How OpenMP handles array in C?

Question

I am just starting playing around with OpenMp and got very confused about this problem. Say if I declare an array int a[10] and use it inside some loop. What is the difference between #pragma omp parallel for private(a) and #pragma omp parallel for shared(a)? To me a is just a pointer to the first element of the array and according to what I have read the private construct will make a copy of the variable for each thread, and also very strangely initialize it with a random value if I do not use firstprivate construct. So my question is if I use private construct, what will happen semantically? Will it make a set of pointers and assign them to every thread created or will it make a set of copies of the whole array and let each thread to use them separately?

Thanks a lot.

Hristo Iliev · Accepted Answer · 2013-10-08 22:52:56Z

2

There is a subtle semantic difference between arrays and pointers in C and C++. An array definition like int a[10] provides the compiler with information both about the type and the size of the array and it (the compiler) can use that information in order to properly allocate a private copy of the same size and possibly initialise it (in the case of firstprivate).

int *a only tells the compiler the type but not the size and therefore in OpenMP such variable is treated simply as a pointer and the pointer itself is made private, not the data that it points to. If you put a pointer in a firstprivate clause then each thread would get a separate copy of the pointer variable but initially all copies would be pointing at the same place in memory.

answered Oct 8, 2013 at 22:52

Hristo Iliev

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