Parallel Programming in Fortran 95 using OpenMP - People

Chapter 3
PRIVATE, SHARED & Co.
In many of the previously presented OpenMP directives it is possible to modify their way
of working by adding so called clauses. Two different kinds of clauses can be identified.
The first kind, called data scope attribute clauses, specify how each variable is handled
and who is allowed to see its value and to change it. The second kind includes all the
clauses which do not fit into the first kind.
Not all of the following data scope attribute clauses are allowed by all directives, but
the clauses that are valid on a particular directive are indicated in the previous descriptions
of the directives.
Although most of the examples presented in the following descriptions make use of the
directive-pair !$OMP PARALLEL/!$OMP END PARALLEL, this does not mean that their use is
only restricted to that directive-pair, but most of the concepts are easier to understand
with it than with other directive-pairs.
3.1 Data scope attribute clauses
3.1.1 PRIVATE(list)
Sometimes certain variables are going to have different values in each thread. This is only
possible if and only if each thread has its own copy of the variable. This clause fixes which
variables are going to be considered as local variables to each thread. For example
!$OMP PARALLEL PRIVATE(a, b)
says, that the variables a and b will have different values in each thread: they will be
local to each thread.
When a variable is said to be private, a new object of the same type is declared once
for each thread in the team and used by each thread inside the scope of the directive-pair
(in the previous example inside the parallel region) instead of the original variable. This
idea is shown in figure 3.1 in a graphical way.
Variables declared as private have an undefined value at the beginning of the scope
of the directive-pair, since they have just been created. Also when the scope of the
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