top command returns 0% CPU usage when I check my process's CPU usage from within the process

Question

I have written a C program which needs to get its own CPU and memory usage. So I have written something like this:

        system("prs_pid=`ps | grep prs-m1 | awk '{print $1}'` \n top -n1 | grep -m1 $prs_pid | \
                    awk '{print \"prs_cpu:\" $7 \"\\nprs_mem:\" $6}' >> /stats");

My application name is prs and I do a PS and get the pid of my process and then want to get CPU usage from running TOP. The program reports it is using 2% memory and 0% CPU. But, running the same command manually on the cmd returns the same memry usage but a valid non-zero CPU that I can verify by running top. What I don't understand is why is the cpu usage always 0% when tried from inside the system?

If you're on Linux, just read from `/proc`: http://stackoverflow.com/q/16726779/8446 — Roger Lipscombe, Nov 02 '16 at 20:44

score 9 · Accepted Answer · answered Nov 02 '16 at 20:10

9

When you're running your system command, the current process is suspended (not sure if it is the proper term, but not running at any rate), waiting for the command you ran to end.

While it's suspended, its CPU usage is 0%, which is expected.

To get the correct information, you have to run your system command in a separate thread or process, so your program can keep running.

answered Nov 02 '16 at 20:10

Jean-François Fabre

137,073
23
153
219

Oh I think you are right, the process state says S when I printed the whole line in top command from within the system. Thank you! – bhargava_sg Nov 02 '16 at 20:29
I used popen() instead of system and now it works! Thanks a lot. The answer was spot on! – bhargava_sg Nov 02 '16 at 20:46
`popen()`! nice hack to avoid using `fork` and threads. I'll keep that in mind. – Jean-François Fabre Nov 02 '16 at 20:48

top command returns 0% CPU usage when I check my process's CPU usage from within the process

1 Answers1