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E. Implementation-Defined Behaviors in OpenMP C/C++

This appendix summarizes the behaviors that are described as "implementation-defined" in this API. Each behavior is cross-referenced back to its description in the main specification.

An implementation is required to define and document its behavior in these cases, but this list may be incomplete.

  • Number of threads: If a parallel region is encountered while dynamic adjustment of the number of threads is disabled, and the number of threads requested for the parallel region exceeds the number that the run-time system can supply, the behavior of the program is implementation-defined (see page 9).

    In Visual C++, for a non-nested parallel region, 64 threads (the maximum) will be provided.

  • Number of processors: The number of physical processors actually hosting the threads at any given time is implementation-defined (see page 10).

    In Visual C++, this number is not constant, and is controlled by the operating system.

  • Creating teams of threads: The number of threads in a team that execute a nested parallel region is implementation-defined.(see page 10).

    In Visual C++, this is determined by the operating system.

  • schedule(runtime): The decision regarding scheduling is deferred until run time. The schedule type and chunk size can be chosen at run time by setting the OMP_SCHEDULE environment variable. If this environment variable is not set, the resulting schedule is implementation-defined (see page 13).

    In Visual C++, schedule type is static with no chunk size.

  • Default scheduling: In the absence of the schedule clause, the default schedule is implementation-defined (see page 13).

    In Visual C++, the default schedule type is static with no chunk size.

  • ATOMIC: It is implementation-defined whether an implementation replaces all atomic directives with critical directives that have the same unique name (see page 20).

    In Visual C++, if data modified by atomic is not on a natural alignment or if it is 1 or 2 bytes long all atomic operations that satisfy that property will use one critical section. Otherwise, critical sections will not be used.

  • omp_get_num_threads: If the number of threads has not been explicitly set by the user, the default is implementation-defined (see page 9, and Section 3.1.2 on page 37).

    In Visual C++, the default number of threads is equal to the number of processors.

  • omp_set_dynamic: The default for dynamic thread adjustment is implementation-defined (see Section 3.1.7 on page 39).

    In Visual C++, the default is FALSE.

  • omp_set_nested: When nested parallelism is enabled, the number of threads used to execute nested parallel regions is implementation-defined (see Section 3.1.9 on page 40).

    In Visual C++, the number of threads is determined by the operating system.

  • OMP_SCHEDULE environment variable: The default value for this environment variable is implementation-defined (see Section 4.1 on page 48).

    In Visual C++, schedule type is static with no chunk size.

  • OMP_NUM_THREADS environment variable: If no value is specified for the OMP_NUM_THREADS environment variable, or if the value specified is not a positive integer, or if the value is greater than the maximum number of threads the system can support, the number of threads to use is implementation-defined (see Section 4.2 on page 48).

    In Visual C++, if value specified is zero or less, the number of threads is equal to the number of processors. If value is greater than 64, the number of threads is 64.

  • OMP_DYNAMIC environment variable: The default value is implementation-defined (see Section 4.3 on page 49).

    In Visual C++, the default is FALSE.

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