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As mex-files do not make the distinction between single and double quoted strings within Octave, there is perhaps less complexity in the use of strings and character matrices in mex-files. An example of their use, that parallels the demo in stringdemo.cc, is given in the file mystring.c, as seen below.
     #include <string.h>
     #include "mex.h"
     
     void
     mexFunction (int nlhs, mxArray *plhs[], int nrhs, 
                  const mxArray *prhs[])
     {
       mwIndex i, j;
       mwSize m, n;
       mxChar *pi, *po;
     
       if (nrhs != 1 || ! mxIsChar (prhs[0]) || 
           mxGetNumberOfDimensions (prhs[0]) > 2)
         mexErrMsgTxt ("expecting char matrix");
     
       m = mxGetM (prhs[0]);
       n = mxGetN (prhs[0]);
       pi = mxGetChars (prhs[0]);
       plhs[0] = mxCreateNumericMatrix (m, n, mxCHAR_CLASS, 
                                        mxREAL);
       po = mxGetChars (plhs[0]);
     
       for (j = 0; j < n; j++)
         for (i = 0; i < m; i++)
           po [j*m + m - 1 - i] = pi [j*m + i];
     }
     
An example of its expected output is
     mystring(["First String"; "Second String"])
     ⇒ s1 = Second String
             First String
   Other functions in the mex interface for handling character strings are
mxCreateString, mxArrayToString, and
mxCreateCharMatrixFromStrings.  In a mex-file, a character string
is considered to be a vector rather than a matrix.  This is perhaps an
arbitrary distinction as the data in the mxArray for the matrix is
consecutive in any case.