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<h4 class="subsection">6.1.16 OpenMP</h4>
<p><a name="index-OpenMP-319"></a>
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>GNU Fortran strives to be compatible to the
<a href="http://www.openmp.org/mp-documents/spec30.pdf">OpenMP Application Program Interface v3.0</a>.
<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, <samp><span class="command">gfortran</span></samp> needs to be invoked with the
<samp><span class="option">-fopenmp</span></samp>. This also arranges for automatic linking of the
GNU OpenMP runtime library <a href="../libgomp/index.html#Top">libgomp</a>.
<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><span class="file">omp_lib.h</span></samp>.
<p>An example of a parallelized loop taken from Appendix A.1 of
the OpenMP Application Program Interface v2.5:
<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>
<p>Please note:
<ul>
<li><samp><span class="option">-fopenmp</span></samp> implies <samp><span class="option">-frecursive</span></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>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
<samp><span class="command">-Wl,--whole-archive -lpthread -Wl,--no-whole-archive</span></samp> is added
to the command line. However, this is not supported by <samp><span class="command">gcc</span></samp> and
thus not recommended.
</ul>
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