How to send Ctrl-C control character or terminal hangup message to child process?

Question

I have a child process which runs in a pseudo terminal. The parent process does not run as root, but the child process does, through su or sudo. Because of this it is not possible to send a signal to the child process to force it to exit. I want to force it to exit by one of these means:

• emulating a Ctrl-C.
• emulating a terminal hangup.

How do I do either of these? I already have a pty master fd, and I've tried something like this:

write(master, &termios.c_cc[VINTR], 1)

but it doesn't do anything.

Answer 1

It seems to me that if you truly have a pty (unless you mean something else by pseudo terminal), that all you have to do is send a Control-C to that FD. As evidence of this, I submit the following code in Python (but fairly close to the C required to do it):

import pty, os, sys, time

pid, fd = pty.fork()
if pid == 0:
   os.execv('/bin/sh', ['/bin/sh', '-c',
         'while true; do date; sleep 1; done'])
   sys.exit(0)
time.sleep(3)
os.write(fd, '^C')
print 'results:', os.read(fd, 1024)

This forks a process under a pty, which runs an infinite loop printing the date. Then the parent waits 3 seconds and sends a control-C.

This results in the following output:

guin:/tmp$ time python /tmp/foo
results: Fri Feb  5 08:28:09 MST 2010
Fri Feb  5 08:28:10 MST 2010
Fri Feb  5 08:28:11 MST 2010

python /tmp/foo  0.02s user 0.01s system 1% cpu 3.042 total
guin:/tmp$

It ran just over 3 seconds, printed out the date 3 times, and exited.

Answer 2

I eventually went with the following solution:

After forking, instead of exec'ing sudo immediately, I exec() a helper child process instead, which in turn forks and execs sudo and calls waitpid on it. So the process hierarchy looks like this:

original process          <---- runs as user
  |
  +-- helper process      <---- runs as user, session leader,
         |                      has own pty, in pty's foreground process group
         |
         +--- sudo        <---- runs as root

By killing the helper process, the pty does not have a foreground process anymore. This will cause the OS to send SIGHUP to the entire foreground process group, regardless of the user, so sudo is SIGHUP'ed too.