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

Oct-files are pieces of C++ code that have been compiled with the Octave API into a dynamically loadable object. They take their name from the file which contains the object which has the extension `.oct`.

Finding a C++ compiler, using the correct switches, adding the right include paths for header files, etc. is a difficult task. Octave automates this by providing the `mkoctfile` command with which to build oct-files. The command is available from within Octave or at the shell command line.

mkoctfile [-options] file …
[output, status] = mkoctfile (…)

  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. Calling it from the Octave prompt simply delegates the call to the shell prompt. The output is stored in the output variable and the exit status in the status variable.

  `mkoctfile` accepts the following options, all of which are optional except for the filename 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 options to the linker like "-Wl,-rpath=…". The quotes are needed since commas are interpreted as command separators.

  ‘`-W…`’

    Pass options to the assembler like "-Wa,OPTION".

  ‘`-c`’

    Compile but do not link.

  ‘`-g`’

    Enable debugging options for compilers.

  ‘`-o FILE`’
  ‘`--output FILE`’

    Output filename. 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                  INCFLAGS
           ALL_CXXFLAGS                INCLUDEDIR
           ALL_FFLAGS                  LAPACK_LIBS
           ALL_LDFLAGS                 LD_CXX
           AR                          LDFLAGS
           BLAS_LIBS                   LD_STATIC_FLAG
           CC                          LFLAGS
           CFLAGS                      LIBDIR
           CPICFLAG                    LIBOCTAVE
           CPPFLAGS                    LIBOCTINTERP
           CXX                         LIBS
           CXXFLAGS                    OCTAVE_HOME
           CXXPICFLAG                  OCTAVE_LIBS
           DEPEND_EXTRA_SED_PATTERN    OCTAVE_LINK_DEPS
           DEPEND_FLAGS                OCTAVE_LINK_OPTS
           DL_LD                       OCTAVE_PREFIX
           DL_LDFLAGS                  OCTINCLUDEDIR
           F77                         OCTLIBDIR
           F77_INTEGER8_FLAG           OCT_LINK_DEPS
           FFLAGS                      OCT_LINK_OPTS
           FFTW3F_LDFLAGS              RANLIB
           FFTW3F_LIBS                 RDYNAMIC_FLAG
           FFTW3_LDFLAGS               READLINE_LIBS
           FFTW3_LIBS                  SED
           FFTW_LIBS                   SPECIAL_MATH_LIB
           FLIBS                       XTRA_CFLAGS
           FPICFLAG                    XTRA_CXXFLAGS

  ‘`--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 following short example which introduces the basics of writing a C++ function that can be linked to Octave.

    #include <octave/oct.h>

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

      // Return empty matrices for any outputs
      octave_value_list retval (nargout);
      for (int i = 0; i < nargout; i++)
        retval(i) = octave_value (Matrix ());

      return retval;
    }

The first critical line is `#include <octave/oct.h>` which makes available most of the definitions necessary for a C++ oct-file. Note that `octave/oct.h` is a C++ header and cannot be directly `#include`’ed in a C source file, nor any other language.

Included by `oct.h` is a definition for the macro `DEFUN_DLD` which creates a dynamically loaded function. This macro takes four arguments:

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 be—and often is—omitted if not used, and
4. The string to use for 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. First, it must be a valid Octave function name and so must be a sequence of letters, digits, and underscores not starting with a digit. Second, 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 would 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 the function shows how to find the number of input arguments, how to print through the Octave pager, and how to 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.

Subsequent sections show how to use specific classes from Octave’s core internals. Base classes like `dMatrix` (a matrix of double values) are found in the directory `liboctave/array`. The definitive reference for how to use a particular class is the header file itself. However, it is often enough simply to study the examples in the manual in order to be able to use a class.