How do I find out if another program is still running?

nefch · 02-17-2004, 09:14 AM

Hi.

What is the best way to find out if another program is still running? The two programs are unrelated. One program knows the pid of another program and wants to know if that other program is still running.

Regards
Christof

tangle · 02-17-2004, 09:25 AM

Look in /var/run and see if a .pid file is there for that program. Also you could do a ps x | grep <program name> to see if it is running.

nefch · 02-17-2004, 09:35 AM

I do not know the name of the program. I only know the pid.

itsjustme · 02-17-2004, 09:42 AM

Try:

ps -A

jim mcnamara · 02-17-2004, 09:50 AM

Code:

#!/bin/sh
# $1 = pid of process to find
# you may have to adjust the cut
# paramters to match where the pid is on your display
# eg., ps -A or whatever

ps -ef | cut -c 9-15 | grep -q "$1"
if [ $? -eq o ] ; then
     echo "Still running"
else
     echo "Not running"
fi

There is one problem - on busy systems another, new process can come along and have the same pid as the one you were looking for. This would give you a false positive.

If you are waiting on the process from inside C, look into wait(), or waitpid().

nefch · 02-17-2004, 09:52 AM

I need to know how to do this in C/C++. Is there no API to do this, like OpenProcess() in Windows?
I don't want to wait for the other process. I just need to know if it is still around.

jim mcnamara · 02-17-2004, 09:59 AM

OpenProcess() in unix is: fork() exec()
Fork returns the pid to the parent process, zero to the child.
The child runs specific code. The parent skips over it, but can save the pid from the fortk() call.

exec() runs the code. There are different flavors of exec, read the man pages to find the one that meets your needs.

Using the pid still has the problem I outlined earlier, especailly if the parent code runs for a while before checking the child.

nefch · 02-17-2004, 10:29 AM

This doesn't make sense to me. I do not want to start another program - it is already running. There is no parent/child relationship between these two programs.
If program A knows the pid of program B, how can A check if B is still running?
What would A need to know about B to be able to identify B for sure?

jtshaw · 02-17-2004, 12:27 PM

ptrace will let you trace other pids. I realize you don't want to trace it, but you could always attempt to connect to it (it returns an error code if it doesn't exist) and then let go of it if it does exist.

Quote:

Description:
The ptrace system call provides a means by which a parent process may observe and control the execution of another process, and examine and change its core image and registers. It is primarily used to implement breakpoint debugging and system call tracing.

kalleanka · 02-17-2004, 03:05 PM

check the kill comman. Sounds strange but kill is to send a signal and not only a killing signal.

main()
{
int pid;

/* ... Get the process ID of this program. */

getpid(pid);

/* ... Check to see if this program is active. */

printf("R = %d\n", kill (pid, 0));
}

nefch · 02-18-2004, 02:49 AM

kill or ptrace seem to be the way to go. Thanks for your help!

Christof

worldmagic · 02-18-2004, 06:36 AM

A nicer (cleaner way) is to ask for the process nicelevel:

int prio = getpriority(PRIO_PROCESS, pid);

if( errno ) {
puts("Unable to get nice level for requested process");
}else{
puts("Process alive and kicking");
}

The downside to kill and ptrace is that they realy do change something for the process..

kalleanka · 02-18-2004, 09:41 AM

worldmagic thats the way to do it.