Calcular tiempo de ejecución

julio@acer ~> time ./programa
  Hello World!
  ./programa  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

#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;
    }
    else
    {
        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;
}

julio@acer ~> ./programa
  0:7674929