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OpenMP: Difference between revisions
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== Description == | == Description == | ||
[http://openmp.org/wp/ OpenMP] (Open Multi-Processing) is an application programming interface (API) for shared memory parallel computing. It is supported on numerous platforms, including Linux and Windows, and is available for the C/C++ and Fortran programming languages. The API consists of a set of directives, a software library, and environment variables. | [http://openmp.org/wp/ OpenMP] (Open Multi-Processing) is an application programming interface (API) for shared memory parallel computing. It is supported on numerous platforms, including Linux and Windows, and is available for the C/C++ and Fortran programming languages. The API consists of a set of directives, a software library, and environment variables. | ||
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OpenMP directives are inserted in Fortran programs using sentinels. A sentinel is a keyword placed immediately after a symbol that marks a comment. For example: | OpenMP directives are inserted in Fortran programs using sentinels. A sentinel is a keyword placed immediately after a symbol that marks a comment. For example: | ||
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<pre> | <pre> | ||
!$OMP directive | !$OMP directive | ||
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*$OMP directive | *$OMP directive | ||
</pre> | </pre> | ||
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In C, directives are inserted using a pragma construct, as follows: | In C, directives are inserted using a pragma construct, as follows: | ||
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<syntaxhighlight lang="c"> | <syntaxhighlight lang="c"> | ||
#pragma omp directive | #pragma omp directive | ||
</syntaxhighlight> | </syntaxhighlight> | ||
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===OpenMP directives=== | ===OpenMP directives=== | ||
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{| class="wikitable" style="font-size: 88%; text-align: left;" | {| class="wikitable" style="font-size: 88%; text-align: left;" | ||
! scope="col" width="260px" | Fortran | ! scope="col" width="260px" | Fortran | ||
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|nowait | |nowait | ||
|} | |} | ||
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== Environment == | == Environment == | ||
There are four main environment variables that influence the execution of an OpenMP program: | There are four main environment variables that influence the execution of an OpenMP program: | ||
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<pre> | <pre> | ||
OMP_SCHEDULE | OMP_SCHEDULE | ||
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OMP_NESTED | OMP_NESTED | ||
</pre> | </pre> | ||
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They can be set and modified using the UNIX command | They can be set and modified using the UNIX command | ||
{{command|export OMP_NUM_THREADS{{=}}12}} | {{command|export OMP_NUM_THREADS{{=}}12}} | ||
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== Example == | == Example == | ||
Here is a ''hello world'' example that shows the use of openMP. | Here is a ''hello world'' example that shows the use of openMP. | ||
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{| border="0" cellpadding="5" cellspacing="0" align="center" | {| border="0" cellpadding="5" cellspacing="0" align="center" | ||
! style="background:#8AA8E5;" | '''C CODE''': <tt>ompHello.c</tt> | ! style="background:#8AA8E5;" | '''C CODE''': <tt>ompHello.c</tt> | ||
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</source> | </source> | ||
|} | |} | ||
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Compiling and running the C code goes as follows: | Compiling and running the C code goes as follows: | ||
litai10:~$ gcc -O3 -fopenmp ompHello.c -o ompHello | litai10:~$ gcc -O3 -fopenmp ompHello.c -o ompHello | ||
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[http://www.openmp.org/ OpenMP.org] publishes the formal [http://www.openmp.org/specifications/ specifications], | [http://www.openmp.org/ OpenMP.org] publishes the formal [http://www.openmp.org/specifications/ specifications], | ||
handy reference cards for the [http://www.openmp.org/wp-content/uploads/OpenMP-4.0-C.pdf C/C++] and [http://www.openmp.org/wp-content/uploads/OpenMP-4.0-Fortran.pdf Fortran] interfaces, and [http://www.openmp.org/wp-content/uploads/openmp-examples-4.0.2.pdf examples]. | handy reference cards for the [http://www.openmp.org/wp-content/uploads/OpenMP-4.0-C.pdf C/C++] and [http://www.openmp.org/wp-content/uploads/OpenMP-4.0-Fortran.pdf Fortran] interfaces, and [http://www.openmp.org/wp-content/uploads/openmp-examples-4.0.2.pdf examples]. | ||
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