> I was writing some flaky tests to see if it's returning true results,
> but the more I think of my assumptions, the harder this gets.  For
> example, is there any filesystem almost guaranteed to be quiescent
> between two calls of it?  /tmp is both iffy and not guaranteed to be a
> separate file system, neither is /.   But maybe adding 32 MiB to
> whatever is /tmp and checking to see if there's less free space is safe
> enough.
>
> Advice is solicited; I have checked the results by hand with df, Linux
> is fine, but either I'm too out of it, or OpenBSD is lying.

Good questions.

Calling stat() on two pathnames and checking whether the device numbers
are equal (st_dev), you can check whether those pathnames are on the
same file system.

Other concerns:

- Do the results reflect the Unix quota system or not?

- Unix tends to reserve n% of each file system's space for the root user
only. The "Avail" column in `df` output reflects the space available for
use by normal plebs I think; if you compute Size-Used you may get more
than that, because root can use more than that. Does statvfs() give the
space free for root or for plebs?