Messages in this thread | | | From | Richard Genoud <> | Date | Wed, 27 Jan 2016 11:26:57 +0100 | Subject | Re: stty blocks forever when the line is already opened (and the tx buffer can't be flushed) |
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2016-01-27 10:14 GMT+01:00 Richard Genoud <richard.genoud@gmail.com>: > 2016-01-26 18:13 GMT+01:00 Peter Hurley <peter@hurleysoftware.com>: >> 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. > Damn ! I forgot about the TCSADRAIN/FLUSH/NOW flags > >> 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)... > Yes, I think such a flag will make sense. In fact, a similar flag exist in the very last version of coreutils (8.25) http://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=cb7d8b90a213c0186a8c8ba66da959e1f5930e78 :)
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