tcsetattr(fd, TCSAFLUSH) will always return -EINTR on a pts device
when O_ASYNC has been set. This is demonstrated by the test program
below - just compile and follow the instructions. You'll get an
infinite stream of SIGIOs and -EINTRs.
The underlying reason is that the pty driver keeps its TTY_THROTTLED
flag set all the time:
static void check_unthrottle(struct tty_struct * tty)
{
if (tty->count &&
test_and_clear_bit(TTY_THROTTLED, &tty->flags) &&
tty->driver->unthrottle)
tty->driver->unthrottle(tty);
}
tty->driver->unthrottle is pty_unthrottle in this case:
static void pty_unthrottle(struct tty_struct * tty)
{
...
tty_wakeup(o_tty);
set_bit(TTY_THROTTLED, &tty->flags);
}
tty_wakeup naturally wakes up any processes sleeping on the device
and also queues a SIGIO to any processes wanting async I/O
notifications. However, the fact that a SIGIO is queued means that
the ioctl underneath tcsetattr returns -EINTR. When userspace sees
this and retries the call, we go around the same loop:
make sure that output is flushed
call the unthrottle routine because TTY_THROTTLED is set
deliver another SIGIO
set TTY_THROTTLED
return -EINTR
The call stack looks like this:
kill_fasync
tty_wakeup
pty_unthrottle
check_unthrottle
n_tty_flush_buffer
set_termios
n_tty_ioctl
tty_ioctl
I'd love to get rid of the set_bit(TTY_THROTTLED, &tty->flags) in
pty_unthrottle, but it's protected by this comment:
/* For PTY's, the TTY_THROTTLED
* flag is always set, to force the line discipline to always call the
* unthrottle routine when there are fewer than TTY_THRESHOLD_UNTHROTTLE
* characters in the queue. This is necessary since each time this
* happens, we need to wake up any sleeping processes that could be
* (1) trying to send data to the pty, or (2) waiting in wait_until_sent()
* for the pty buffer to be drained.
*/
Failing that, there should be a relevant state change in the device
before it will deliver a SIGIO. I just have no idea where to put it.
Jeff
--
Work email - jdike at linux dot intel dot com
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define __USE_GNU /* Needed to get F_SETSIG */
#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <termios.h>
int pts_fd;
void sigio(int sig)
{
struct pollfd fd;
int err;
printf("SIGIO\n");
fd.fd = pts_fd;
fd.events = POLLIN | POLLPRI | POLLOUT | POLLERR | POLLHUP | POLLNVAL;
fd.revents = 0;
err = poll(&fd, 1, 0);
if (err < 0) {
perror("poll");
exit(1);
}
printf("poll returns revents = 0x%x\n", fd.revents);
}
int main(int argc, char **argv)
{
struct termios save;
int err;
char c;
pts_fd = open("/dev/ptmx", O_RDWR);
if (pts_fd < 0) {
perror("Opening /dev/ptmx");
exit(1);
}
err = grantpt(pts_fd);
if (err) {
perror("grantpt");
exit(1);
}
if (unlockpt(pts_fd) < 0) {
perror("unlockpt");
exit(1);
}
if((fcntl(pts_fd, F_SETFL, O_ASYNC | O_NONBLOCK) < 0) ||
(fcntl(pts_fd, F_SETSIG, SIGIO) < 0) ||
(fcntl(pts_fd, F_SETOWN, getpid()) < 0)){
perror("F_SETFL, F_SETSIG, or F_SETOWN");
exit(1);
}
signal(SIGIO, sigio);
err = tcgetattr(pts_fd, &save);
if (err) {
perror("tcgetattr");
exit(1);
}
printf("Attach screen to %s and hit return here\n", ptsname(pts_fd));
read(0, &c, 1);
do {
err = tcsetattr(pts_fd, TCSAFLUSH, &save);
if (!err)
break;
err = errno;
perror("tcsetattr");
} while(err == EINTR);
}
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