For beginners, the following code seems good.
int main()
{
omp_set_num_threads(4);
int A = 100;
#pragma omp parallel for private(A)
for (int i = 0; i < 10; i++)
{
int num = omp_get_thread_num();
printf("Thread %i got %d\n", num, i + A);
}
return 0;
}
But there will be an error if you run it.
What causes this error?
It is important to know that in OpenMP, Private variables are undefined on entry that means, this code is the same to
int main()
{
omp_set_num_threads(4);
// remove A
#pragma omp parallel for private(A)
for (int i = 0; i < 10; i++)
{
int num = omp_get_thread_num();
printf("Thread %i got %d\n", num, i + A);
}
return 0;
}
Of course there will be an error!
The way to fix this problem is to define A inside and outside the loop.
int main()
{
// you have to declare A outside, too. Otherwise there will be an error with #pragma omp parallel for private(A)
int A;
omp_set_num_threads(4);
#pragma omp parallel for private(A)
for (int i = 0; i < 10; i++)
{
int A = 100;
int num = omp_get_thread_num();
printf("Thread %i got %d\n", num, i + A);
}
return 0;
}
If you want to see what A is outside for loop, you need to initialize A with a number(I use 0 as example)
int main()
{
// you have to declare it outside, too. Otherwise there will be an error with #pragma omp parallel for private(A)
int A = 0;
omp_set_num_threads(4);
#pragma omp parallel for private(A)
for (int i = 0; i < 10; i++)
{
int A = 100;
int num = omp_get_thread_num();
printf("Thread %i got %d\n", num, i + A);
}
printf("Here you will see previous A: %i\n", A);
return 0;
}
Result:
A will be the value we defined before for loop if we print it outside.
