Parallel Programming in Fortran 95 using OpenMP - People

Chapter 4
The OpenMP run-time library
In some of the examples presented in previous chapters it has been mentioned the existence
of a run-time library. It is the aim of the present chapter to introduce the so called
OpenMP Fortran API run-time library, describe the subroutines and functions it
includes and to explain its capabilities as well as its limitations.
The OpenMP run-time library is meant to serve as a control and query tool for the
parallel execution environment, which the programmer can use from inside its program.
Therefore, the run-time library is a set of external procedures with clearly defined interfaces.
These interfaces are explicitly delivered by the OpenMP-implementation through
a Fortran 95 module named omp lib. In the following descriptions of the different routines,
interfaces for them are shown: these may vary from one OpenMP-implementation
to another in the precision/size of the variables, but in essence will be all equal.
4.1 Execution environment routines
The following functions and subroutines, part of the OpenMP Fortran API run-time
library, allow to modify, from inside of a program, the conditions of the run-time environment
that controls the parallel execution of the program:
4.1.1 OMP set num threads
This subroutine sets the number of threads to be used by subsequent parallel regions.
Therefore, it can only be called from outside of a parallel region. Its interface declaration
looks as follows:
subroutine OMP_set_num_threads(number_of_threads)
integer(kind = OMP_integer_kind), intent(in) :: number_of_threads
end subroutine OMP_set_num_threads
where number of threads is the number of threads to be set. If dynamic adjustment is
disabled, the value of number of threads is used as the number of threads for all subsequent
parallel regions prior to the next call to this subroutine; otherwise, if dynamic adjustment
is enabled, number of threads is the maximum number of threads that is allowed.
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