The total size of the file system in 512-byte units. The exact
meaning of this figure is implementation-defined, but should
include <space used>, <space free>, plus any space reserved by
the system not normally available to a user.
The total amount of space available within the file system for
the creation of new files by unprivileged users, in 512-byte
units. When this figure is less than or equal to zero, it shall
not be possible to create any new files on the file system with-
out first deleting others, unless the process has appropriate
privileges. The figure written may be less than zero.