How to call mexCallMATLAB on a function that has inputs of different types?

I would like to call the following function from a mex file:

    norm(X, 'fro')

X is a double matrix. 'fro' is obviously a character array. How would I construct an input array, such to be able to use mexCallMATLAB, i.e.:

    mexCallMATLAB(1, normOutputs, 2, normInputs, "norm")

Since the inputs have different types (mxArray and character array), I'm not sure how to construct an array of pointers to them. Help would be greatly appreciated!

Answer 1

[James Tursa]

mxArray *normInputs[2];
mxArray *normOutputs[1];
normInputs[0] = mxCreateDoubleMatrix(2,2,mxREAL); // a double matrix
// code to fill in normInputs[0]
normInputs[1] = mxCreateString("fro");
mexCallMATLAB(1,normOutputs,2,normInputs,"norm");
mxDestroyArray(normInputs[1]);
mxDestroyArray(normInputs[0]);
// use normOutputs[0]
mxDestroyArray(normOutputs[0]);

[Robert]

Thanks! Trying it out now...

I have a small "side question" though. I noticed that many people use "//" to mark single-line comments, but my compiler complains about these. Is there some setup step needed to allow "//" to denote comments?

[James Tursa]

You might try adding the c99 flag. E.g. add this to the mex command:

   CFLAGS="\$CFLAGS -std=c99"

[Robert]

Thanks, that worked! Back to the original question...

I ran into a problem where MATLAB was hanging after executing the compiled mex function. In my debugger, I get this error:

* glibc detected * /usr/local/MATLAB/R2013a/bin/glnxa64/MATLAB: double free or corruption (fasttop): 0x00007fffdc0279f0 *

This occurs on the line "mxDestroyArray(normOutputs[0]);". Any ideas what might be the problem?

[James Tursa]

Hmmm ... I thought norm always returned an answer. You can try this:

if( normOutputs[0] ) {
    mxDestroyArray(normOutputs[0]);
}

Or just remove the mxDestroyArray(normOutputs[0]) line completely and let the garbage collection handle it.

Neither of these answers is satisfactory to me, however, since to my understanding of the mexCallMATLAB call you should get a normOutputs[0] result. Don't know why this would not be the case. Are you sure you haven't already called mxDestroyArray(normOutputs[0]) prior to this line?

[Robert]

I figured out the problem! I am running this code in a loop, and the problem actually occurred the second time mxDestroyArray(normOutputs[0]) was called. So, if I put this line outside of the loop, the error goes away.

Answer 2

[Jan]

No, the inputs do not have different types and 'fro' is not a char vector, although it looks like it is one. Both variables must be mxArrays to be valid input arguments for mxCallMATLAB. So the 2nd normInput must be an mxArray pointer of the type mxCHAR (as shonw in James' example already):

normInput[1] = mxCreateString('fro');