OpenMP Application Programming Interface Examples

Fortran
1
2
3
4
5
6
7
8
The following example is non-conforming because the initialization (a = 0) of the original list
item a is not synchronized with the update of a as a result of the reduction computation in the for
loop. Therefore, the example may print an incorrect value for a.
To avoid this problem, the initialization of the original list item a should complete before any
update of a as a result of the reduction clause. This can be achieved by adding an explicit
barrier after the assignment a = 0, or by enclosing the assignment a = 0 in a single directive
(which has an implied barrier), or by initializing a before the start of the parallel region.
Example reduction.6.c
C / C++
S-1 #include
S-2
S-3 int main (void)
S-4 {
S-5 int a, i;
S-6
S-7 #pragma omp parallel shared(a) private(i)
S-8 {
S-9 #pragma omp master
S-10 a = 0;
S-11
S-12 // To avoid race conditions, add a barrier here.
S-13
S-14 #pragma omp for reduction(+:a)
S-15 for (i = 0; i < 10; i++) {
S-16 a += i;
S-17 }
S-18
S-19 #pragma omp single
S-20 printf ("Sum is %d\n", a);
S-21 }
S-22 return 0;
S-23 }
C / C++
CHAPTER 7. DATA ENVIRONMENT 237