Hi Richard, On 01/26/2016 08:19 AM, Richard Genoud wrote: > [ sorry for the noise, I forgot to Cc the lkml ] > > Hi, > I've found a case were calling > stty -F /dev/ttyS1 clocal > blocks forever. > And I don't know if it's a very old bug or if it's meant to be like that. > > Here is how to reproduce the lock : > NB: there's NO modem on ttyS1 > stty -F /dev/ttyS1 clocal cread crtscts > cat < /dev/ttyS1 > > #on another terminal : > echo "dummy" > /dev/ttyS1 # This call doesn't block > > stty -F /dev/ttyS1 -crtscts # this blocks forever on ioctl(TCSETSW ) > > > looking at tty_port_close_start(), it's pretty clear that nothing is > flushed until the last user, so it explains why the "echo dummy" > returns directly, despite the crtscts flags. > And in tty_mode_ioctl(), there are the lines: > case TCSETSW: > return set_termios(real_tty, p, TERMIOS_WAIT | TERMIOS_OLD); > That explain why the stty blocks. > > But this behavior seems really strange. > ... Or it's meant to be like that ?
Yeah, meant to be like that. When mgetty writes the login prompt but h/w flow control is enabled and nothing's connected, the output is buffered. Since stty uses tcsetattr(TCSADRAIN), the attempt to turn off h/w flow control blocks, waiting for output to empty. In this situation, stopping mgetty will allow the other process to unblock and advance. Hmmm, I could add a -f,--force flag to stty so it uses tcsetattr(TCSANOW)... Regards, Peter Hurley > Regards, > Richard > > NB: This is actually a real life use case with mgetty, a modem losing > its power and another process trying to speak to the modem. >
