FLUSH Directive

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

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ID 767251

Date 11/07/2023

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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

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FLUSH Directive

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