LabVIEW
MIDI in using DLL callback function
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.
@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:The main problem is I don't know how to program the Call Library Function Node properly in order to perform the callplus set it to start receiving callback data, being midi messages. I have tried creating and registering a User Event andpassing the Event ref to the dll's "dwCallback" and then trapping the callback in an Event Structure, but nothing happens.
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
