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

Question

I want to programmatically  in C  calculate CPU usage   for a given process ID in Linux   How can we get the realtime CPU usage   for a given process   To make it further clear    I should be able to determine the CPU usage for the provided processid or process    The process need not be the child process    I want the solution in  C  language

User · Answer

Use strace found the CPU usage need to be calculated by a time period     top -b -n 1 -p 3889 top - 16 46 37 up  1 04   3 users   load average  0 00  0 01  0 02 Tasks    1 total    0 running    1 sleeping    0 stopped    0 zombie  Cpu s    0 0 us   0 0 sy   0 0 ni 100 0 id   0 0 wa   0 0 hi   0 0 si   0 0 st KiB Mem    5594496 total   5158284 free    232132 used    204080 buff cache KiB Swap   3309564 total   3309564 free         0 used   5113756 avail Mem     PID USER      PR  NI    VIRT    RES    SHR S   CPU  MEM     TIME  COMMAND  3889 root      20   0  162016   2220   1544 S   0 0  0 0   0 05 77 top     strace top -b -n 1 -p 3889         stat   proc 3889    st mode S IFDIR 0555  st size 0          0 open   proc 3889 stat   O RDONLY          7 read 7   3889  top  S 3854 3889 3854 3481      1024    342        nanosleep  0  150000000   NULL            0       stat   proc 3889    st mode S IFDIR 0555  st size 0          0 open   proc 3889 stat   O RDONLY          7 read 7   3889  top  S 3854 3889 3854 3481      1024    342

User · Answer

Instead of parsing this from proc  one can use functions like getrusage   or clock gettime   and calculate the cpu usage as a ratio or wallclock time and time the process thread used on the cpu

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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

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I wrote two little C function based on cafs answer to calculate the user kernel cpu usage of of an process  https   github com fho code snippets blob master c getusage c

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Install psacct or acct package   Then use the sa command to display CPU time used for various commands   sa man page  A nice howto from the nixCraft site

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You can read the manpage for proc for more detail  but in summary you can read  proc  number  stat to get the information about a process  This is also used by the  ps  command   All the fields and their scanf format specifiers are documented in the proc manpage   Here are some of the information from the manpage copied  it is quite long              pid  d The process ID             comm  s                  The  filename of the executable  in parentheses   This is                  visible whether or not the executable is swapped out             state  c                  One character from the string  RSDZTW  where  R  is  run                    ning   S is sleeping in an interruptible wait  D is wait                    ing in uninterruptible disk sleep   Z  is  zombie   T  is                  traced or stopped  on a signal   and W is paging             ppid  d                  The PID of the parent             pgrp  d                  The process group ID of the process             session  d                  The session ID of the process             tty nr  d                  The tty the process uses             tpgid  d                  The  process group ID of the process which currently owns                  the tty that the process is connected to

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I think it s worth looking at GNU  time  command source code  time It outputs user system cpu time along with real elapsed time  It calls wait3 wait4 system call  if available  and otherwise it calls times system call  wait  system call returns a  rusage  struct variable and times system call returns  tms   Also  you can have a look at getrusage system call which also return very interesting timing information  time

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Easy step to step for beginners like me    Read the first line of  proc stat to get total cpu usage1        sscanf line    s  llu  llu  llu  llu   amp user  amp nice  amp system  amp idle       total cpu usage1   user   nice   system   idle     Read  proc pid stat where pid is the PID of the process you want to know the CPU usage  like this        sscanf line         d   s   c   d    pid command state ppid         d   d   d   d   u   lu   lu   lu   lu         lu  lu    usertime systemtime         ld   ld   ld   ld   ld   ld   llu          lu     virtual memory size in bytes              Now sum usertime and systemtime and get proc times1 Now wait 1 second or more Do it again  and get total cpu usage2 and proc times2   The formula is    number of processors     proc times2 - proc times1    100    float   total cpu usage2 - total cpu usage1    You can get the amount of CPU s from  proc cpuinfo

