This issue is a bit dated, but here goes: 

   On Unix, the function `statvfs()` is called. `path` should be a path on a mounted filesystem (it should not be a device).

   On Windows, the function `GetDiskFreeSpaceExW()` is called. `path` is any directory on a disk volume.


Looking through the rest of the page, no other functions get into such specifics; maybe this is outside the scope of the document. Maybe the issue should be closed.