I have to implement a program A that can prevent process B from being killed. And I can kill B with A by passing A some command...
BTW, Both A and B are background application.

I don't know how to do it.

Regards.

What do you mean by program a preventing process b from being killed? Do you mean that program a is going to grab a semiphore that program b needs to have to exit (or something along these lines) or do you mean that program a is suppose to catch all the signals designated for program b so that no outsiders can pass a SIGKILL into program b?

jtshaw,
thank you.

Sorry for my unclear meaning.

It's not so complicated. What I mean is that A is a monitor that queries B's status. If B has been killed illegally, A start B again.
And I can kill A with A by passing A somm command...

Regards

Ahh... ok, I get it.

What I would do is create a little program that has two threads... one which checks for use input on the command line (like a Q for quit or something) and one that checks a particular pid (perhaps that is passed in on the command line when the program is envoked) and checks to make sure it is still running.

You can check to see if a process pid is still running by seeing if it's directory still exists in /proc. The first time you check it you would want to save the value of /proc/<pid>/cmdline as well as the value of /proc/pid/cwd. This will tell you both the directory the pid was run from as well as the cmdline used to run it. You could then fork and exec to restart the program if it wasn't running anymore.

The probelm is this: I don't know how to passing command to a background application.
Any method?

It create a pipe (man 2 pipe) if I wanted to be able to pass information into a backgrounded process.

jtshaw,

thanks a lot.

I'll try it, thank you!