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From: tj@mks.UUCP (T. J. Thompson)
Newsgroups: comp.unix.questions
Subject: Re: distinguishing from pipes and files
Message-ID: <320@mks.UUCP>
Date: Mon, 23-Nov-87 08:35:59 EST
Article-I.D.: mks.320
Posted: Mon Nov 23 08:35:59 1987
Date-Received: Thu, 26-Nov-87 20:46:48 EST
References: <454@rocksanne.UUCP>
Organization: Mortice Kern Systems, Waterloo, Ont.
Lines: 31
Keywords: files, pipes
Summary: possible, but probably a bad idea

In article <454@rocksanne.UUCP>, fuss@rocksanne.UUCP (William Fuss) writes:
> Can a process can recognize the "source" of its input/output ???
> for example, given the scenerio below:
> (1)
>     foo < input > file2 && bar < file2 > file3 && foobar < file3 > output
> (2)
>     cat input | foo | bar | foobar > output
> 
> Can "bar" figure out that its standard input is coming from:
> 
> 	a file in example (1),
> 		vs
> 	a pipe in example (2),

	lseek(0, 0L, 1)

will return -1 with errno set to ESPIPE if standard input is a pipe,
and succeed if standard input is a regular file.
I am sure many people will be ready to point this out.
What i wonder is why you want to know the difference.
I strongly suggest that any program which behaves differently
when its standard input is redirected from a file rather than piped
is mis-designed. The only counter-example that springs to mind is
a file pager that allows backward scrolling, so copies its input
to a temporary file if coming from a pipe (which behaviour is reasonable,
being invisible to the user).
-- 
     ll  // // ,'/~~\'   T. J. Thompson              uunet!watmath!mks!tj
    /ll/// //l' `\\\     Mortice Kern Systems Inc.         (519) 884-2251
   / l //_// ll\___/     35 King St. N., Waterloo, Ont., Can. N2J 2W9
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