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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                  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")
     {
       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.