Share a variable between C and Labview?

Question

What is the best way to permit C code to regularly access the instantaneous value of an integer generated from a separate Labview program?

I have time-critical C code that controls a scientific experiment and records data once every 20ms. I also have some labview code that operates a different instrument and outputs an integer value ever 100ms. I want my C code to be able to record the value from labview. What is the best way to do this?

One idea is to have Labview write the integer to file in a loop, and have the C code read the value of the file in a loop. (I could add a second thread to my C code if necessary.) Labview can also link to C dll's. So I might be able to write a DLL in C that somehow facilitates sharing between the two programs. Is that advisable? How would I do that?

Answer 1

I have a similar application here and use TCP sockets with the TCP_NO_DELAY option set (disables the Nagle algorythm which does some sort of packet buffering). Sockets should allow for a 100mSec update rate without problems, although the actual network delay will always remain an unknown variable. For my application this does not matter as long as it stays under a certain limit (this is also checked for by sending a timestamp with each packet and big red dialog boxes if timestamp delta becomes too large :]). Does it matter for your application? Ie, is it important that whenever the LV instrument acquires a new sample it's value has to make it to the C app within x mSec?

You might get the dll approach working, but it's not as straightforward as sockets and it will make the two applications more dependant of each other. Variable acces will be pretty much instantaneous though. I see at least two possibilities:

• put your entire C app in a dll (might seem a weird approach at first but it works), and have LV load it and call methods on it. Eg to start your app LV calls dll's Start() method, then in the loop LV acquires it's samples it calls the dll's NewSampleValue(0 method or so. Also means your app cannot run standalone unless you write a seperate host process for it.
• look into shared process memory, and have the C app and another dll share common memory. LV will load that dll and call a method on it to write a value to the shared memory, then the C app can read it after polling a flag (which needs a lock!).
• it might also be possible to have the C app call the LV program using dll/activeX/? calls but I don't know how that system works..

I would definitely stay away from the file approach: disk I/O can be a real bottleneck and it also has the locking problem which is messy to solve with files. C app cannot read the file while LV is writing it and vice-versa which might introduce extra delays.

On a sidenote, you can see that each of the approaches above either use a push or pull model (the TCP one can be implemented in both ways), this might affect your final decision of which way to go.. Push = LV signals the C app directly, pull = C app has to poll a flag or ask LV for the value.

Answer 2

I'm an employee at National Instruments and I wanted to make sure you didn't miss the Network Variable API that is provided with LabWindows/CVI, the National Instruments C development environment. The the Network Variable API will allow you to easily communicate with the LabVIEW program over Shared Variables (http://zone.ni.com/devzone/cda/tut/p/id/4679). While reading these links, note that a Network Variable and a Shared Variable are the same thing - the different names are unfortunate...

The nice thing about the Network Variable API is that it allows easy interoperability with LabVIEW, it provides a strongly typed communication mechanism, and it provides a callback model for notification when the Network/Shared variable's properties (such as value) change.

You can obtain this API by installing LabWindows/CVI, but it is not necessary to use the LabWindows/CVI environment. The header file is available at C:\Program Files\National Instruments\CVI2010\include\cvinetv.h, and the .lib file located at C:\Program Files\National Instruments\CVI2010\extlib\msvc\cvinetv.lib can be linked in with whatever C development tools you are using.