The GNU Fortran Compiler: OpenMP

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#### 6.1.16 OpenMP

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.

GNU Fortran strives to be compatible to the OpenMP Application Program Interface v4.0.

To enable the processing of the OpenMP directive `!$omp` in free-form source code; the `c$omp`, `*$omp` and `!$omp` directives in fixed form; the `!$` conditional compilation sentinels in free form; and the `c$`, `*$` and `!$` sentinels in fixed form, `gfortran` needs to be invoked with the `-fopenmp`. This also arranges for automatic linking of the GNU OpenMP runtime library libgomp in GNU OpenMP runtime library.

The OpenMP Fortran runtime library routines are provided both in a form of a Fortran 90 module named `omp_lib` and in a form of a Fortran `include` file named `omp_lib.h`.

An example of a parallelized loop taken from Appendix A.1 of the OpenMP Application Program Interface v2.5:

    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

Please note:

- `-fopenmp` implies `-frecursive`, 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.
- 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 `-Wl,--whole-archive -lpthread -Wl,--no-whole-archive` is added to the command line. However, this is not supported by `gcc` and thus not recommended.

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