Xref: utzoo comp.unix.questions:11953 comp.unix.xenix:5173
Path: utzoo!attcan!uunet!lll-winken!csd4.milw.wisc.edu!mailrus!uflorida!haven!adm!cmcl2!phri!marob!daveh
From: daveh@marob.MASA.COM (Dave Hammond)
Newsgroups: comp.unix.questions,comp.unix.xenix
Subject: Re: How to determine file being redirected to in C
Keywords: redirection
Message-ID: <574@marob.MASA.COM>
Date: 1 Mar 89 12:35:40 GMT
References: <10@sherpa.UUCP>
Reply-To: daveh@marob.masa.com (Dave Hammond)
Followup-To: comp.unix.questions
Distribution: na
Organization: ESCC  New York City
Lines: 22

In article <10@sherpa.UUCP> rac@sherpa.UUCP (Roger A. Cornelius) writes:
>..................  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 [...]
>
>	cat catfile > catfile
>produces the error "cat: input catfile is output".

1. Determine if your stdout has been redirected (isatty(fileno(stdout)) will
   fail if stdout is redirected).

2. Do an fstat() on fileno(stdout), and a stat() on the other file in question.

3. Compare the st_ino and st_dev members of the two stat structures.  If the
   inode (st_ino) and device-of-directory-entry (st_dev) for the two match,
   your standard output and the other file are the same.

Hope this helps.

--
Dave Hammond
daveh@marob.masa.com