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<a name="Calling-External-Code-from-Oct_002dFiles"></a>
<div class="header">
<p>
Next: <a href="Allocating-Local-Memory-in-Oct_002dFiles.html#Allocating-Local-Memory-in-Oct_002dFiles" accesskey="n" rel="next">Allocating Local Memory in Oct-Files</a>, Previous: <a href="Calling-Octave-Functions-from-Oct_002dFiles.html#Calling-Octave-Functions-from-Oct_002dFiles" accesskey="p" rel="prev">Calling Octave Functions from Oct-Files</a>, Up: <a href="Oct_002dFiles.html#Oct_002dFiles" accesskey="u" rel="up">Oct-Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Calling-External-Code-from-Oct_002dFiles-1"></a>
<h4 class="subsection">A.1.9 Calling External Code from Oct-Files</h4>

<p>Linking external C code to Octave is relatively simple, as the C
functions can easily be called directly from C++.  One possible issue is
that the declarations of the external C functions may need to be explicitly
defined as C functions to the compiler.  If the declarations of the
external C functions are in the header <samp>foo.h</samp>, then the tactic to
ensure that the C++ compiler treats these declarations as C code is
</p>
<div class="example">
<pre class="example">#ifdef __cplusplus
extern &quot;C&quot;
{
#endif
#include &quot;foo.h&quot;
#ifdef __cplusplus
}  /* end extern &quot;C&quot; */
#endif
</pre></div>

<p>Calling Fortran code, however, can pose more difficulties.  This is due to
differences in the manner in which compilers treat the linking of Fortran code
with C or C++ code.  Octave supplies a number of macros that allow consistent
behavior across a number of compilers.
</p>
<p>The underlying Fortran code should use the <code>XSTOPX</code> function to
replace the Fortran <code>STOP</code> function.  <code>XSTOPX</code> uses the Octave
exception handler to treat failing cases in the Fortran code
explicitly.  Note that Octave supplies its own replacement <small>BLAS</small>
<code>XERBLA</code> function, which uses <code>XSTOPX</code>.
</p>
<p>If the code calls <code>XSTOPX</code>, then the <code><span class="nolinebreak">F77_XFCN</span></code><!-- /@w -->
macro should be used to call the underlying Fortran function.  The Fortran
exception state can then be checked with the global variable
<code>f77_exception_encountered</code>.  If <code>XSTOPX</code> will not be called,
then the <code><span class="nolinebreak">F77_FCN</span></code><!-- /@w --> macro should be used instead to call the Fortran
code.
</p>
<p>There is no great harm in using <code><span class="nolinebreak">F77_XFCN</span></code><!-- /@w --> in all cases, except that
for Fortran code that is short running and executes a large number of times,
there is potentially an overhead in doing so.  However, if <code><span class="nolinebreak">F77_FCN</span></code><!-- /@w -->
is used with code that calls <code>XSTOP</code>, Octave can generate a
segmentation fault.
</p>
<p>An example of the inclusion of a Fortran function in an oct-file is
given in the following example, where the C++ wrapper is
</p>
<div class="example">
<pre class="verbatim">#include &lt;octave/oct.h&gt;
#include &lt;octave/f77-fcn.h&gt;

extern &quot;C&quot;
{
  F77_RET_T
  F77_FUNC (fortransub, FORTSUB)
    (const int&amp;, double*, F77_CHAR_ARG_DECL F77_CHAR_ARG_LEN_DECL);
}

DEFUN_DLD (fortrandemo, args, , &quot;Fortran Demo&quot;)
{
  octave_value_list retval;
  int nargin = args.length ();

  if (nargin != 1)
    print_usage ();
  else
    {
      NDArray a = args(0).array_value ();
      if (! error_state)
        {
          double *av = a.fortran_vec ();
          octave_idx_type na = a.numel ();
          OCTAVE_LOCAL_BUFFER (char, ctmp, 128);

          F77_XFCN (fortransub, FORTSUB,
                    (na, av, ctmp F77_CHAR_ARG_LEN (128)));

          retval(1) = std::string (ctmp);
          retval(0) = a;
        }
    }
  return retval;
}
</pre><pre class="example">

</pre></div>

<p>and the Fortran function is
</p>
<div class="example">
<pre class="verbatim">      subroutine fortransub (n, a, s)
      implicit none
      character*(*) s
      real*8 a(*)
      integer*4 i, n, ioerr
      do i = 1, n
        if (a(i) .eq. 0d0) then
          call xstopx ('fortransub: divide by zero')
        else
          a(i) = 1d0 / a(i)
        endif
      enddo
      write (unit = s, fmt = '(a,i3,a,a)', iostat = ioerr)
     $       'There are ', n,
     $       ' values in the input vector', char(0)
      if (ioerr .ne. 0) then
        call xstopx ('fortransub: error writing string')
      endif
      return
      end

</pre><pre class="example">

</pre></div>

<p>This example demonstrates most of the features needed to link to an
external Fortran function, including passing arrays and strings, as well
as exception handling.  Both the Fortran and C++ files need to be compiled
in order for the example to work.
</p>
<div class="example">
<pre class="example">mkoctfile fortrandemo.cc fortransub.f
[b, s] = fortrandemo (1:3)
&rArr;
  b = 1.00000   0.50000   0.33333
  s = There are   3 values in the input vector
[b, s] = fortrandemo (0:3)
error: fortrandemo: fortransub: divide by zero
</pre></div>

<hr>
<div class="header">
<p>
Next: <a href="Allocating-Local-Memory-in-Oct_002dFiles.html#Allocating-Local-Memory-in-Oct_002dFiles" accesskey="n" rel="next">Allocating Local Memory in Oct-Files</a>, Previous: <a href="Calling-Octave-Functions-from-Oct_002dFiles.html#Calling-Octave-Functions-from-Oct_002dFiles" accesskey="p" rel="prev">Calling Octave Functions from Oct-Files</a>, Up: <a href="Oct_002dFiles.html#Oct_002dFiles" accesskey="u" rel="up">Oct-Files</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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