MIDI in using DLL callback function

Question

I am trying to get MIDI into LV. The dll used is winmm.dll and the function midiinopen (plus others) is described here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/multimed/htm/_win32_midiinopen.asp
The main problem is I don't know how to program the Call Library Function Node properly in order to perform the call
plus set it to start receiving callback data, being midi messages. I have tried creating and registering a User Event and
passing the Event ref to the dll's "dwCallback" and then trapping the callback in an Event Structure, but nothing happens.
I have studied the "Communicating with a Windows MIDI Device in LabVIEW" example but it gives no hint since midi out
does not require the use of callbacks.
Please advice,
Stefan

Answer 1

Vedeja wrote:
I am trying to get MIDI into LV. The dll used is winmm.dll and the function midiinopen (plus others) is described here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/multimed/htm/_win32_midiinopen.asp
The main problem is I don't know how to program the Call Library Function Node properly in order to perform the call
plus set it to start receiving callback data, being midi messages. I have tried creating and registering a User Event and
passing the Event ref to the dll's "dwCallback" and then trapping the callback in an Event Structure, but nothing happens.
Damn! Need to make this post shorter as this message
board just silently told me that it needs to be shorter than 5000 words
and ate up my lengthy repsonse with no way to get it back.
You can't configure a Call Library Node to pass a Callback function to
another function. Callback functions have been alien to LabVIEW for a
long time with good reasons and what it has now as callback function in
LabVIEw 7.1 and newer is not directly compatible with C callback
functions.
Basically as you want to get data from the callback function back into
LabVIEW there is really no way around some intermediate software layer
which in this case almost surely means your own specific wrapper DLL
written in C.
If you use LabVIEW 7.1 you could use user events but not in the way you
describe. Attached is an example of how you can use user events from
external code. Note the extra DLL you will have to write. You have to
watch out what data you pass back to the user event as it has to match
exactly the type you configured the user event for, otherwise LabVIEW
will simply crash on you.
For numerics this is quite simply and also shown in the example. For
strings you can't just pass back a C string pointer but you will have
to allocate a LabVIEW string handle with
handle = DSNewHandle(sizeof(int32) + <length of C string>)
and then copy the C string into it. For specifics about how to do this
you should refer to the External Code reference manual in your Online
Bookshelf. Similar rules apply for arrays or clusters for that matter.
If you want or need to do this for LabVIEW < 7.1 there are two
possible approaches but both are even less trivial. You could either
create a wrapper DLL that translates your callback events into LabVIEW
occurrences and for the data transfer back to LabVIEW you would have to
implement your own queing too, or you could use the Windows Message
Queue example somewhere here in the NI examples and adapt it to return
your specific data. That would solve the data queueing more or less for
you without having to worry about that.
Rolf Kalbermatter
Message Edited by rolfk on 05-22-2006 11:22 AM
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions
Attachments:
userevent.zip ‏27 KB