quotactl() question

[ok at quintus]

I am trying to put together a wrapper for open() which will be less
unhelpfully vague about errors.  For example, instead of just mumbling
"no such file or directory" it will tell you *which* directory doesn't
exist, and so on.  Opening files for input was tricky, but now I am
trying to hack O_WRONLY and O_RDWR.  There are four ways you can run
out of space: ENOSPC (not enough blocks left or not enough inodes left
on device, but quota not exhausted) and EDQUOT (quota of blocks or of
inodes exhausted, but room left on device).  I have figured out how to
disambiguate ENOSPC.  I'm now trying to disambiguate EDQUOT.  I have
three questions:

(a) given the path-name of a directory which does exist, how can I find
    out the path-name of the block special device on which is mounted
    the file system containing that directory?  (That's what quotactl()
    is said to want for its second argument.)
(b) the manual page I'm working from doesn't list ESRCH as a possible
    error return from quotactl(), but I sometimes get that result.
(c) When the directory is accessed via NFS, the block special device
    /dev/whatever is on some other machine, and I _can't_ get at it.
    Given a directory accessed via NFS and a uid, how do you get the
    appropriate struct dqblk?

Better still, tell me I'm wasting my time and that no-one needs to know
whether they ran out of inodes or blocks...