static extern UInt32 midiOutPrepareHeader(IntPtr hMidiOut, IntPtr lpMidiOutHdr, UInt32 cbMidiOutHdr); // LD83

Declare Function midiOutPrepareHeader Lib "winmm.dll" (hMidiOut AS Intptr, lpMidiOutHdr AS IntPtr, cbMidiOutHdr) AS UInt32 ' LD83

This function is complementary to the midiOutPrepareHeader function. You must call midiOutUnprepareHeader before freeing the buffer. After passing a buffer to the device driver with the midiOutLongMsg function, you must wait until the device driver is finished with the buffer before calling midiOutUnprepareHeader.