#### A.1.1 Getting Started with Oct-Files

The basic command to build oct-files is `mkoctfile` and it can be call from within octave or from the command line.

— Command: mkoctfile [-options] file ...

> The `mkoctfile` function compiles source code written in C, C++, or Fortran. Depending on the options used with `mkoctfile`, the compiled code can be called within Octave or can be used as a stand-alone application.

> `mkoctfile` can be called from the shell prompt or from the Octave prompt.

> `mkoctfile` accepts the following options, all of which are optional except for the file name of the code you wish to compile:

> ‘`-I DIR`’
>   Add the include directory DIR to compile commands.
> ‘`-D DEF`’
>   Add the definition DEF to the compiler call.
> ‘`-l LIB`’
>   Add the library LIB to the link command.
> ‘`-L DIR`’
>   Add the library directory DIR to the link command.
> ‘`-M`’
> ‘`--depend`’
>   Generate dependency files (.d) for C and C++ source files.
> ‘`-R DIR`’
>   Add the run-time path to the link command.
> ‘`-Wl,...`’
>   Pass flags though the linker like "-Wl,-rpath=...". The quotes are needed since commas are interpreted as command separators.
> ‘`-W...`’
>   Pass flags though the compiler like "-Wa,OPTION".
> ‘`-c`’
>   Compile but do not link.
> ‘`-g`’
>   Enable debugging options for compilers.
> ‘`-o FILE`’
> ‘`--output FILE`’
>   Output file name. Default extension is .oct (or .mex if ‘`--mex`’ is specified) unless linking a stand-alone executable.
> ‘`-p VAR`’
> ‘`--print VAR`’
>   Print the configuration variable VAR. Recognized variables are:

>                         ALL_CFLAGS                FFTW_LIBS
>                         ALL_CXXFLAGS              FLIBS
>                         ALL_FFLAGS                FPICFLAG
>                         ALL_LDFLAGS               INCFLAGS
>                         BLAS_LIBS                 LDFLAGS
>                         CC                        LD_CXX
>                         CFLAGS                    LD_STATIC_FLAG
>                         CPICFLAG                  LFLAGS
>                         CPPFLAGS                  LIBCRUFT
>                         CXX                       LIBOCTAVE
>                         CXXFLAGS                  LIBOCTINTERP
>                         CXXPICFLAG                LIBREADLINE
>                         DEPEND_EXTRA_SED_PATTERN  LIBS
>                         DEPEND_FLAGS              OCTAVE_LIBS
>                         DL_LD                     RDYNAMIC_FLAG
>                         DL_LDFLAGS                RLD_FLAG
>                         F2C                       SED
>                         F2CFLAGS                  XTRA_CFLAGS
>                         F77                       XTRA_CXXFLAGS
>                         FFLAGS

> ‘`--link-stand-alone`’
>   Link a stand-alone executable file.
> ‘`--mex`’
>   Assume we are creating a MEX file. Set the default output extension to ".mex".
> ‘`-s`’
> ‘`--strip`’
>   Strip the output file.
> ‘`-v`’
> ‘`--verbose`’
>   Echo commands as they are executed.
> ‘`file`’
>   The file to compile or link. Recognized file types are

>                                        .c    C source
>                                        .cc   C++ source
>                                        .C    C++ source
>                                        .cpp  C++ source
>                                        .f    Fortran source (fixed form)
>                                        .F    Fortran source (fixed form)
>                                        .f90  Fortran source (free form)
>                                        .F90  Fortran source (free form)
>                                        .o    object file
>                                        .a    library file

Consider the short C++ example:

         #include <octave/oct.h>

         DEFUN_DLD (helloworld, args, nargout,
           "Hello World Help String")
         {
           int nargin = args.length ();
           octave_stdout << "Hello World has " << nargin
                 << " input arguments and "
                 << nargout << " output arguments.\n";
           return octave_value_list ();
         }

This example although short introduces the basics of writing a C++ function that can be dynamically linked to Octave. The easiest way to make available most of the definitions that might be necessary for a C++ oct-file in Octave is to use the `#include <octave/oct.h>` header. Note that `octave/oct.h` is a C++ header and cannot be directly `#include`'ed in a C source file, nor any other language. What follows is mostly C++, with a discussion of other languages in section Calling External Code from Oct-Files.

The macro that defines the entry point into the dynamically loaded function is `DEFUN_DLD`. This macro takes four arguments, these being

1. The function name as it will be seen in Octave,
2. The list of arguments to the function of type `octave_value_list`,
3. The number of output arguments, which can and often is omitted if not used, and
4. The string that will be seen as the help text of the function.

The return type of functions defined with `DEFUN_DLD` is always `octave_value_list`.

There are a couple of important considerations in the choice of function name. Firstly, it must be a valid Octave function name and so must be a sequence of letters, digits and underscores, not starting with a digit. Secondly, as Octave uses the function name to define the filename it attempts to find the function in, the function name in the `DEFUN_DLD` macro must match the filename of the oct-file. Therefore, the above function should be in a file `helloworld.cc`, and it should be compiled to an oct-file using the command

         mkoctfile helloworld.cc

This will create a file called `helloworld.oct`, that is the compiled version of the function. It should be noted that it is perfectly acceptable to have more than one `DEFUN_DLD` function in a source file. However, there must either be a symbolic link to the oct-file for each of the functions defined in the source code with the `DEFUN_DLD` macro or the autoload (Function Files) function should be used.

The rest of this function then shows how to find the number of input arguments, how to print through the octave pager, and return from the function. After compiling this function as above, an example of its use is

         helloworld (1, 2, 3)
         -| Hello World has 3 input arguments and 0 output arguments.