Re: Repost: Bug with select?

[Marco Roeland <marco.roeland@xs4all.nl>]

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'!

>     len = write(fd, "hi\n", 3);

You don't check the exit status here, but when you press Ctrl-C (stdout
blocked) it will indicate an error here (exit status -1) with errno set
to EAGAIN, meaning you should try again, which is the appropriate result
for a non-blocking descriptor or socket here. Anyway, the call "succeeds" and
we loop back into the while(1), indeed as you say creating a busy loop.
No surprises there I'd say.

>   }
>   fcntl(fd, F_SETFL, flags);
> }

You might start by checking for EAGAIN as result of the write, and then
reacting according to your needs (waiting a while or exiting the
program or whatever).