Messages in this thread | | | From | Richard Genoud <> | Date | Wed, 27 Jan 2016 10:14:31 +0100 | Subject | Re: stty blocks forever when the line is already opened (and the tx buffer can't be flushed) |
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2016-01-26 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.
Thank for your enlightenment !
Richard.
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