Vi reference = function pointer for external DLL?

Question

So I'm porting this simple program from C to LabVIEW. All it does is getting some signals from an haptics hardware (Sensable Phantom Omni). It should be simple, but it has shown several complications. (been stuck for 2 weeks )
I successfully imported the hardware's DLL with all of its functions (using LV 8.2, because 8.5 and 8.6 's wizard sucks).
But there is a function whose argument is a function pointer:
hUpdateHandle = hdScheduleAsynchronous(updateDeviceCallback, 0, HD_MAX_SCHEDULER_PRIORITY);
 I already have the corresponding VI for updateDeviceCallback (which gets a (void *) that does not really use, and returns an int).
hdScheduleAsynchronous is part of the hardware's API (I can't mess with it, nor I know what's inside), and as first argument expects a function pointer.
Can I use the Open VI reference node to get the 'pointer' for my VI and then feed it as an argument for the external DLL? How do I properly cast the VI ref datatype into a function pointer?
Solved!
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Attachments:
QueryDevice.c ‏9 KB
updateDeviceCallback.vi ‏14 KB

Answer 1

JavierRuiz wrote:
So I'm porting this simple program from C to LabVIEW. All it does is getting some signals from an haptics hardware (Sensable Phantom Omni). It should be simple, but it has shown several complications. (been stuck for 2 weeks )
I successfully imported the hardware's DLL with all of its functions (using LV 8.2, because 8.5 and 8.6 's wizard sucks).
But there is a function whose argument is a function pointer:
hUpdateHandle = hdScheduleAsynchronous(updateDeviceCallback, 0, HD_MAX_SCHEDULER_PRIORITY);
 I already have the corresponding VI for updateDeviceCallback (which gets a (void *) that does not really use, and returns an int).
hdScheduleAsynchronous is part of the hardware's API (I can't mess with it, nor I know what's inside), and as first argument expects a function pointer.
Can I use the Open VI reference node to get the 'pointer' for my VI and then feed it as an argument for the external DLL? How do I properly cast the VI ref datatype into a function pointer?
LabVIEW has no concept that can translate seemlessly to function pointers and there is certainly no sensible way to make the Call Library Node support something like this.
There are possibilities to create a DLL from your callback VI and import the function using Windows API calls to pass it as a function pointer to your other function. But this is involved and quite dirty and most importantenly a total nightmare to maintain, since things go completely awry if your DLL is created in a different LabVIEW version than the LabVIEW version you call it with. This may seem like a piece of cake now but whoever will have to upgrade your software to a new LabVIEW version later will love you to the point that they will feel like sending you as the original programmer a nice little present that blows in your face when opened 
The only feasable way is to write a wrapper DLL in C that translates between something more LabVIEW friendly like a user event and the actual callback function mechanisme.
Rolf Kalbermatter
Message Edited by rolfk on 06-09-2009 10:24 AM
Rolf Kalbermatter
CIT Engineering Netherlands
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