Issue #
Explicitly declare the scope of each variable used in a OpenMP parallel region to prevent an invalid default scoping being assigned to it.
Relevance #
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.
Actions #
Declare the scope of each variable through the appropriate OpenMP clause.
Code example #
In the following code, a variable factor is used in each iteration of the loop to initialize the array result.
void foo() {
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 foo() {
int factor = 42;
int result[10];
#pragma omp parallel for shared(result, factor)
for (int i = 0; i < 10; i++) {
result[i] = factor * i;
}
}
Related resources #
- PWR004 examples at GitHub
- PWR005: Disable default OpenMP scoping
- OpenMP 4.5 Complete Specifications, November 2015 [last checked May 2019]
