Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!yale!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.unix.questions
Subject: Re: How do I tell if STDIN is a PIPE?
Message-ID: <16281@smoke.brl.mil>
Date: 27 May 91 03:26:37 GMT
References: <1991May26.172328.713@arizona.edu>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 18

In article <1991May26.172328.713@arizona.edu> jjr@ace.ece.arizona.edu (Jeffrey J. Rodriguez) writes:
>How do I tell whether stdin is coming from a pipe?
>There must be some system call I can use from a C program.

The simplest method is to attempt an lseek(0,0L,1) and see if it
reports failure:  If you can't seek to relative location 0, the
file is undoubtedly not seekable in general.  (If you CAN seek to
relative location 0, doing so will not mess anything up.)

>If stdin is coming from a pipe, then what is the best way to do a seek?

There isn't one.

>Please let me know if there's a better way to handle this.

Why do you think you need to seek?  Perhaps what you need to do is
copy the standard input into a temporary (regular) file and switch
to that instead of trying to use stdin directly.