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SubjectRe: Repost: Bug with select?
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).
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