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 [EMAIL PROTECTED] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/