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From: Ben Greear <greearb@candelatech.com>
To: Marco Roeland <marco.roeland@xs4all.nl>
Cc: Linux Kernel Development <linux-kernel@vger.kernel.org>,
	Eli Barzilay <eli@barzilay.org>
Subject: Re: Repost: Bug with select?
Date: Fri, 25 Jul 2003 17:20:13 -0700	[thread overview]
Message-ID: <3F21C93D.6000005@candelatech.com> (raw)
In-Reply-To: <20030725134155.GA19211@localhost>

Marco Roeland wrote:
> On Thursday July 24th 2003 at 01:28 uur Eli Barzilay wrote:
> 
> 
>>When I run the following program, and block the terminal's output
>>(C-s), the `select' doesn't seem to have any effect, resulting in a
>>100% cpu usage (this is on a RH8, with 2.4.18).  I wouldn't be
>>surprised if I'm doing something stupid, but it does seem to work fine
>>on Solaris.
>>
>>Is there anything wrong with this, or is this some bug?
>>
>>======================================================================
>>#include <unistd.h>
>>#include <fcntl.h>
>>int main() {
>>  int flags, fd, len; fd_set writefds;
>>  fd = 1;
>>  flags = fcntl(fd, F_GETFL, 0);
>>  fcntl(fd, F_SETFL, flags | O_NONBLOCK);
> 
> 
> You use non-blocking mode here.
> 
> 
>>  while (1) {
>>    FD_ZERO(&writefds);
>>    FD_SET(fd, &writefds);
>>    len = select(fd + 1, NULL, &writefds, NULL, NULL);
> 
> 
> A select with no timeout, so it will immediately return.
> 
> 
>>    if (!FD_ISSET(fd,&writefds)) exit(0);
> 
> 
> This might be what Solaris does differently, by _not_ including '1' in
> the returned descriptors? Linux will say (rightly) that a following call
> will not block, which is something very different than 'will not fail'!

I thought select is supposed to tell you when you can read/write at least something without
failing.  Otherwise it would be worthless when doing non-blocking IO because you can
both read and write w/out blocking at all times.  If you run similar code on a tcp
socket instead of std-out, do you see the same busy spin?  (To do it right, make
sure the network between source and destination is slower than the CPU can handle,
ie 10bt hub.)


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com



  reply	other threads:[~2003-07-26  0:05 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-06-18  8:49 Problem " Eli Barzilay
2003-07-24  5:28 ` Repost: Bug " Eli Barzilay
2003-07-25 13:41   ` Marco Roeland
2003-07-26  0:20     ` Ben Greear [this message]
2003-07-26  9:05       ` Marco Roeland
2003-07-26  0:35   ` Philippe Troin
2003-07-26 14:29     ` Eli Barzilay
2003-07-26 14:25   ` Eli Barzilay
2003-07-26 15:37     ` Marco Roeland
2003-07-27 19:29 Manfred Spraul

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