A.2.5 Structures with Mex-Files

The basic function to create a structure in a mex-file is mxCreateStructMatrix which creates a structure array with a two dimensional matrix, or mxCreateStructArray.

mxArray *mxCreateStructArray (int ndims, int *dims,
                              int num_keys,
                              const char **keys);
mxArray *mxCreateStructMatrix (int rows, int cols,
                               int num_keys,
                               const char **keys);

Accessing the fields of the structure can then be performed with mxGetField and mxSetField or alternatively with the mxGetFieldByNumber and mxSetFieldByNumber functions.

mxArray *mxGetField (const mxArray *ptr, mwIndex index,
                     const char *key);
mxArray *mxGetFieldByNumber (const mxArray *ptr,
                             mwIndex index, int key_num);
void mxSetField (mxArray *ptr, mwIndex index,
                 const char *key, mxArray *val);
void mxSetFieldByNumber (mxArray *ptr, mwIndex index,
                         int key_num, mxArray *val);

A difference between the oct-file interface to structures and the mex-file version is that the functions to operate on structures in mex-files directly include an index over the elements of the arrays of elements per field; Whereas, the oct-file structure includes a Cell Array per field of the structure.

An example that demonstrates the use of structures in a mex-file can be found in the file mystruct.c shown below.

#include "mex.h"

void
mexFunction (int nlhs, mxArray *plhs[],
             int nrhs, const mxArray *prhs[])
{
  int i;
  mwIndex j;
  mxArray *v;
  const char *keys[] = { "this", "that" };

  if (nrhs != 1 || ! mxIsStruct (prhs[0]))
    mexErrMsgTxt ("ARG1 must be a struct");

  for (i = 0; i < mxGetNumberOfFields (prhs[0]); i++)
    for (j = 0; j < mxGetNumberOfElements (prhs[0]); j++)
      {
        mexPrintf ("field %s(%d) = ", mxGetFieldNameByNumber (prhs[0], i), j);
        v = mxGetFieldByNumber (prhs[0], j, i);
        mexCallMATLAB (0, NULL, 1, &v, "disp");
      }

  v = mxCreateStructMatrix (2, 2, 2, keys);

  mxSetFieldByNumber (v, 0, 0, mxCreateString ("this1"));
  mxSetFieldByNumber (v, 0, 1, mxCreateString ("that1"));
  mxSetFieldByNumber (v, 1, 0, mxCreateString ("this2"));
  mxSetFieldByNumber (v, 1, 1, mxCreateString ("that2"));
  mxSetFieldByNumber (v, 2, 0, mxCreateString ("this3"));
  mxSetFieldByNumber (v, 2, 1, mxCreateString ("that3"));
  mxSetFieldByNumber (v, 3, 0, mxCreateString ("this4"));
  mxSetFieldByNumber (v, 3, 1, mxCreateString ("that4"));

  if (nlhs)
    plhs[0] = v;
}

An example of the behavior of this function within Octave is then

a(1).f1 = "f11"; a(1).f2 = "f12";
a(2).f1 = "f21"; a(2).f2 = "f22";
b = mystruct (a);
⇒  field f1(0) = f11
    field f1(1) = f21
    field f2(0) = f12
    field f2(1) = f22
b
⇒ 2x2 struct array containing the fields:

     this
     that

b(3)
⇒ scalar structure containing the fields:

     this = this3
     that = that3