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| <h4 class="subsection">6.1.16 OpenMP</h4> |
| |
| <p><a name="index-OpenMP-343"></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/spec31.pdf">OpenMP Application Program Interface v3.1</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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