Explicitly declare the scope of each variable used in a
OpenMP parallel region to prevent an invalid
default scoping being
assigned to it.
Declare the scope of each variable through the appropriate OpenMP clause.
When the scope for a variable is not specified in an OpenMP parallel region, a
default scope is assigned to it. This default scope can be wrong, for instance
sharing a variable that should be private, which can lead to a race condition.
Furthermore, it ensures that the scope of each variable has been determined and
improves code readability.
In the following code, a variable factor is used in each iteration of the
loop to initialize the array result:
void example() {
int factor = 42;
int result[10];
#pragma omp parallel for
for (int i = 0; i < 10; i++) {
result[i] = factor * i;
}
}Having the scope declared explicitly for each variable improves readability, since it makes explicit the scope of all the variables within the parallel region:
void example() {
int factor = 42;
int result[10];
#pragma omp parallel for default(none) shared(result, factor) private(i)
for (int i = 0; i < 10; i++) {
result[i] = factor * i;
}
}In the following code, a variable factor is used in each iteration of the
loop to initialize the array result:
subroutine example()
integer :: factor
integer :: result(10)
factor = 42
!$omp parallel do
do i = 1, 10
result(i) = factor * i
end do
end subroutine exampleHaving the scope declared explicitly for each variable improves readability, since it makes explicit the scope of all the variables within the parallel region:
subroutine example()
integer :: factor
integer :: result(10)
factor = 42
!$omp parallel do default(none) shared(factor, result) private(i)
do i = 1, 10
result(i) = factor * i
end do
end subroutine example-
OpenMP 4.5 Complete Specifications, November 2015 [last checked May 2019]