCallback parameter is a thread identifier.
CALLBACK_WINDOW The dwCallback parameter is a window handle.
MIDI_IO_STATUS When this parameter also specifies CALLBACK_FUNCTION, MIM_MOREDATA messages are sent to the callback function as well as MIM_DATA messages. Or, if this parameter also specifies CALLBACK_WINDOW, MM_MIM_MOREDATA messages are sent to the window as well as MM_MIM_DATA messages. This flag does not affect event or thread callbacks.

Most applications that use a callback mechanism will specify CALLBACK_FUNCTION for this parameter.

Return value

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

Return code Description
The specified resource is already allocated.
The specified device identifier is out of range.
The flags specified by dwFlags are invalid.
The specified pointer or structure is invalid.
The system is unable to allocate or lock memory.


To determine the number of MIDI input devices present in the system, use the midiInGetNumDevs function. The device identifier specified by wDeviceID varies from zero to one less than the number of devices present.

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 input: MM_MIM_OPEN, MM_MIM_CLOSE, MM_MIM_DATA, MM_MIM_LONGDATA, MM_MIM_ERROR, MM_MIM_LONGERROR, and MM_MIM_MOREDATA.

If a function is chosen to receive callback information, the following messages are sent to the function to indicate the progress of MIDI input: MIM_OPEN, MIM_CLOSE, MIM_DATA, MIM_LONGDATA, MIM_ERROR, MIM_LONGERROR, and MIM_MOREDATA.


Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header mmeapi.h (include Windows.h)
Library Winmm.lib
DLL Winmm.dll

See also

MIDI Functions