#### 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.

— Function File: mex [options] file ...

> Compile source code written in C, C++, or Fortran, to a MEX file. This is equivalent to `mkoctfile --mex [options] file`.

> See also: mkoctfile.

— Function File: mexext ()

> Return the filename extension used for MEX files.

> See also: mex.

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`.