Idle time of a process in Linux

Question

I need to calculate CPU usage (user mode, system mode, idle time) of a process in Linux. I am able to calculate usage in user and system mode using utime and

Answer 1

I don't know much about it but maybe the following works:

1) Get the process start up time. Im sure thats possible
2) Generate time difference (dTime = CurrentTime - TimeProcessStarted)
3) Substract the time the process is running ( dTime - (usageSystemMode + usageUserMode))

Hope this helps! :D