UNIX for Dummies Questions & Answers

Hello all,

I have a very basic question. I have a requirement where in, I have a main process which forks a child process, which execs and runs a c executable corresponding to a daemon. In the c executable corresponding to a daemon, as everyone does, I fork another child process, and as part of child, run the daemon functionality by its parent exiting, so that the child(daemon) becomes a child of init. Now, from the main process(grand parent), I would like to wait for the daemon process(grand child).

Just to explain more clearly, I have a process p1, which forks a new process p2. In p2 functionality, I fork another process p3 which execs and runs a daemon, p2 exits to make p3 a child of init. Now, from p1, I want to wait for p3. I can not use waitpid in the grand parent process to wait for the grand child I believe. How to proceed?

I would be very glad if you can help me.

Thanks in advance,
Sai

You could write a waiting loop, like this:

Code:

start_p2 &
while [ $(ps -fe | grep start_p3 | wc -l) -le 1 ] ; then
     print - "process not started yet"
     sleep 1
done
print - "process start_p3 has started"

Notice that you need to query for more than one process because "grep start_p3" will be found by "grep start_p3" as a process too.

Alternatively add a "grep -v grep" to the pipeline and change the comparison to "-lt 1".

I hope this helps.

Or you try this

Code:

start_p2 &
while [ $(ps -fe | grep [s]tart_p3 | wc -l) -le 1 ] ; then
     print - "process not started yet"
     sleep 1
done
print - "process start_p3 has started"

If the parent PID becomes 1, then your are no longer the grandparent. You could watch for it in a loop if you can determine the PID to look for, for example you could write the PID to a file in /var, which is common.



Robin

I find the stdout/stderr are great at tracking progeny processes:

Code:

( ... ) 2>&1 | cat >log_file

When all processes that did not redirect both stdout and stderr exit, then stdin on cat goes to EOF and it exits.

I wouldn't bat that the own process is always present in the proc table.

Thank you all for your replies. But, I forgot to post another requirement of mine. I need to capture the return status of the grand child and the signals it received at the time of the daemon(grand child) exiting. Can you suggest a solution with this additional requirement?

Thank you all in advance,
Sai