MATLAB: Struct as input to mex file

MATLABmexstruct

I want to provide a struct as an input to mex function in C and then use the data stored as in a particular field in the computational routine. The C code is as follows:
#include "mex.h"
#include <math.h>
#include <string.h>
double times_two(double W0[]){
    double A;
    return A = 2*W0[0];
}
void mexFunction(int nlhs, mxArray * plhs[], int nrhs,
                 const mxArray * prhs[]) {
    double A, *W0;
    double *tmp;
    mwIndex idx = 1;
    int ifield, nfields;
    const char **fname;       /* pointers to field names */
      nfields = mxGetNumberOfFields(prhs[0]);
      fname = mxCalloc(nfields, sizeof(*fname));
      for (ifield=0; ifield< nfields; ifield++){
          fname[ifield] = mxGetFieldNameByNumber(prhs[0],ifield);
          if (strcmp(fname[ifield],"W0") == 0){
              tmp = mxGetPr(mxGetFieldByNumber(prhs[0],idx,ifield));
              W0 = tmp;
          }
      }
      A = times_two(W0);
      mexPrintf("A = %%f",A);
  }
The matlab code for calling this would be something like:
a.W0 = 1.5;
a.somethingElse = 2;
test_struct(a); % Mex function
The mex function compiles successfully. However, at runtime, I get a segmentation fault at W0 = tmp; What am I doing wrong here?

Best Answer

You are passing in a 1x1 struct, so the only valid index inside a mex routine is 0 since the indexing is 0-based. So change this line:
    mwIndex idx = 1;
to this:
    mwIndex idx = 0;
And to make your program more robust to unexpected inputs, I would advise putting in many other checks. E.g., check to see that the number of inputs and outputs is as required, that the input variable is in fact a struct, that the number of elements of this struct is as expected, that the field element extracted is not NULL, that the field element is a non-empty double, etc etc.
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