Matlab vs. C DLL

Question

I have an algorithm written in Matlab that I need to integrate into a LabVIEW VI.  In our current version of the software, we use a DLL written in C that I compiled in Visual Studio through a long period of trial and error.  I need to make some significant updates to our code and would much prefer to keep it in the Matlab environment that I’m more comfortable with.  Unfortunately, using a MathScript node is not an option due to the complexity of the algorithm and our desired processing speeds.
Initially I thought I would buy the Matlab Compiler and use it to create the DLL, which I’d then call in LabVIEW, problem solved.  However, after reading through several threads about Matlab DLLs on the discussion board, I am concerned about all of the issues that people seem to have had when trying to do this.  Can anyone offer some insight into whether these Matlab DLLs usually cause problems in LabVIEW VIs or if these are isolated incidents?  Do they always need to be called through a secondary C DLL?  Thanks in advance!

Answer 1

It's been a little while since the last time I did this, but I think I can answer some of your questions. One, there's a good chance that the c-dll will run faster than the Matlab dll, unless you are using some functions that were heavily optimized for Matlab. You can use the compiler to create a dll and call it from LabVIEW, but you will have to create some sort of interface to convert the datatypes (outside of LabVIEW). You may have better luck if you use an older version of Matlab. 6.5 and prior would actually generate compiled code, and as long as you have the 90 or so dlls installed correctly, would run very quickly. These compiled dll's were also easier to call, but still require an interface to LabVIEW. Since Matlab 7 (R14), the compiled code will run exactly like it does in Matlab (i.e., with the same performance), and is easier to get compiled and running (just install the runtime). Previously, not everything would compile. Now, if it runs in Matlab, you can use the Matlab compiler to create a dll. We've created an internal c-library of functions that allow us to create and handle the pointers to the Matlab datatypes so we can work with Matlab generated code all in LabVIEW, but it is still a hassle. Unfortunately, I can't share this library.
A better option, but not always feasible is to use Real Time Workshop to create c code from your Matlab code. This can be done quickly, and will generate an actual dll that has traditional datatypes. These can be called directly from LabVIEW. Unfortunately, RTW requires Simulink and Matlab, and all of that will set you back ~$15k if you don't already have it.
Of course, there is always the Matlab script node, which works fine as long as you don't mind having Matlab installed on your test machine. This is the way we usually go for initial testing or quick demonstrations. It's much easier to modify the Matlab code this way during debugging. I'm not positive, but I think you can build an executable in LabVIEW and still call the Matlab script node, which can target a script explicitly somewhere in the file system. That way, you can tweak the "analysis" portion of the code on a built executable even without rebuilding in LabVIEW.
Finally, the Mathscipt nodes. I wish they would work, but we rarely actually get anything to work in them. The problem is that everyone developing m files does so in Matlab, not Mathscript. Then when we try to take something they've built and get it running in Mathscript we are bled to death by a million little cuts. I don't think structures or cell arrays work, and it just seems like we always run up against a little annoying thing that keeps us from using them. We are even paying for the Mathscript RT licenses now (or soon anyway) just in case someday we get something to work. I'm not a Matlab guy myself, so I can't really explain the problems clearly, but it seems like the code always has to be re-done for Mathscript, and that is not something the Matlab guys are too enthusiastic about.
sorry if I rambled a bit.
Chris
edit: wow, all my linefeeds disappeared.. FIXED