API changes in R2019a?

I have a (Win64) C# software package allowing users to transfer data to MATLAB for user defined processing through a MATLAB script and transfer back the results.

It relies on the MATLAB API with libeng.dll etc, and was working fine with R2018b, but has stopped working with R2019a. I changed nothing, so assume something changed with the MATLAB API, but can find no mention of such a change here or on the 'News' pages.

Any bright ideas?

[James Tursa]

Did you recompile it with R2019a, or are you using the previously compiled code?

Answer 1

[James Tursa]

I don't know if this is the cause of your problems, but there was a change to the low level mxArray variable structure in R2019a. So code might have to be recompiled in order to work.

[Bo]

Thanks for the feedback. I recompiled the C# program (in VisualStudio 2017), changing nothing in the code that worked together with MATLAB R2018b.

Trying to singlestep my way through it seems to crash when I call engOpen from Libeng.dll. No error message or other hint about the nature of the problem, it just crashes. So I never even reach the part of my code that might be affected by changes in the lowlevel mxArray structure.

But where did you find that info, perhaps there are other changes that I'm not aware of?

[Bruno Luong]

James, what is the low-level change of mxArray? I did not notice and my old mex files are still working without recompile them.

[James Tursa]

Documented in my mex MATLAB API version FEX submission here (in the header file):

https://www.mathworks.com/matlabcentral/fileexchange/67016-c-mex-matlab-version

And here is the low level change for R2019a:

 * R2019a - First version where the CrossLink field in the mxArray header
 *          has been moved next to the reverse CrossLink field. I.e.,
 *          R2018b-:
 *                  mxArray *RevCrossLink;
 *                  mxClassID ClassID;
 *                  int VariableType;
 *                  mxArray *CrossLink;
 *                     :
 *                    etc.
 *          R2019a+:
 *                  mxArray *RevCrossLink;
 *                  mxArray *CrossLink;
 *                  mxClassID ClassID;
 *                  int VariableType;
 *                     :
 *                    etc.

If you are only calling API functions that deal with this in the background then I can easily see that this may not affect your mex routines.

[Bruno Luong]

OK so they change the order of the fields. I can't see the motivation. The aligment is more or less identical for 64-bit pointer (all MATLAB) no?

That looks odd.

[James Tursa]

Since ClassID and VariableType are both ints they occupy 4 bytes each. Having them next to each other means they won't mess up any 8-byte boundary alignments for the stuff around them. So, yes, I agree that the compiler won't need to add any padding in either the old or the new definitions to maintain 8-byte boundary alignments of the nearby fields.

Motivation for the change? I suppose the new definition looks nicer having the crosslink stuff next to each other. But I suspect the real motivation is just to irritate me and mess up my mex hacking code ...