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2.6.5 flush Directive

  • Article

The flush directive, whether explicit or implied, specifies a "cross-thread" sequence point at which the implementation is required to ensure that all threads in a team have a consistent view of certain objects (specified below) in memory. This means that previous evaluations of expressions that reference those objects are complete and subsequent evaluations have not yet begun. For example, compilers must restore the values of the objects from registers to memory, and hardware may need to flush write buffers to memory and reload the values of the objects from memory.

The syntax of the flush directive is as follows:

#pragma omp flush [(variable-list)]  new-line

If the objects that require synchronization can all be designated by variables, then those variables can be specified in the optional variable-list. If a pointer is present in the variable-list, the pointer itself is flushed, not the object the pointer refers to.

A flush directive without a variable-list synchronizes all shared objects except inaccessible objects with automatic storage duration. (This is likely to have more overhead than a flush with a variable-list.) A flush directive without a variable-list is implied for the following directives:

  • barrier

  • At entry to and exit from critical

  • At entry to and exit from ordered

  • At entry to and exit from parallel

  • At exit from for

  • At exit from sections

  • At exit from single

  • At entry to and exit from parallel for

  • At entry to and exit from parallel sections

The directive is not implied if a nowait clause is present. It should be noted that the flush directive is not implied for any of the following:

  • At entry to for

  • At entry to or exit from master

  • At entry to sections

  • At entry to single

A reference that accesses the value of an object with a volatile-qualified type behaves as if there were a flush directive specifying that object at the previous sequence point. A reference that modifies the value of an object with a volatile-qualified type behaves as if there were a flush directive specifying that object at the subsequent sequence point.

Note that because the flush directive does not have a C language statement as part of its syntax, there are some restrictions on its placement within a program. See Appendix C for the formal grammar. The example below illustrates these restrictions.

/* ERROR - The flush directive cannot be the immediate
*          substatement of an if statement.
*/
if (x!=0)
   #pragma omp flush (x)
...

/* OK - The flush directive is enclosed in a
*      compound statement
*/
if (x!=0) {
   #pragma omp flush (x)
}

Restrictions to the flush directive are as follows:

  • A variable specified in a flush directive must not have a reference type.