How to calculate the CPU usage of a process by PID in Linux from C?

You need to parse out the data from /proc/<PID>/stat. These are the first few fields (from Documentation/filesystems/proc.txt in your kernel source):

Table 1-3: Contents of the stat files (as of 2.6.22-rc3)
..............................................................................
 Field          Content
  pid           process id
  tcomm         filename of the executable
  state         state (R is running, S is sleeping, D is sleeping in an
                uninterruptible wait, Z is zombie, T is traced or stopped)
  ppid          process id of the parent process
  pgrp          pgrp of the process
  sid           session id
  tty_nr        tty the process uses
  tty_pgrp      pgrp of the tty
  flags         task flags
  min_flt       number of minor faults
  cmin_flt      number of minor faults with child's
  maj_flt       number of major faults
  cmaj_flt      number of major faults with child's
  utime         user mode jiffies
  stime         kernel mode jiffies
  cutime        user mode jiffies with child's
  cstime        kernel mode jiffies with child's

You're probably after utime and/or stime. You'll also need to read the cpu line from /proc/stat, which looks like:

cpu  192369 7119 480152 122044337 14142 9937 26747 0 0

This tells you the cumulative CPU time that's been used in various categories, in units of jiffies. You need to take the sum of the values on this line to get a time_total measure.

Read both utime and stime for the process you're interested in, and read time_total from /proc/stat. Then sleep for a second or so, and read them all again. You can now calculate the CPU usage of the process over the sampling time, with:

user_util = 100 * (utime_after - utime_before) / (time_total_after - time_total_before);
sys_util = 100 * (stime_after - stime_before) / (time_total_after - time_total_before);

Make sense?

getrusage() can help you in determining the usage of current process or its child

Update: I can't remember an API. But all details will be in /proc/PID/stat, so if we could parse it, we can get the percentage.

EDIT: Since CPU % is not straight forward to calculate, You could use sampling kind of stuff here. Read ctime and utime for a PID at a point in time and read the same values again after 1 sec. Find the difference and divide by hundred. You will get utilization for that process for past one second.

(might get more complex if there are many processors)