finding NFS dirs in csh?

[Guy Harris]

> NFS breaks the directory-as-a-file paradigm 

DIRECTORIES AREN'T FILES!!!!!!!!  They may happen to be implemented on UNIX
in such a fashion that they can be treated as such in some cases, but they
can't always be treated as such.

When you read a directory entry, you want the i-number (and any other
numbers in the directory entry) to be presented in the format of the machine
and operating system doing the *reading*, *not* the format of the machine
and operating system on which they are stored.  (Otherwise, it would
*really* be non-transparent.)  As such, you must use a system call that
knows you're trying to read directory entries, so that they can be put into
a standard form at the sending site and put into the native form at the
receiving site.

If "read" worked on directories over NFS, any program that tried to use
"read" to read directory entries from a directory, rather than
"getdirentries" (SunOS) or "getdents" (System V, Release 3, and probably
some future SunOS release) would be quite likely to get wrong answers.  In
fact, if the two machines are, say, a Sun and a VAX, it would pretty much be
*guaranteed* to get wrong answers!

> so an attempt to open() and read() a directory will produce errors.  Since
> standard I/O insulates programs from these errors,

Standard I/O does no such thing.  If "read" gets an I/O error, the standard
I/O call doing the reading will return whatever its error/EOF indication is.
The program can then use "ferror" or "feof" to distinguish between error and
EOF.  A program using standard I/O can do the same error checking that a
program using "open()" and "read()" can.

> a remote directory will look like an empty file to many programs.

Only to *broken* and *incorrect* programs, i.e. programs that assume that
any error/EOF indication from standard I/O is an EOF indication.  Such
programs should be fixed!

FYI, the way "find" does it is to "stat" the file in question and search
through the mount table (see GETMNTENT(3), at least in the SunOS
documentation), "stat"ing the "mnt_dir" field for each entry in the mount
table and comparing its "st_dev" with the "st_dev" for the file in question.
When they're equal, it's found the file system on which the file in question
appears; the "mnt_type" entry contains the file system type for that file
system.  "nfs" indicates an NFS file system.
-- 
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy at sun.com (or guy at sun.arpa)