Messages in this thread | | | From | Alan Curry <> | Subject | tty O_NONBLOCK spooky action at a distance | Date | Sat, 30 Jun 2007 03:35:07 -0400 (EDT) |
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Short version: writing to a tty with O_NONBLOCK will block if there is another, unrelated process already blocking inside a write() to the same tty.
Long version:
Take this test program, nbhello.c
#include <stdio.h> #include <unistd.h> #include <fcntl.h>
int main(int argc, char **argv) { int fd;
if(argc!=2) { fprintf(stderr, "Usage: %s tty\n", argv[0]); return 2; } fd=open(argv[1], O_WRONLY|O_NONBLOCK); if(fd<0) { perror("open"); return 1; } if(write(fd, "hello world\n", 12)<0) { perror("write"); return 1; } return 0; }
Open a tty for testing purposes; I do this by logging in on tty11, but starting a new xterm works just as well. Press ^S on the test tty to block output. Back on your original tty:
$ cc nbhello.c -o nbhello $ ./nbhello /dev/tty11 This will report an EAGAIN, as it should.
$ echo block > /dev/tty11 This will block, as it should. ^C to kill it.
$ echo block > /dev/tty11 & $ ./nbhello /dev/tty11 With a background process blocking in an attempt to write() on the tty, the non-blocking write also blocks! If you kill the background process first, the "non-blocking" write will wake up and return EAGAIN.
I've been surprised before by the way O_NONBLOCK propagates from a forked child back up to the parent's file descriptors, but this not the same thing. The file descriptors involved here have come from 2 completely separate open()s.
(Real-world impact of this bug: wall(1) uses O_NONBLOCK to avoid getting stuck if a user has paused a tty with ^S, but the kernel doesn't respect the flag and wall gets stuck anyway. When the user finally hits ^Q -- hours, days, or weeks later -- he gets the message and so does everyone who was after him in utmp.)
-- Alan Curry pacman@world.std.com - 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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