Run a process to particular/dedicated pid only

Question

I have a c program executable or shell script which I want to run very often. If I want to stop/pause or to notify something I will send signal to that process. So every time I have to check the pid of that process and I have to use kill to send a signal.

Every time I have to check pid and remembering that upto system shutdown, really bad. I want that process has to run on particular pid only like init always run on 1.

Is there any C api for that? and Also needed script for bash program.

Answer 1

I don't think you can reserve or assign PIDs. However, you could start your process in a script like this:

myprocess &
echo "$!" > /tmp/myprocess.pid

This creates a "pid file", as some other people have referred to it. You can then fetch that in bash with, e.g., $(</tmp/myprocess.pid) or $(cat /tmp/myprocess.pid).

Just beware when you do this that if the process died and the pid was recycled, you'll be signalling the wrong thing. You can check with:

pid=$(cat /tmp/myprocess.pid)
if [ "$(ps -o comm= -p "$pid")" = "myprocess" ]; then
    ...send your signal...
else echo "Myprocess is dead!"
fi

See comments if "$(ps -o comm= -p "$pid")" looks strange to you. You may want to do a more vigorous validation if there is a chance of someone doing something devious with the content of /tmp/myprocess.pid (which should not be writeable by other users!).

Answer 2

Fixing the pid is definitely the wrong solution to your problem, but note that with some versions of Linux, you can get a better chance to obtain the pid you'd like by writing a value to /proc/sys/kernel/ns_last_pid:

echo 9999 | sudo tee /proc/sys/kernel/ns_last_pid; ps -C ps
9999
  PID TTY          TIME CMD
10000 pts/3    00:00:00 ps

That only works if the pid 10000 is not already in use (and there's been no pid or thread creation between the time you write to ns_last_pid and you spawn a process/thread).

Otherwise, you can always fork until you get the pid you like.

Answer 3

Something similar to what you want to do is normally done by the process early in its lifecycle writing out its own pid (which can be obtained through getpid(2)) to a file with a known name. In general-use daemons the name of this file is often configurable, but in a special-use software you can probably get along with hardcoding it. (I strongly suggest at least using a macro for it, however.)

PID files are normally placed in /var/run or /run, but can be placed in other locations as well including /tmp. The "proper" location according to the Filesystem Hierarchy Standard is in /run, but /var/run also sees significant use (and on many modern systems is the same as /run) and /tmp don't require root privileges on startup (which system daemons very often have before they drop privileges).

That file can then be read through a variety of means to obtain the PID of the process in question, in order to send a signal to it process in question.

Answer 4

You can not set the PID, but you can set the PGID: create or join a process group. Then you can send signals to this dedicated process group.

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I had the impression that the new systemd init system has some automation on this part, which is superior to having the process to write its PID to a PID file and then using it for controlling it.

systemd seems to switch to a "process group" (as I can understand this) before starting a controlled process, and then everything is in this group. So, you can control all the child processes by remembering the special "group".

• If it functions like, this is superior to having the process to write out its PID, because you don't need to modify the program.

• It might also be better then:

  myprocess & echo $! > /tmp/myprocess.pid

because this approach captures all the children of that process, too.

I don't have a detailed documentation at hand to support my words, but here is the general idea of what systemd needs from cgroups,a nd this seems to match my impression:

Control Groups are two things: (A) a way to hierarchally group and label processes, and (B) a way to then apply resource limits to these groups. systemd only requires the former (A), and not the latter (B). That means you can compile your kernel without any control group resource controllers (B) and systemd will work perfectly on it. However, if you in addition disable the grouping feature entirely (A) then systemd will loudly complain at boot and proceed only reluctantly with a big warning and in a limited functionality mode.

Answer 5

System level daemon are assigned as per the sequence of start during boot time. I believe assigning a fixed pid to particular process is not possible because it is getting assigned by the Kernel as per the system load and other dependencies.

But using system call we may assign a fixed PID to the process. But no idea how can it be achieved.