GNU Octave: Character Strings in Mex-Files

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#### A.2.3 Character Strings in Mex-Files

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 shown below.

    #include <string.h>
    #include "mex.h"

    void
    mexFunction (int nlhs, mxArray *plhs[],
                 int nrhs, const mxArray *prhs[])
    {
      mwSize m, n;
      mwIndex i, j;
      mxChar *pi, *po;

      if (nrhs != 1 || ! mxIsChar (prhs[0])
          || mxGetNumberOfDimensions (prhs[0]) > 2)
        mexErrMsgTxt ("ARG1 must be a 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.