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From: terryl@tekcrl.TEK.COM
Newsgroups: comp.unix.questions
Subject: Re: File descriptors
Message-ID: <1673@tekcrl.TEK.COM>
Date: Thu, 21-May-87 17:45:50 EDT
Article-I.D.: tekcrl.1673
Posted: Thu May 21 17:45:50 1987
Date-Received: Sat, 23-May-87 14:13:06 EDT
References: <2532@ncoast.UUCP> <5875@brl-smoke.ARPA>
Reply-To: terryl@tekcrl.tek.com
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 34

In article <5875@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>In article <2532@ncoast.UUCP> robertd@ncoast.UUCP (Rob DeMarco) writes:
>> Can some one tell me what a "File Descriptor" is? Thank you.
>
>It's just an index into an open-file table maintained inside the
>kernel; the open-file table is used to keep track of the state of
>the open file (such as, where is the actual data, and how far into
>the data is the file position pointer associated with this F.D.).
>Think of it as a "handle" on the file that the kernel gives you when
>you open it.  For further information read Ken Thompson's "UNIX
>Implementation" (used to be in Vol. 2 of the UNIX manual set).

     I can't believe this. Doug, you really blew it on this one. Close but
no cigar.

     A file descriptor (since we haven't established an official definition of
a file descriptor, let me propose one: A file descriptor is the value you get
back from an "open" or "creat" system call. If that's not Doug's definition,
then he could be right) is just an index into a data structure; this data struc-
ture is part of the PER PROCESS information, not part of the system wide as
what Doug is alluding to. Now, this data structure in the per process infor-
mation does have pointers to the system wide data structures (what Doug refers
to as the "open-file table"), and everything else is as Doug described it
(well, there are a couple of other things, but they really don't have anything
to do with the original question).

     All of this is true for Berkeley 4.2/4.3 (I just looked at the code just
to make sure 4.3 didn't change what file descriptors mean). I have absolutely
no idea if this is the way System V does things.


				Terry Laskodi
				     of
				Tektronix