How to set process ID in Linux for a specific program

Question

I was wondering if there is some way to force to use some specific process ID to Linux to some application before running it. I need to know in advance the process ID.

Answer 1

Actually, there is a way to do this. Since kernel 3.3 with CONFIG_CHECKPOINT_RESTORE set(which is set in most distros), there is /proc/sys/kernel/ns_last_pid which contains last pid generated by kernel. So, if you want to set PID for forked program, you need to perform these actions:

1. Open /proc/sys/kernel/ns_last_pid and get fd
2. flock it with LOCK_EX
3. write PID-1
4. fork

Voilà! Child will have PID that you wanted. Also, don't forget to unlock (flock with LOCK_UN) and close ns_last_pid.

You can checkout C code at my blog here.

Answer 2

As many already suggested you cannot set directly a PID but usually shells have facilities to know which is the last forked process ID.

For example in bash you can lunch an executable in background (appending &) and find its PID in the variable $!. Example:

$ lsof >/dev/null & [1] 15458 $ echo $! 15458