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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.

Command: mkoctfile [-options] file …
Function 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 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                FFTW3F_LIBS
   ALL_CXXFLAGS              FLIBS
   ALL_FFLAGS                FPICFLAG
   ALL_LDFLAGS               INCFLAGS
   BLAS_LIBS                 LAPACK_LIBS
   CC                        LDFLAGS
   CFLAGS                    LD_CXX
   CPICFLAG                  LD_STATIC_FLAG
   CPPFLAGS                  LFLAGS
   CXX                       LIBOCTAVE
   CXXFLAGS                  LIBOCTINTERP
   CXXPICFLAG                LIBS
   DEPEND_EXTRA_SED_PATTERN  OCTAVE_LIBS
   DEPEND_FLAGS              OCTAVE_LINK_DEPS
   DL_LD                     OCT_LINK_DEPS
   DL_LDFLAGS                RDYNAMIC_FLAG
   EXEEXT                    READLINE_LIBS
   F77                       SED
   F77_INTEGER_8_FLAG        XTRA_CFLAGS
   FFLAGS                    XTRA_CXXFLAGS
   FFTW3_LDFLAGS
   FFTW3_LIBS
   FFTW3F_LDFLAGS

--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")
{
  int nargin = args.length ();

  octave_stdout << "Hello World has "
                << nargin << " input arguments and "
                << nargout << " output arguments.\n";

  return octave_value_list ();
}

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 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 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 just to study the examples in the manual in order to be able to use the class.


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