Calculate Execution Time


julio@acer ~> time ./program
  Hello World!
  ./program  1.54s user 0.03s system 97% cpu 1.611 total
julio@acer ~> type -a time
  time is a reserved word
julio@acer ~> which time
  time: shell reserved word
julio@acer ~> sudo pacman -S time
julio@acer ~> /usr/bin/time ./programa
  Hello World!
  1.39user 0.00system 0:01.41elapsed 99%CPU (0avgtext+0avgdata 8940maxresident)k
  0inputs+0outputs (0major+758minor)pagefaults 0swaps
julio@acer ~> /usr/bin/time -v ./programa
  Hello World!
      Command being timed: "./programa"
      User time (seconds): 1.57
      System time (seconds): 0.05
      Percent of CPU this job got: 99%
      Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.63
      Average shared text size (kbytes): 0
      Average unshared data size (kbytes): 0
      Average stack size (kbytes): 0
      Average total size (kbytes): 0
      Maximum resident set size (kbytes): 8936
      Average resident set size (kbytes): 0
      Major (requiring I/O) page faults: 0
      Minor (reclaiming a frame) page faults: 758
      Voluntary context switches: 1
      Involuntary context switches: 181
      Swaps: 0
      File system inputs: 0
      File system outputs: 3160
      Socket messages sent: 0
      Socket messages received: 0
      Signals delivered: 0
      Page size (bytes): 4096
      Exit status: 0
julio@acer ~> /usr/bin/time -p ./programa
Hello World!
real 1.45
user 1.44
sys 0.00

alias time='/usr/bin/time -p'


The precision of time and /usr/bin/time is not very good if you want to do serious profiling or want to check the time spent by a specific part of the code and not the whole program. For these cases, on Linux and FreeBSD, we can use the function clock_gettime from time.h.

Looking at the file /usr/include/time.h int clock_gettime(clockid_t clk_id, struct timespec *tp);

CLOCK_REALTIME: System clock. CLOCK_MONOTONIC: CLOCK_PROCESS_CPUTIME_ID: Timer provided by the CPU to each process. CLOCK_THREAD_CPUTIME_ID: Timer provided by the CPU to each thread.

#include <ctime>

timespec diff(timespec start, timespec end)
    timespec temp;
    if (end.tv_nsec - start.tv_nsec < 0)
        temp.tv_sec = end.tv_sec - start.tv_sec-1;
        temp.tv_nsec = 1000000000 + end.tv_nsec - start.tv_nsec;
        temp.tv_sec = end.tv_sec - start.tv_sec;
        temp.tv_nsec = end.tv_nsec - start.tv_nsec;
    return temp;

int main()
    timespec time1, time2;
    clock_gettime(CLOCK_REALTIME, &time1);

    // ... ... ...

    clock_gettime(CLOCK_REALTIME, &time2);

    cout << diff(time1,time2).tv_sec << ":"
            << diff(time1,time2).tv_nsec << endl;

    return 0;

This program should be compiled using g++ -lrt -o program program.cpp, the result will be something like this:

julio@acer ~> ./program
Julio Batista Silva
Julio Batista Silva
Data Engineer

I’m a computer engineer passionate about science, technology, photography, and languages. Currently working as a Data Engineer in Germany.

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