From owner-freebsd-standards@FreeBSD.ORG  Sat May 21 20:52:29 2005
Return-Path: <owner-freebsd-standards@FreeBSD.ORG>
Delivered-To: freebsd-standards@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 12F6C16A4CE
	for <freebsd-standards@freebsd.org>;
	Sat, 21 May 2005 20:52:29 +0000 (GMT)
Received: from aiolos.otenet.gr (aiolos.otenet.gr [195.170.0.23])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 696D643D1F
	for <freebsd-standards@freebsd.org>;
	Sat, 21 May 2005 20:52:28 +0000 (GMT)
	(envelope-from keramida@freebsd.org)
Received: from kane.otenet.gr (kane.otenet.gr [195.170.0.27])
	j4LKqQAX003339;	Sat, 21 May 2005 23:52:26 +0300
Received: from gothmog.gr (patr530-a033.otenet.gr [212.205.215.33])
	j4LKoZNq014454;	Sat, 21 May 2005 23:50:36 +0300
Received: from gothmog.gr (gothmog [127.0.0.1])
	by gothmog.gr (8.13.3/8.13.3) with ESMTP id j4LKqMix001516;
	Sat, 21 May 2005 23:52:22 +0300 (EEST)
	(envelope-from keramida@freebsd.org)
Received: (from giorgos@localhost)
	by gothmog.gr (8.13.3/8.13.3/Submit) id j4LKqMEH001515;
	Sat, 21 May 2005 23:52:22 +0300 (EEST)
	(envelope-from keramida@freebsd.org)
Date: Sat, 21 May 2005 23:52:22 +0300
From: Giorgos Keramidas <keramida@freebsd.org>
To: Steve Kargl <sgk@troutmask.apl.washington.edu>
Message-ID: <20050521205222.GA1014@gothmog.gr>
References: <20050521193605.GB51782@troutmask.apl.washington.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20050521193605.GB51782@troutmask.apl.washington.edu>
cc: freebsd-standards@freebsd.org
Subject: Re: Read /dev/null differs from POSIX
X-BeenThere: freebsd-standards@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Standards compliance <freebsd-standards.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-standards>,
	<mailto:freebsd-standards-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-standards>
List-Post: <mailto:freebsd-standards@freebsd.org>
List-Help: <mailto:freebsd-standards-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-standards>,
	<mailto:freebsd-standards-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sat, 21 May 2005 20:52:29 -0000

On 2005-05-21 12:36, Steve Kargl <sgk@troutmask.apl.washington.edu> wrote:
> >From http://www.opengroup.org/onlinepubs/009695399/toc.htm
>
>    /dev/null -- An infinite data source and data sink.  Data
>    written to /dev/null shall be discarded.  Reads from
>    /dev/null shall always return end-of-file (EOF).
>
> This program
>
>   #include <stdio.h>
>   int main(void) {
>      int i=0,j;
>      FILE *fp;
>      fp = fopen("/dev/null", "r");
>      while(fread(&j, sizeof(int), 1, fp) != 1) {
>        i++;
>        if (i == 5) break;
>      }

The condition of the while loop above is bogus.  fread() of any amount
of data from /dev/null returns 0 items, but the loop will still iterate,
increasing i as if some data was read.

Rewriting the loop as shown below reveals that /dev/null indeed works as
expected:

        for (i = 0; fread(&j, sizeof(j), 1, fp) == 1; i++)
                printf("step %d read %lu bytes\n", i,
                    (unsigned long)sizeof(j));

This *NEVER* prints a "step X read Y bytes" message from /dev/null.

>         if (j == EOF)
>                 printf("EOF\n");
>         else
>                 printf("j = %d\n", j);

This is bogus.  fread() doesn't set the output buffer to EOF but marks
the `fp' stream as end-of-file'd.  The correct way to check for
end-of-file on stdio.h streams is:

	if (feof(fp))
		... ;