MEX Function - Using mexCallMATLAB function in conjunction with "PolyFit"

I am just starting to work with MEX functions, and have run into a situation that I am currently unable to solve. I am trying to use the mexGetMATLAB function with "polyfit" and an not able to pass in the correct array of pointers for the input. My call is as follows :

mexCallMATLAB(0,NULL,3,Plot_Pointer,"polyfit");

where Plot_Pointer is defined as

mxArray *Plot_Pointer[3] = {pm, pp, fit_order}; and pm, pp and fit_order are defined as mxArray *pm[1], *pp[1], *fit_order[1]

The snipit of code that sets up all the required inputs (sample data for now just to try and get it working) is as follows:

        mwSize m, n;
  mxArray *rhs[1], *lhs[1], *pm[1], *pp[1], *fit_order[1];
        mxArray *Plot_Pointer[3] = {pm, pp, fit_order};
  pm[0] = mxCreateDoubleMatrix(10, 1, mxREAL);
  pp[0] = mxCreateDoubleMatrix(10, 1, mxREAL);
  fit_order[0] = mxCreateDoubleMatrix(1, 1, mxREAL);
  pmp = mxGetPr(pm[0]);
  ppp = mxGetPr(pp[0]);
  myarray = mxGetPr(fit_order[0]);
  for (i=0; i<m; i++)
    pmp[i] = i*(4*3.14159/10);
  for (i=0; i<m; i++)
    ppp[i] = -i*(4*3.14159/10);
  myarray[0] = 2.0;
  Plot_Pointer[0] = pm;
  Plot_Pointer[1] = pp;
  Plot_Pointer[2] = fit_order;
        mexCallMATLAB(0,NULL,3,Plot_Pointer,"polyfit");

This code compiles without issue, but causes Matlab to crash when executed. I am confident that it is an issue with the definition of Plot_Pointer, but I do not have enough knowledge of the subject at this point to figure out what. I am able to execute other mexGetMATLAB functions that only require 1 input, but have failed to figured out how to handle inputs that require more than one value. Any help would be appreciated.

Answer 1

[Michael Stackpole]

Well, I managed to figure the problem out. The updated code is below :

   Plot_Pointer[0] = *pm;
  Plot_Pointer[1] = *pp;
  Plot_Pointer[2] = *fit_order;

        mexCallMATLAB(3,OutFit,3,Plot_Pointer,"polyfit");

Everything works as expected now. Just wanted to put the final solution out there in case anyone runs into the same problem that I did and wants to call the PolyFit function from MEX:

  mxArray *rhs[1], *lhs[1], *pm[1], *pp[1], *fit_order[1];
        mxArray *Fit_Pointer[3] ;

  mxArray *OutFit[3];
  pm[0] = mxCreateDoubleMatrix(m, n, mxREAL);
  pp[0] = mxCreateDoubleMatrix(m, n, mxREAL);
  fit_order[0] = mxCreateDoubleScalar(2.0);   
  pmp = mxGetPr(pm[0]);
  ppp = mxGetPr(pp[0]);

  for (i=0; i<m; i++)
    pmp[i] = i*(4*3.14159/10);
  for (i=0; i<m; i++)
    ppp[i] = -i*(4*3.14159/10);

  Fit_Pointer[0] = *pm;
  Fit_Pointer[1] = *pp;
  Fit_Pointer[2] = *fit_order; 

  mexCallMATLAB(3,OutFit,3,Fit_Pointer,"polyfit");

Answer 2

[Friedrich]

Hi,

not 100% sure but polyfit return always at least one variable which you don't capture. Maybe this causes the crash?

Answer 3

[Michael Stackpole]

Friedrich,

I did notice that and updated the code as follows :

     mxArray *rhs[1], *lhs[1], *pm[1], *pp[1], *fit_order[1];
     mxArray *Plot_Pointer[3] ;
     mxArray *OutFit[3];
  pm[0] = mxCreateDoubleMatrix(m, n, mxREAL);
  pp[0] = mxCreateDoubleMatrix(m, n, mxREAL);
  fit_order[0] = mxCreateDoubleScalar(2.0);   
  pmp = mxGetPr(pm[0]);
  ppp = mxGetPr(pp[0]);
  for (i=0; i<m; i++)
    pmp[i] = i*(4*3.14159/10);
  for (i=0; i<m; i++)
    ppp[i] = -i*(4*3.14159/10);
  Plot_Pointer[0] = pm;
  Plot_Pointer[1] = pp;
  Plot_Pointer[2] = fit_order; 
        mexCallMATLAB(3,OutFit,3,Plot_Pointer,"polyfit");

but still have the same issue. I am still trying some other changes, but still a little over my head right now.

Answer 4

[James Tursa]

You don't need to make pm, pp, and fit_order arrays. Try this:

       double *pmp, *ppp;
       mxArray *rhs[1], *lhs[1], *pm, *pp, *fit_order;
       mxArray *Plot_Pointer[3] ;
       mxArray *OutFit[3];
    pm = mxCreateDoubleMatrix(m, n, mxREAL);
    pp = mxCreateDoubleMatrix(m, n, mxREAL);
    fit_order = mxCreateDoubleScalar(2.0);   
    pmp = mxGetPr(pm);
    ppp = mxGetPr(pp);
    for (i=0; i<m; i++)
      pmp[i] = i*(4*3.14159/10);
    for (i=0; i<m; i++)
      ppp[i] = -i*(4*3.14159/10);
    Plot_Pointer[0] = pm;
    Plot_Pointer[1] = pp;
    Plot_Pointer[2] = fit_order; 
          mexCallMATLAB(3,OutFit,3,Plot_Pointer,"polyfit");

Also, I would note that you are not setting the entire contents of pm and pp with data (they are sized m*n but you are only filling m elements) ... so the remainder is garbage which you then pass to polyfit.