Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!tektronix!sequent!bills
From: bills@sequent.UUCP (Bill Sears)
Newsgroups: comp.lang.c
Subject: Re: File descriptors and streams and copying thereof.
Message-ID: <15039@sequent.UUCP>
Date: 25 Apr 89 02:31:34 GMT
Reply-To: bills@sequent.UUCP (Bill Sears)
Followup-To: comp.lang.c
Organization: Sequent Computer Systems, Inc
Lines: 23


In article <8450@xanth.cs.odu.edu> kremer@cs.odu.edu (Lloyd Kremer) writes:
>	close(0);
>
>	if(open("my_filename", O_RDONLY) == -1)
>	/* attempt to open your file.  If open succeeds, it will return
>		file descriptor 0, since it always returns the earliest
>		one, and we just closed 0 */

And in article <731@root44.co.uk> gwc@root.co.uk (Geoff Clare) writes:
>It is, of course, safe to assume the stream returned by fopen() and
>friends has the lowest available file descriptor (i.e. fileno(stream)),
>...

It is NOT safe to assume that the file descriptor returned either
by open(2) or the fopen(3) family will be the lowest available file
descriptor.  Although this may be true on many machines, it is not
guaranteed to be the case (I have worked with a machine where this
did not always work).  Nowhere in the manuals that I have seen (v7,
SYS V.2, BSD4.2, BSD4.3) does the open(2) system call or the fopen(3)
library function guarantee that the file descriptor returned will
be the lowest one available, only the dup(2) system call makes that
guarantee.