Parallel Programming in Fortran 95 using OpenMP - People

Chapter 5
The environment variables
The parallel environment, under which an OpenMP-parallel application runs, is controlled
by environment variables or equivalent platform-specific mechanisms. These variables
can be set from the command-line of the operating system or by calls to specific subroutines
from the OpenMP Fortran API run-time library.
The way in which these environment variables are set from within the commandline
of the operating system depends on the operating system itself. For example, on a
Linux/Unix machine using csh as shell, the setenv command does the job:
> setenv OMP_NUM_THREADS 4
If the sh, ksh or bash shells are used, then the previous example looks as follows:
> OMP_NUM_THREADS=4
> export OMP_NUM_THREADS
In both cases it is possible to see the value of a given environment variable with the
command echo:
> echo $OMP_NUM_THREADS
4
>
From within Microsoft Windows NT/2000/XP, the environment variables can be specified
through the Control Panel (User Environment Variables) or by adding the corresponding
lines in the AUTOEXEC.BAT file (AUTOEXEC.BAT Environment variables). In
both cases the syntax is very simple:
OMP_NUM_THREADS = 4
The names of the environment variables must always be written in uppercase, while
the characters assigned to them are case insensitive and may have leading or trailing white
spaces.
In the rest of the present chapter the different environment variables are described,
giving their possible values and effects on the behavior of an OpenMP-parallel program:
69