Safely echoing MIDI data in C#

Question

As a side project, I am implementing a MIDI matrix to interconnect a number of MIDI keyboards with sound sources. The key requirement here is to echo MIDI data received on an input port to a selected output port.

Having created the necessary P/Invoke declarations & wrappers, I notice that the Win32 MIDI documentation for MidiInProc states: "Applications should not call any multimedia functions from inside the callback function, as doing so can cause a deadlock".

Given that it is unsafe to call midiOutShortMsg from within a MidiInProc, my current solution is to write MIDI data to a queue and set an event. A worker thread waits on the event and calls midiOutShortMsg. The general idea is this:-

static void InputCallback( int hMidiIn, uint wMsg, uint dwInstance, uint dwParam1, uint dwParam2 )
{
    if( wMsg == MM_MIM_DATA )
    {
        data.EnQueue( dwParam1 );    //data is a Queue<uint>
        dataReady.Set();      //dataReady is AutoResetEvent
    }
}


void ThreadProc
{
    while( !_done )
    {
        dataReady.WaitOne();
        midiOutShortMsg( hMidiOut, data.DeQueue() );
    }
}

However, whilst this has been working fine in testing, there appears to be a window of opportunity between the call to dataReady.Set() and InputCallBack returning during which preemption could allow the worker thread to call midiOutShortMsg (albeit in another thread).

Is this approach safe?

Answer 1

Your question might be a bit domain-specific for StackOverflow. If no one here answers it, check out: http://groups.google.com/group/mididev?hl=en&lnk=

Answer 2

Looks safe to me: even if your two threads are running at the same time, it's still only the worker thread that might block, so this won't deadlock your input callback.