Re: [bug-ncurses] Reset clears CLOCAL flag breaking some hardware serial ports

[Thomas Dickey]

On Thu, Oct 05, 2023 at 09:35:04AM +0200, Dr. Werner Fink wrote:
> On 2023/10/04 10:40:35 +0000, Michal Suchanek wrote:
> > Hello,
> > 
> > on some development boards and server BMCs the hardware apparently
> > lacks the modem control lines.
> > 
> > The OS correctly sets the CLOCAL flag and the terminal works. However,
> > when there is a need to reset the terminal after adjusting the TERM
> > variable the reset tool clears the CLOCAL flag. This is not documented
> > in the man page nor can I see any reason in the git history why this
> > functionality was added (in ncurses 4.1 AFAICS).
> 
> That is back more than 25 years ... ncurses-4.1/progs/tset.c

"more than 25 years" actually goes back to 1995 (28 years), as mentioned here

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=60377

call that ncurses 1.9.6:

https://invisible-island.net/ncurses/ncurses-license.html#ncurses_1_9_6

Just changing it may be disruptive, which prompts caution.

In my response on the Debian bug report, I had in mind making a table for
the manual to explain the change, and perhaps make it an option rather
than surprise everyone.
 
>  static void
>  reset_mode(void)
> 
>  [...]
> 
>  mode.c_cflag &= ~(CSIZE | CSTOPB | PARENB | PARODD | CLOCAL);
> 
> 
> > 
> > On the affected devices (such as Raspberry Pi) the symptom is that
> > terminal applications using the alternate screen such as less or vim
> > keep waiting in the kernel and never start.
> > 
> > I don't think that the CLOCAL flag is part of what the reset tool
> > should change much like the terminal baud rate - this is property of
> > how the terminal is connected, and not the terminal type.
> 
> I suggest to check if the open file descriptor of the the tty line has
> set O_NONBLOCK via fcntl/F_GETFL and if so not to clear CLOCAL.
> Normally opening a a port to a modem blocks until the modem reports
> carrier detection, therefore O_NONBLOCK is used.  Then the first read
> or write call will wait for carrier to be established.
> 
> Werner
> 
> -- 
>   "Having a smoking section in a restaurant is like having
>           a peeing section in a swimming pool." -- Edward Burr



-- 
Thomas E. Dickey <dickey@invisible-island.net>
https://invisible-island.net

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