Editing Fortran

This page describes an example of how to call liboctave functions from a Fortran program.
In the example we will load a single matrix, stored in ASCII format, from a data file.
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

=== Data file ===

{{File|data.txt|<syntaxhighlight lang="text">
 1.59797350e-01 5.41307474e-01 1.12127655e-01 2.09249248e-01
 3.22564589e-01 7.94307947e-01 8.25924316e-01 5.38352076e-01
 3.63990736e-01 1.90371212e-02 2.89370865e-01 1.30131246e-01
 6.28360462e-01 1.98831330e-01 6.89539723e-01 6.91062597e-01
</syntaxhighlight>}}

The file was generated with

<syntaxhighlight lang="octave">
A = rand (4);
save -ascii data.txt A
</syntaxhighlight>

=== C++ function ===

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

{{File|octave_file_io.cc|<syntaxhighlight lang="C++">
// 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;
}
</syntaxhighlight>}}

=== Header file ===

Header file with C interface to {{Path|octave_file_io.cc}}.

{{File|octave_file_io.h|<syntaxhighlight lang="C">
#ifndef OCTAVE_FILE_IO_H
#define OCTAVE_FILE_IO_H

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

#ifdef __cplusplus
}
#endif

#endif
</syntaxhighlight>}}

=== Fortran Code ===

{{Code|octave_file_io_example.f90|<syntaxhighlight lang="fortran" style="font-size:13px">
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
</syntaxhighlight>}}




=== Compiling the code ===

Generate {{Path|octave_file_io.o}} from {{Path|octave_file_io.cc}}.

 mkoctfile -I. -c octave_file_io.cc

Generate {{Path|octave_file_io_example.exe}} from {{Path|octave_file_io_example.f90}} including {{Path|octave_file_io.o}}.

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

If you receive errors about missing libraries, make sure your <code>LD_LIBRARY_PATH</code> is set correctly to find all Octave libraries, e.g.

 $ ldd ./octave_file_io_example.exe
 ...
        libgfortran.so.4 => /usr/lib64/libgfortran.so.4 (0x00007fe9eb62b000)
        liboctinterp.so.8 => not found
        liboctave.so.8 => not found
 ...

Then find {{Path|liboctinterp.so.8}} and {{Path|liboctave.so.8}} on your system and type

 export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/lib/octave/{{Release}}/ 


[[Category:Examples]]