A.2.1 Getting Started with Mex-Files

The basic command to build a mex-file is either mkoctfile --mex or mex. The first can either be used from within Octave or from the command line. However, to avoid issues with matlab's own mex command, the use of the command mex is limited to within Octave.

One important difference with the use of mex between matlab and Octave is that the header file "matrix.h" is implicitly included through the inclusion of "mex.h". This is to avoid a conflict with the Octave file "Matrix.h" with operating systems and compilers that don't distinguish between filenames in upper and lower case

Consider the short example:

     #include "mex.h"
     
     void
     mexFunction (int nlhs, mxArray *plhs[], int nrhs, 
                  const mxArray *prhs[])
     {
       mxArray *v = mxCreateDoubleMatrix (1, 1, mxREAL);
       double *data = mxGetPr (v);
       *data = 1.23456789;
       plhs[0] = v;
     }

This simple example demonstrates the basics of writing a mex-file. The entry point into the mex-file is defined by mexFunction. Note that the function name is not explicitly included in the mexFunction and so there can only be a single mexFunction entry point per-file. Also the name of the function is determined by the name of the mex-file itself. Therefore if the above function is in the file firstmexdemo.c, it can be compiled with

     mkoctfile --mex firstmexdemo.c

which creates a file firstmexdemo.mex. The function can then be run from Octave as

     firstmexdemo()
     ⇒ 1.2346

It should be noted that the mex-file contains no help string for the functions it contains. To document mex-files, there should exist an m-file in the same directory as the mex-file itself. Taking the above as an example, we would therefore have a file firstmexdemo.m that might contain the text

     %FIRSTMEXDEMO Simple test of the functionality of a mex-file.

In this case, the function that will be executed within Octave will be given by the mex-file, while the help string will come from the m-file. This can also be useful to allow a sample implementation of the mex-file within the Octave language itself for testing purposes.

Although we cannot have multiple entry points into a single mex-file, we can use the mexFunctionName function to determine what name the mex-file was called with. This can be used to alter the behavior of the mex-file based on the function name. For example if

     #include "mex.h"
     
     void
     mexFunction (int nlhs, mxArray *plhs[], int nrhs, 
                  const mxArray *prhs[])
     {
       const char *nm;
       nm = mexFunctionName ();
       mexPrintf ("You called function: %s\n", nm);
       if (strcmp (nm, "myfunc") == 0)
         mexPrintf ("This is the principal function\n", nm);
       return; 
     }

is in file myfunc.c, and it is compiled with

     mkoctfile --mex myfunc.c
     ln -s myfunc.mex myfunc2.mex

Then as can be seen by

     myfunc()
     ⇒ You called function: myfunc
         This is the principal function
     myfunc2()
     ⇒ You called function: myfunc2

the behavior of the mex-file can be altered depending on the functions name.

Allow the user should only include mex.h in their code, Octave declares additional functions, typedefs, etc., available to the user to write mex-files in the headers mexproto.h and mxarray.h.