Question

I’m trying to use “rusage” statistics in my program to get data similar to that of the time tool. However, I’m pretty sure that I’m doing something wrong. The values seem about right but can be a bit weird at times. I didn’t find good resources online. Does somebody know how to do it better?

Sorry for the long code.

class StopWatch { public:     void start() {         getrusage(RUSAGE_SELF, &m_begin);         gettimeofday(&m_tmbegin, 0);     }      void stop() {         getrusage(RUSAGE_SELF, &m_end);         gettimeofday(&m_tmend, 0);         timeval_sub(m_end.ru_utime, m_begin.ru_utime, m_diff.ru_utime);         timeval_sub(m_end.ru_stime, m_begin.ru_stime, m_diff.ru_stime);         timeval_sub(m_tmend, m_tmbegin, m_tmdiff);     }      void printf(std::ostream& out) const {         using namespace std;          timeval const& utime = m_diff.ru_utime;         timeval const& stime = m_diff.ru_stime;          format_time(out, utime);         out << 'u ';         format_time(out, stime);         out << 's ';         format_time(out, m_tmdiff);     }  private:     rusage m_begin;     rusage m_end;     rusage m_diff;     timeval m_tmbegin;     timeval m_tmend;     timeval m_tmdiff;      static void timeval_add(timeval const& a, timeval const& b, timeval& ret) {         ret.tv_usec = a.tv_usec + b.tv_usec;         ret.tv_sec = a.tv_sec + b.tv_sec;         if (ret.tv_usec > 999999) {             ret.tv_usec -= 1000000;             ++ret.tv_sec;         }     }      static void timeval_sub(timeval const& a, timeval const& b, timeval& ret) {         ret.tv_usec = a.tv_usec - b.tv_usec;         ret.tv_sec = a.tv_sec - b.tv_sec;         if (a.tv_usec < b.tv_usec) {             ret.tv_usec += 1000000;             --ret.tv_sec;         }     }      static void format_time(std::ostream& out, timeval const& tv) {         using namespace std;         long usec = tv.tv_usec;         while (usec >= 1000)             usec /= 10;         out << tv.tv_sec << '.' << setw(3) << setfill('0') << usec;     } }; // class StopWatch 

Answer 1

What is the purpose of:

while (usec >= 1000)     usec /= 10; 

I gather that you want the most significant three digits of the usec; in that case, the most straightforward way I can think of is to divide usec by 1000, and be done with that.

Test cases:

• 999999 ⇒ 999
• 99999 ⇒ 999 (should be 099)
• 9999 ⇒ 999 (should be 009)
• 999 ⇒ 999 (should be 000)