How do I terminate backgrounded ssh instances - c

Question

I have written C program that uses fork(2) and execl(3) to run ssh for port forwarding purposes.
The ssh's are run in the background the -f option.
When the C program exits, I want it to send SIGTERM to the ssh instances it spawned.
I have tried
// creating the ssh process
ssh_pid = fork();
if (ssh_pid == 0)
execl("/usr/bin/ssh", "/usr/bin/ssh", "-f", other options, NULL)
// ...
printf("Killing %d\n", ssh_pid); // <--- output the PID
kill(ssh_pid, 15);
sleep(1); // give it a chance to exit properly
if (kill(ssh_pid, 0) == 0)
kill(ssh_pid, 9); // the "shotgun" approach
However, this doesn't work (even with the SIGKILL).
If I run ps before the program exits
ps aux | grep ssh | grep -v sshd | grep -v grep
I see something like this:
user 27825 0.2 0.0 0 0 pts/0 Z+ 18:23 0:00 [ssh] <defunct>
user 27834 0.0 0.0 41452 1176 ? Ss 18:23 0:00 /usr/bin/ssh -f [other options]
When the program prints the PID it is killing, I see this:
Killing 27825
Subsequently repeating the ps gives me:
user 27834 0.0 0.0 41452 1176 ? Ss 18:23 0:00 /usr/bin/ssh -f [other options]
It seems that the original ssh has forked itself in order to become a background process.
So I changed my call to kill(2) to attempt to kill all processes spawned by the original ssh:
kill(-ssh_pid, 15);
But this appears to have no effect. I suspect it is because the original ssh is no longer the parent of the backgrounded ssh.
So, how do I safely kill the backgrounded ssh? Is it even possible?

Answer 1

The solution I have just found is not to use the -f option at all, and background the ssh myself.
ssh_pid = fork();
if (ssh_pid == 0)
{
freopen("/dev/null", "r", stdin);
freopen("/dev/null", "w", stdout);
freopen("/dev/null", "w", stderr);
execl("/usr/bin/ssh", "/usr/bin/ssh", other options, NULL);
}
Because when ssh gets the -f option it creates a child, and signals sent to the parent won't get passed to the child.