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When you want monitor specified process  usually it is done by scripting  Here is perl example  This put percents as the same way as top  scalling it to one CPU  Then when some process is active working with 2 threads  cpu usage can be more than 100   Specially look how cpu cores are counted  D then let me show my example      usr bin perl  my  pid 1234   insert here monitored process PID   returns current process time counters or single undef if unavailable  returns   1  process counter    2  system counter   3  total system cpu cores sub GetCurrentLoads       my  pid shift      my  fh      my  line      open  fh   lt     proc  pid stat  or return undef       line  lt  fh gt       close  fh      return undef unless  line     d             S  d   d   d   d  -  d   d   d   d   d   d    d     d         my  TimeApp  1  2      my  TimeSystem 0      my  CpuCount 0      open  fh   lt     proc stat  or return undef      while  defined  line  lt  fh gt              if   line    cpu s                 foreach my  nr   line    d  g     TimeSystem   nr                 next                      CpuCount   if  line    cpu d             close  fh      return undef if  TimeSystem  0      return  TimeApp  TimeSystem  CpuCount     my   currApp  currSys  lastApp  lastSys  cores   while            currApp  currSys  cores  GetCurrentLoads  pid       printf  Load is   5 1f   n    currApp- lastApp    currSys- lastSys   cores 100 if defined  currApp and defined  lastApp and defined  currSys and defined  lastSys        lastApp  lastSys    currApp  currSys       sleep 1      I hope it will help you in any monitoring  Of course you should use scanf or other C functions for converting any perl regexpes I ve used to C source  Of course 1 second for sleeping is not mandatory  you can use any time  effect is  you will get averrage load on specfied time period  When you will use it for monitoring  of course last values you should put outside  It is needed  because monitoring usually calls scripts periodically  and script should finish his work asap

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You need to parse out the data from  proc  lt PID gt  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

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Take a look at the  pidstat  command  sounds like exactly what you require

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This is my solution        this program is looking for CPU Memory Procs also u can look glibtop header there was a lot of usefull function have fun   systeminfo c     include  lt stdio h gt   include  lt glibtop h gt   include  lt glibtop cpu h gt   include  lt glibtop mem h gt   include  lt glibtop proclist h gt     int main     glibtop init     glibtop cpu cpu  glibtop mem memory  glibtop proclist proclist   glibtop get cpu   amp cpu   glibtop get mem  amp memory     printf  CPU TYPE INFORMATIONS  n n   Cpu Total    ld  n   Cpu User    ld  n   Cpu Nice    ld  n   Cpu Sys    ld  n   Cpu Idle    ld  n   Cpu Frequences    ld  n    unsigned long cpu total   unsigned long cpu user   unsigned long cpu nice   unsigned long cpu sys   unsigned long cpu idle   unsigned long cpu frequency    printf   nMEMORY USING n n   Memory Total    ld MB n   Memory Used    ld MB n   Memory Free    ld MB n   Memory Buffered    ld MB n   Memory Cached    ld MB n   Memory user    ld MB n   Memory Locked    ld MB n    unsigned long memory total  1024 1024    unsigned long memory used  1024 1024    unsigned long memory free  1024 1024    unsigned long memory shared  1024 1024    unsigned long memory buffer  1024 1024    unsigned long memory cached  1024 1024    unsigned long memory user  1024 1024    unsigned long memory locked  1024 1024     int which arg  glibtop get proclist  amp proclist which arg   printf   ld n ld n ld n    unsigned long proclist number   unsigned long proclist total   unsigned long proclist size   return 0     makefile is CC gcc CFLAGS -Wall -g CLIBS -lgtop-2 0 -lgtop sysdeps-2 0 -lgtop common-2 0  cpuinfo cpu c   CC    CFLAGS  systeminfo c -o systeminfo   CLIBS  clean  rm -f systeminfo

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

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