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From: karl@haddock.UUCP (Karl Heuer)
Newsgroups: comp.unix.wizards
Subject: Re: Seek on a pipe
Message-ID: <633@haddock.UUCP>
Date: Fri, 26-Jun-87 16:23:39 EDT
Article-I.D.: haddock.633
Posted: Fri Jun 26 16:23:39 1987
Date-Received: Sat, 27-Jun-87 11:07:39 EDT
References: <7359@brl-adm.ARPA> <983@bobkat.UUCP> <5872@brl-smoke.ARPA> <791@mcgill-vision.UUCP> <7074@mimsy.UUCP>
Reply-To: karl@haddock.ISC.COM.UUCP (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 16
Summary: Seek on a character-special file

In article <7074@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>In article <791@mcgill-vision.UUCP> mouse@mcgill-vision.UUCP (der Mouse) writes:
>>A program that expects to do seeks should ensure that the object can seek
>
>... which is exactly why I wrote a routine to do this.
>	return (lseek(fd, 0L, 1) >= 0 && !isatty(fd));

"isatty" is not sufficient; character-special files in general are incapable
of seeking.  I ran into this same question a couple of years ago, reluctantly
decided that I had to use fstat(), and wrote it that way in my library.  (I
called it "canseek".)

I wish lseek would just fail, instead of being a no-op, on nonseekable files
other than pipes.  Does anyone know what POSIX says about the matter?

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint