On Sun, Jun 09, 2019 at 09:51:32PM +0200, Pavel Machek wrote: > Hi! > > > > > This patch series introduces TTY keyboard status request, a feature of > > > > the n_tty line discipline that reserves a character in struct termios > > > > (^T by default) and reacts to it by printing a short informational line > > > > to the terminal and sending a Unix signal to the tty's foreground > > > > process group. The processes may, in response to the signal, output a > > > > textual description of what they're doing. > > > > > > > > The feature has been present in a similar form at least in > > > > Free/Open/NetBSD; it would be nice to have something like this in Linux > > > > as well. There is an LKML thread[1] where users have previously > > > > expressed the rationale for this. > > > > > > > > The current implementation does not break existing kernel API in any > > > > way, since, fortunately, all the architectures supported by the kernel > > > > happen to have at least 1 free byte in the termios control character > > > > array. > > > > > > I like the idea... I was often wondering "how long will this dd take". > > > (And in > > > case of dd, SIGUSR1 does the job). > > > > > > I assume this will off by default, so that applications using ^T today > > > will not > > > get surprise signals? > > > > If any of isig, icanon and iexten is disabled on the tty, the signal is > > not sent. > > As expected. > > > Any application that wants to handle raw terminal input events itself, > > e.g. vim, mutt, libreadline, anything ncurses-based, etc., has to turn > > off the tty's cooked mode, i.e. at least icanon. This means those > > applications are unaffected. > > Agreed, those are unaffected. > > But if I have an application doing read() from console (without > manipulating tty), am I going to get surprise signal when user types > ^T? > > Pavel
As of now, that application will indeed receive a signal that is guaranteed to be ignored by default. This is similar to SIGWINCH, which is default-ignored as well: if the terminal width/height changes (like when a terminal emulator window is resized), its foreground pgrp gets a surprise signal as well, and the processes that don't care about WINCH (and thus have default disposition) do not get confused. E.g. 'strace cat' demonstrates this quite clearly.
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