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To: Eli Barzilay <eli@barzilay.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Repost: Bug with select?
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From: Philippe Troin <phil@fifi.org>
Date: 25 Jul 2003 17:35:17 -0700
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Eli Barzilay <eli@barzilay.org> writes:

> [This is a second post, since I didn't get any replies the first time.
> It looks more like a bug now, which sounds strange for something that
> common...]
> 
> 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);
>   while (1) {
>     FD_ZERO(&writefds);
>     FD_SET(fd, &writefds);
>     len = select(fd + 1, NULL, &writefds, NULL, NULL);
>     if (!FD_ISSET(fd,&writefds)) exit(0);
>     len = write(fd, "hi\n", 3);
>   }
>   fcntl(fd, F_SETFL, flags);
> }
> ======================================================================

Looks like a bug to me.
Strace says:

select(2, NULL, [1], NULL, NULL)        = 1 (out [1])
write(1, "hi\n", 3)                     = -1 EAGAIN (Resource temporarily unavailable)

forever.

Then select() should not return fd 1 as writable, at least not
reapeatedly.

Phil.