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| <title>The GNU Fortran Compiler: OpenMP</title> |
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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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