Why aren't unsigned OpenMP index variables allowed?

Question

I have a loop in my C++/OpenMP code that looks like this:

#pragma omp parallel for for(unsigned int i=0; i<count; i++) {     // do stuff } 

When I compile it (with Visual Studio 2005) I get the following error:

error C3016: 'i' : index variable in OpenMP 'for' statement must have signed integral type

I understand that the error occurs because i is unsigned instead of signed, and changing i to be signed removed this error. What I want to know is why is this an error? Why aren't unsigned index variables allowed? Looking at the MSDN page for this error gives me no clues.

Answer 1

According to the OpenMP 2.0 C/C++ API specification (pdf), section 2.4.1, that's one of the restrictions of the for loop. No reason is given for it, but I suspect it's just to simplify the assumptions that the code and compiler have to make, since there's special code to ensure that the range doesn't overflow the maximum value of the type.

OpenMP 3.0 apparently allows for unsigned types too, but I haven't seen it in action yet.

Answer 2

The Microsoft C/C++ Compiler 12.0 integrated with Visual Studio 2013 still only support OpenMP 2.5 and doesn't allow unsigned int for the loop counter.

GCC support OpenMP 3.0 since its version 4.4 and allows unsigned int for the loop counter.