The GNU Fortran Compiler: OpenMP

Next: OpenACC, Previous: CONVERT specifier, Up: Extensions implemented in GNU Fortran [Contents][Index]

#### 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 Offloading and Multi Processing Runtime Library libgomp in GNU Offloading and Multi Processing 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.

Next: OpenACC, Previous: CONVERT specifier, Up: Extensions implemented in GNU Fortran [Contents][Index]