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<title>The GNU Fortran Compiler: OpenMP</title>
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<a name="OpenMP"></a>
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<p>
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Next: <a href="OpenACC.html#OpenACC" accesskey="n" rel="next">OpenACC</a>, Previous: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="p" rel="prev">CONVERT specifier</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="OpenMP-1"></a>
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<h4 class="subsection">6.1.16 OpenMP</h4>
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<a name="index-OpenMP-1"></a>
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<p>OpenMP (Open Multi-Processing) is an application programming
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interface (API) that supports multi-platform shared memory
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multiprocessing programming in C/C++ and Fortran on many
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architectures, including Unix and Microsoft Windows platforms.
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It consists of a set of compiler directives, library routines,
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and environment variables that influence run-time behavior.
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</p>
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<p>GNU Fortran strives to be compatible to the
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<a href="http://openmp.org/wp/openmp-specifications/">OpenMP Application Program Interface v4.0</a>.
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</p>
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<p>To enable the processing of the OpenMP directive <code>!$omp</code> in
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free-form source code; the <code>c$omp</code>, <code>*$omp</code> and <code>!$omp</code>
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directives in fixed form; the <code>!$</code> conditional compilation sentinels
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in free form; and the <code>c$</code>, <code>*$</code> and <code>!$</code> sentinels
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in fixed form, <code>gfortran</code> needs to be invoked with the
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<samp>-fopenmp</samp>. This also arranges for automatic linking of the
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GNU Offloading and Multi Processing Runtime Library
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<a href="http://gcc.gnu.org/onlinedocs/libgomp/index.html#Top">libgomp</a> in <cite>GNU Offloading and Multi Processing Runtime
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Library</cite>.
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</p>
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<p>The OpenMP Fortran runtime library routines are provided both in a
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form of a Fortran 90 module named <code>omp_lib</code> and in a form of
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a Fortran <code>include</code> file named <samp>omp_lib.h</samp>.
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</p>
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<p>An example of a parallelized loop taken from Appendix A.1 of
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the OpenMP Application Program Interface v2.5:
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</p><div class="smallexample">
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<pre class="smallexample">SUBROUTINE A1(N, A, B)
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INTEGER I, N
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REAL B(N), A(N)
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!$OMP PARALLEL DO !I is private by default
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DO I=2,N
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B(I) = (A(I) + A(I-1)) / 2.0
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ENDDO
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!$OMP END PARALLEL DO
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END SUBROUTINE A1
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</pre></div>
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<p>Please note:
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</p><ul>
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<li> <samp>-fopenmp</samp> implies <samp>-frecursive</samp>, i.e., all local arrays
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will be allocated on the stack. When porting existing code to OpenMP,
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this may lead to surprising results, especially to segmentation faults
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if the stacksize is limited.
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</li><li> On glibc-based systems, OpenMP enabled applications cannot be statically
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linked due to limitations of the underlying pthreads-implementation. It
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might be possible to get a working solution if
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<code>-Wl,--whole-archive -lpthread -Wl,--no-whole-archive</code> is added
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to the command line. However, this is not supported by <code>gcc</code> and
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thus not recommended.
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</li></ul>
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Next: <a href="OpenACC.html#OpenACC" accesskey="n" rel="next">OpenACC</a>, Previous: <a href="CONVERT-specifier.html#CONVERT-specifier" accesskey="p" rel="prev">CONVERT specifier</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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