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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.unix.questions
Subject: Re: File descriptors
Message-ID: <19531@sun.uucp>
Date: Fri, 22-May-87 02:42:21 EDT
Article-I.D.: sun.19531
Posted: Fri May 22 02:42:21 1987
Date-Received: Sat, 23-May-87 14:47:02 EDT
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>      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.

Well, this happens to be one of the things that has remained pretty
much constant in UNIX implementations since at least V6 (and probably
back further than that).  4.3 didn't change anything of consequence
(which isn't really surprising - there's really little that needs
changing), and neither 4.2BSD, nor System III, nor System V,
introduced any major changes.  (The only change 4.2BSD made was to
have objects other than inodes attached to a file table entry; Bill
Shannon noted that, given a system with multiple "file system types",
it might have been possible to use that mechanism instead.)

The distinction between a file descriptor (i.e., either the small
number you get back from "open", "dup", etc.)  and a file table entry
(the entry in the system-wide table that indicates things like the
current seek pointer) is not significant in most cases, so Doug's
description is, at worst, a slight over-simplification.  The only
per-file-descriptor state in the system is the "close on 'exec'"
flag.  Most operations treat all file descriptors that refer to the
same file table entry as equivalent.  To quote from the S5R3 manual
page DUP(2):

	"dup" returns a new file descriptor having the following in
	common with the original:

		Same open file (or pipe).

		Same file pointer (i.e., both file descriptors share
		one file pointer).

		Same access mode (read, write or read/write).

which describes most of the state stored with a file descriptor.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com