How to determine file being redirected to in C

[Guy Harris]

>Anyway, can someone tell me how to determine (in C) if output is being
>redirected, or more precisely, how to determine the file output is
>being redirected to.  The cat command does this (at
>least the Xenix version of cat, I don't think the BSD version does),

Yes, the BSD version does.  So does the System V Release 3 version, and
I think every version since the V7 version does. 

However:

	1) It doesn't "determine that output is being redirected"; all
	   it does is determine that one of the input files is the same
	   as the output file.

	2) It doesn't do the latter by finding the path name for any of
	   the files; it does an "fstat" on the file (descriptor) in
	   question, and compares the "st_dev" and "st_ino" fields of
	   the results.  That's all it needs to do, since the "st_dev"
	   and "st_ino" fields are sufficient to uniquely identify a
	   file; it doesn't have to find the file's name (and that
	   wouldn't help anyway, since the same file can have more than
	   one name).

So if you really want the name of the file, you'll have to work *very*
hard to get it (you basically have to scan all mounted file systems, or
at least the ones whose mount points have an "st_dev" that matches that
of the file in question), and you may not get it anyway if the input is
a pipe, or a file that has been unlinked.