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This page describes an example of how to call liboctave functions from a Fortran program. In the example we will load an Octave array from a file in Octave's native ASCII format, it consists of two steps:

  • write a C++ function with a C compatible interface and C linkage that reads a variable from an Octave ASCII file
  • write Fortran code using the "iso_c_binding" intrinsic module to call the C++ function

C++ function

Code: C++ function to load a matrix from an ASCII file in Octave native format
#include <octave_file_io.h>

#include <octave/oct.h>
#include <octave/ov.h>
#include <fstream>
#include <ls-oct-ascii.h>
#include <octave/octave.h>
#include <octave/parse.h>
#include <octave/toplev.h>
#include <octave/load-save.h>
#include <octave/oct-map.h>
#include <cstring>

int octave_load (const char* filename, const char* varname, double** data, int* numel)
  string_vector argv (1);
  load_save_format format = LS_ASCII;

  install_types ();  
  argv(0) = varname;
  std::ios::openmode mode = std::ios::in | std::ios::binary;
  std::string fname (filename);

  std::ifstream file (fname.c_str (), mode);
  octave_scalar_map m = do_load (file, fname, format, oct_mach_info::flt_fmt_unknown,
                                 false, false, true, argv, 0, 1, 1).scalar_map_value ();

  file.close ();  

  Array<double> M = m.contents (varname).array_value ();
  if (error_state)
    return -1;

  *data = (double*) malloc (M.numel () * sizeof (double));
  *numel = M.numel ();

  memcpy (*data, M.fortran_vec (), *numel * sizeof (double));

  return 0;  

Header file

Code: octave_file_io.h: header file with C interface to

#ifdef __cplusplus
extern "C" {
  int octave_load (const char* filename, const char* varname, double** data, int* numel);

#ifdef __cplusplus


Fortran Code

Code: octave_file_io_example.f90
program octave_file_io_example
  use iso_c_binding
  implicit none

     function octave_load (filename, varname, data, numel) bind(c, name="octave_load")
       use iso_c_binding
       implicit none

       integer(c_int) :: octave_load
       character(kind=c_char), intent(in) :: filename(*)
       character(kind=c_char), intent(in) :: varname(*)
       type(c_ptr), intent(out) :: data
       integer(c_int), intent(out) :: numel
     end function octave_load
  end interface
  integer(c_int) :: res 
  type(c_ptr) :: data
  real(c_double), pointer :: fdata(:)
  integer(c_int) :: numel

  res = octave_load (c_char_'pippo.octxt' // c_null_char, c_char_'A' // c_null_char, data, numel)
  call c_f_pointer (data, fdata, (/numel/))
  write (*,*) fdata

end program octave_file_io_example

Compiling the code

 mkoctfile -I. 
 mkoctfile -I. --mkoctfile --link-stand-alone octave_file_io_example.f90 octave_file_io.o -o octave_file_io_example