Fortran

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

C++ function to load a single matrix, stored in ASCII format, from a data file.

File: octave_file_io.cc

// Octave header
#include <octave/oct.h>
#include <octave/ls-mat-ascii.h>

// Custom header containing the C compatible interface
#include <octave_file_io.h>

//! Load a single matrix, stored in ASCII format, from a data file.
//!
//! @param file_name name of the data file.
//! @param data pointer to the read-in matrix stored as fortran vector
//!             (column-major order).
//! @param numel number of elements in @p data.

int octave_load (const char* file_name, double** data, int* numel)
{
  // Define variable to hold the read data.
  octave_value read_data;

  // Read a plain ASCII matrix from data file.
  std::ifstream in_file_stream (file_name, std::ios::binary);
  read_mat_ascii_data (in_file_stream, file_name, read_data);
  in_file_stream.close ();

  // Convert read data to numerical array (matrix).
  NDArray A = read_data.array_value ();

  // Extract number of elements in matrix A.
  *numel = A.numel ();

  // Allocate memory to pointer to returned values.
  *data  = (double*) malloc (A.numel () * sizeof (double));

  // Copy the content of matrix A to data structure Fortran can handle.
  memcpy (*data, A.fortran_vec (), A.numel () * sizeof (double));

  return 0;
}

Header file

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

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

Fortran Code

Code: octave_file_io_example.f90

program octave_file_io_example use iso_c_binding implicit none interface 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. octave_file_io.cc 
 mkoctfile -I. --mkoctfile --link-stand-alone octave_file_io_example.f90 octave_file_io.o -o octave_file_io_example