Messages in this thread | | | Date | Fri, 25 Jul 2003 15:41:55 +0200 | From | Marco Roeland <> | Subject | Re: Repost: Bug with select? |
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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). - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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