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<a name="OpenMP-1"></a>
<h4 class="subsection">6.1.16 OpenMP</h4>
<a name="index-OpenMP-1"></a>

<p>OpenMP (Open Multi-Processing) is an application programming
interface (API) that supports multi-platform shared memory 
multiprocessing programming in C/C++ and Fortran on many 
architectures, including Unix and Microsoft Windows platforms.
It consists of a set of compiler directives, library routines,
and environment variables that influence run-time behavior.
</p>
<p>GNU Fortran strives to be compatible to the 
<a href="http://openmp.org/wp/openmp-specifications/">OpenMP Application Program Interface v4.0</a>.
</p>
<p>To enable the processing of the OpenMP directive <code>!$omp</code> in
free-form source code; the <code>c$omp</code>, <code>*$omp</code> and <code>!$omp</code>
directives in fixed form; the <code>!$</code> conditional compilation sentinels
in free form; and the <code>c$</code>, <code>*$</code> and <code>!$</code> sentinels
in fixed form, <code>gfortran</code> needs to be invoked with the
<samp>-fopenmp</samp>.  This also arranges for automatic linking of the
GNU Offloading and Multi Processing Runtime Library
<a href="http://gcc.gnu.org/onlinedocs/libgomp/index.html#Top">libgomp</a> in <cite>GNU Offloading and Multi Processing Runtime
Library</cite>.
</p>
<p>The OpenMP Fortran runtime library routines are provided both in a
form of a Fortran 90 module named <code>omp_lib</code> and in a form of
a Fortran <code>include</code> file named <samp>omp_lib.h</samp>.
</p>
<p>An example of a parallelized loop taken from Appendix A.1 of
the OpenMP Application Program Interface v2.5:
</p><div class="smallexample">
<pre class="smallexample">SUBROUTINE A1(N, A, B)
  INTEGER I, N
  REAL B(N), A(N)
!$OMP PARALLEL DO !I is private by default
  DO I=2,N
    B(I) = (A(I) + A(I-1)) / 2.0
  ENDDO
!$OMP END PARALLEL DO
END SUBROUTINE A1
</pre></div>

<p>Please note:
</p><ul>
<li> <samp>-fopenmp</samp> implies <samp>-frecursive</samp>, i.e., all local arrays
will be allocated on the stack.  When porting existing code to OpenMP,
this may lead to surprising results, especially to segmentation faults
if the stacksize is limited.

</li><li> On glibc-based systems, OpenMP enabled applications cannot be statically
linked due to limitations of the underlying pthreads-implementation.  It
might be possible to get a working solution if 
<code>-Wl,--whole-archive -lpthread -Wl,--no-whole-archive</code> is added
to the command line.  However, this is not supported by <code>gcc</code> and
thus not recommended.
</li></ul>

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