GetDiskFreeSpaceExA not access value function

Question

hello guys, 

I would enterder GetDiskFreeSpaceExA not because the function accesses the disk size, even being a function of the native api Metro.

see the link of MSDN http://msdn.microsoft.com/en-us/library/windows/apps/aa364937.aspx

I created a project Metro Apps Store in C + +, with the following code

PULARGE_INTEGER VerifyDiskSpace()

{

      PULARGE_INTEGER a =0, b = 0, c = 0;

      BOOL ret = GetDiskFreeSpaceExA((LPCSTR)"C:",a,b,c);

     //the problem is with the variable a, b and c, are always 0, Zero

}

someone could help me, please hel me....