All zeros from mex function evaluation

Hello everyone,

I would like to create one mex function that implements a function defined in a C file.

The C code of the desired function is the following:

/* myFunc */

INTEGER myFunc(type_Md Mode, FLOAT* u1, FLOAT* u2)
{ ...
    return status;
}

This function is defined in a C file scr.c with some #include statements which I didn't write.

type_Md is an enumeration type.

To create the mex function I run a Matlab script that includes the needed C files and evaluates the mexFunction:

% Compile myFunc_mex
mexfile = 'myFunc_mex.c';
mexname = 'myFunc_mex';    

% Including all files
.....
    
libfile = 'library.lib';

% Compile in single precision
eval(['mex -Lsrc -l' libfile(1:end-4) ' ' mexfile ' -output ' mexname '_single']);

Based on the definition of myFunc(), I create the C file myFunc_mex.c like this:

#include "mex.h"
#include "math.h"
#include "scr.c"
#include ....

#define MXCLASS mxSINGLE_CLASS

void mexFunction(int nlhs, mxArray *plhs[],
                 int nrhs, const mxArray *prhs[])
{
    type_Md Mode;
    
    INTEGER status;
    FLOAT *u1, *u2;
    
    Mode = (type_Md)mxGetScalar(prhs[0]);
    
    plhs[0] = mxCreateNumericMatrix((mwSize)1,1,mxINT16_CLASS,0);  /* status */
    plhs[1] = mxCreateNumericMatrix((mwSize)1,1,MXCLASS,mxREAL);   /* u1 */
    plhs[2] = mxCreateNumericMatrix((mwSize)1,1,MXCLASS,mxREAL);   /* u2 */
    
    status = (INTEGER)mxGetScalar(plhs[0]);
    u1 = (FLOAT *)mxGetPr(plhs[1]);
    u2 = (FLOAT *)mxGetPr(plhs[2]);
    
    status = myFunc(Mode, u1, u2);

}

Running the Matlab script I don't get any error, the creation of the mex function is successful. But, evaluating the mex function,

[status,u1,u2] = feval('myFunc_mex_single',Mode1)

I don't get the right result, I get that status, u1 and u2 are ALL ZEROS.

Moreover, if in myFunc_mex.c I substitute the last row with

status = 5;

again I get all zeros. Then I would say that the problem is not (only) in the evaluation of myFunc(), but I'm wrong in defining myFunc_mex.c.

This is the first time I see a mex function, I don't know where the error would be.

Answer 1

[Walter Roberson]

In order to return values, you need to store the values inside the mx arrays that you are creating.

Your u1 and u2 look appropriate to be pointers to the area to write into, but you do not show any code that writes into them.

Your mxGetScalar is reading a value out of the mxINT16_CLASS array that you created. You did not store anything in there, so status is going to be 0 after the mxGetScalar. You then overwrite the local variable status with whatever the function returned, but you never write that status into the memory of that first array -- you would have needed an mxGetPr to get the pointer to it, and then you would have had to store status indirect on that pointer.

[Claudia Chiaro]

Thank you for your answer.

u1 and u2 are written inside myFunc() but are not returned.

I noticed that the mex function didn't get the input variable Mode. This is because I should have casted it into a double in the mex function call. With the cast u1 and u2 are correctly computed by the mex function and delivered to Matlab.

I checked also that status is right in the C code. How can I store status on that pointer?

[Walter Roberson]

#include "mex.h"
#include "math.h"
#include "scr.c"
#include ....

#define MXCLASS mxSINGLE_CLASS

void mexFunction(int nlhs, mxArray *plhs[],
                 int nrhs, const mxArray *prhs[])
{
    type_Md Mode;
    
    INTEGER *status_ptr;
    FLOAT *u1, *u2;
    
    Mode = (type_Md)mxGetScalar(prhs[0]);
    
    plhs[0] = mxCreateNumericMatrix((mwSize)1,1,mxINT16_CLASS,0);  /* status */
    plhs[1] = mxCreateNumericMatrix((mwSize)1,1,MXCLASS,mxREAL);   /* u1 */
    plhs[2] = mxCreateNumericMatrix((mwSize)1,1,MXCLASS,mxREAL);   /* u2 */
    
    status_ptr = (INTEGER*)mxGetPtr(plhs[0]);
    u1 = (FLOAT *)mxGetPr(plhs[1]);
    u2 = (FLOAT *)mxGetPr(plhs[2]);
    
    status_ptr[0] = myFunc(Mode, u1, u2);

}

[Claudia Chiaro]

Thank you very much. I forgot to update the post, it works.