Intel® Fortran Compiler Classic and Intel® Fortran Compiler Developer Guide and Reference

ID 767251
Date 11/07/2023
Public

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FLUSH Directive

OpenMP* Fortran Compiler Directive: Identifies synchronization points at which the threads in a team must provide a consistent view of memory.

!$OMP FLUSH [(list)]

list

Is the name of one or more variables to be flushed. Names must be separated by commas.

The binding thread set for a FLUSH construct is the encountering thread.

The FLUSH directive must appear at the precise point in the code at which the synchronization is required. To avoid flushing all variables, specify a list.

Thread-visible variables are written back to memory at the point at which this directive appears. Modifications to thread-visible variables are visible to all threads after this point. Subsequent reads of thread-visible variables fetch the latest copy of the data.

Thread-visible variables include the following data items:

  • Globally visible variables (common blocks and modules)

  • Local variables that do not have the SAVE attribute but have had their address taken and saved or have had their address passed to another subprogram

  • Local variables that do not have the SAVE attribute that are declared shared in a parallel region within the subprogram

  • Dummy arguments

  • All pointer dereferences

The FLUSH directive is implied for the following directives (unless the NOWAIT keyword is used):

  • ATOMIC and END ATOMIC

  • BARRIER

  • CRITICAL and END CRITICAL

  • END DO

  • END PARALLEL

  • END SECTIONS

  • END SINGLE

  • END WORKSHARE

  • ORDERED and END ORDERED

  • PARALLEL and END PARALLEL

  • PARALLEL DO and END PARALLEL DO

  • PARALLEL SECTIONS and END PARALLEL SECTIONS

  • TARGET

  • TARGET DATA

  • TARGET ENTER DATA on entry

  • TARGET EXIT DATA on exit

  • TARGET UPDATE on entry if TO is present and on exit if FROM is present

These directives are only available on Linux* systems: TARGET, TARGET DATA, TARGET ENTER DATA, TARGET EXIT DATA, TARGET UPDATE.

Example

The following example uses the FLUSH directive for point-to-point synchronization between pairs of threads:

  !$OMP PARALLEL DEFAULT(PRIVATE) SHARED(ISYNC)
        IAM = OMP_GET_THREAD_NUM( )
        ISYNC(IAM) = 0
  !$OMP BARRIER
        CALL WORK( )
  C I AM DONE WITH MY WORK, SYNCHRONIZE WITH MY NEIGHBOR
        ISYNC(IAM) = 1
  !$OMP FLUSH(ISYNC)
  C WAIT TILL NEIGHBOR IS DONE
        DO WHILE (ISYNC(NEIGH) .EQ. 0)
  !$OMP FLUSH(ISYNC)
        END DO
  !$OMP END PARALLEL