Geek asks and answers

Why won't CTRL + C work when using RS-232 on Linux?

First of all, I don’t know if it should stay in SO or go in SU: tell me. The decision may be related to programming.

I run Linux on an embedded device and communicate with it using RS-232 @ 9600 baud. Everything works fine using PuTTY on Windows: I have a shell and you can enter and execute commands.

The problem is this: when I run the command, I cannot use CTRL + C. For example, when pinging some machine, ping goes into an infinite loop, and I cannot stop it using CTRL + C. If at the Bash prompt, CTRL + C works and moves on to the next line (so it gets passed). I also noticed that the terminal shows ^Cwhen I execute CTRL + C during command execution. When connected via Telnet, CTRL + C works great anywhere.

I tried using the "special command" PuTTY, but it does not work. I also tried different terminal emulators, same problem.

So, I think the problem is with the kernel. Is there anything that I could study in this regard?

Change . I am running BusyBox v1.13.2. Output Signal stty -a(RS-232):

speed 9600 baud; rows 24; columns 80;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W;
lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon ixoff
-iuclc -ixany -imaxbel
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
echoctl echoke

stty -a (Telnet):

speed 38400 baud; rows 24; columns 80;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W;
lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 -hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff
-iuclc -ixany -imaxbel
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab3 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
echoctl echoke

, ls -la /bin, , , , CTRL + C, , . . ping.

, seq 1 1000, CTRL + C, , - :

93
94
95
^C6
897
898
899

ls -la /bin:

lrwxrwxrwx    1 10042    2223            7 May  6  2012 dmesg -> busybox
lrwxrwxrwx    1 10042    2223            7 May  6  2012 dos2unix -> busybox
lrwxrwxrwx    1 10042    2223            7^C          7 May  6  2012 ipcrm -> busybox
lrwxrwxrwx    1 10042    2223            7 May  6  2012 ipcs -> busybox
lrwxrwxrwx    1 10042    2223            7 May  6  2012 iplink -> busybox
+3
1

, , , . , stty ( ) , ioctl().

http://linux.die.net/man/1/stty

stty sane

, . "" . ,

stty raw

Linux , , ctrl-C prints-but-do-nothing, .

stty , - telnet.

: - busybox BRKINT -, :

Date: Thu, 31 Jan 2002 13:34:34 -0800
From: Scott Anderson <scott_anderson at [removed]>
Cc: linuxppc-dev at lists.linuxppc.org
Subject: Re: why is tty->pgrp set to -1 for console?

>   What is the correct procedure to follow to get around this problem
> and get ctrl-c working on console?

It looks like everyone is taking a swing at this one, so I think I'll
join in.  First off, the easiest way I've found to track down why
ctrl-c doesn't work is to just run "ps -j".  For ctrl-c to work, you
need a controlling terminal (the TTY column) and a process group.  If
you have a '?' in the TTY column, ctrl-c won't work.  In the past I
have seen this happen because of this code in drivers/char/tty_io.c:
        if (device == SYSCONS_DEV) {
                struct console *c = console_drivers;
                while(c && !c->device)
                        c = c->next;
                if (!c)
                        return -ENODEV;
                device = c->device(c);
                filp->f_flags |= O_NONBLOCK; /* Don't let /dev/console block */
                noctty = 1;
        }
Note that O_NOCTTY (no controlling terminal) is forced on whenever
/dev/console is opened (noctty = 1).  Possible workarounds:
  1) Run getty on something other than /dev/console.  For example,
     if you console is on the first serial port, run getty on /dev/ttyS0.
     I believe this is the "correct" answer.
  2) You could also change getty to do a TIOCSCTTY ioctl explicitly after
     it has opened the terminal.
  3) You could remove the forcing of noctty on from tty_io.c
+2

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