在C / C++中以高精度测量执行时间

执行时间：给定任务的执行时间或CPU时间定义为系统以其他方式执行该任务所花费的时间，您可以说程序正在运行的时间。
有多种方法可以衡量程序的执行时间，在本文中，我将讨论5种不同的方法来测量程序的执行时间。
测量程序的执行时间。

1. 在C＆C++中使用time()函数。

   time() : time() function returns the time since the Epoch(jan 1 1970) in seconds.
   Header File : “time.h”
   Prototype / Syntax : time_t time(time_t *tloc);
   Return Value : On success, the value of time in seconds since the Epoch is returned, on error -1 is returned.

   下面的程序演示如何使用time()函数测量执行时间。

   #include 
   using namespace std;
     
   // A sample function whose time taken to
   // be measured
   void fun()
   {
       for (int i=0; i<10; i++)
       {
       }
   }
     
   int main()
   {
       /* Time function returns the time since the 
           Epoch(jan 1 1970). Returned time is in seconds. */
       time_t start, end;
     
       /* You can call it like this : start = time(NULL);
        in both the way start contain total time in seconds 
        since the Epoch. */
       time(&start);
     
       // unsync the I/O of C and C++.
       ios_base::sync_with_stdio(false);
     
       fun();
     
       // Recording end time.
       time(&end);
     
       // Calculating total time taken by the program.
       double time_taken = double(end - start);
       cout << "Time taken by program is : " << fixed
            << time_taken << setprecision(5);
       cout << " sec " << endl;
       return 0;
   }
   

   输出：

   Time taken by program is : 0.000000 sec
   

2. 在C和C++中使用clock()函数。

   clock() : clock() returns the number of clock ticks elapsed since the program was launched.
   Header File : “time.h”
   Prototype / Syntax : clock_t clock(void);
   Return Value : On success, the value returned is the CPU time used so far as a clock_t; To get the number of seconds used, divide by CLOCKS_PER_SEC.on error -1 is returned.

   下面的程序演示了如何使用clock()函数测量执行时间。您也可以看到此代码

   #include 
   using namespace std;
     
   // A sample function whose time taken to
   // be measured
   void fun()
   {
       for (int i=0; i<10; i++)
       {
       }
   }
     
   int main()
   {
       /* clock_t clock(void) returns the number of clock ticks
          elapsed since the program was launched.To get the number 
          of seconds used by the CPU, you will need to divide by 
          CLOCKS_PER_SEC.where CLOCKS_PER_SEC is 1000000 on typical
          32 bit system.  */
       clock_t start, end;
     
       /* Recording the starting clock tick.*/
       start = clock();
     
       fun();
     
       // Recording the end clock tick.
       end = clock();
     
       // Calculating total time taken by the program.
       double time_taken = double(end - start) / double(CLOCKS_PER_SEC);
       cout << "Time taken by program is : " << fixed 
            << time_taken << setprecision(5);
       cout << " sec " << endl;
       return 0;
   }
   

   输出：

   Time taken by program is : 0.000001 sec
   

3. 在C和C++中使用gettimeofday()函数。

   gettimeofday() : The function gettimeofday() can get the time as well as timezone.
   Header File : “sys/time.h”.
   Prototype / Syntax : int gettimeofday(struct timeval *tv, struct timezone *tz);
   The tv argument is a struct timeval and gives the number of seconds and micro seconds since the
   Epoch.
   struct timeval {
   time_t tv_sec; // seconds
   suseconds_t tv_usec; // microseconds
   };
   Return Value : return 0 for success, or -1 for failure.

   下面的程序演示如何使用gettimeofday()函数测量执行时间。

   #include 
   #include 
   using namespace std;
     
   // A sample function whose time taken to
   // be measured
   void fun()
   {
       for (int i=0; i<10; i++)
       {
       }
   }
     
   int main()
   {
       /* The function gettimeofday() can get the time as
          well as timezone. 
          int gettimeofday(struct timeval *tv, struct timezone *tz);
         The tv argument is a struct timeval and gives the
         number of seconds and micro seconds since the Epoch.
         struct timeval {
                  time_t      tv_sec;     // seconds 
                  suseconds_t tv_usec;    // microseconds
              };    */
       struct timeval start, end;
     
       // start timer.
       gettimeofday(&start, NULL);
     
       // unsync the I/O of C and C++.
       ios_base::sync_with_stdio(false);
     
       fun();
     
       // stop timer.
       gettimeofday(&end, NULL);
     
       // Calculating total time taken by the program.
       double time_taken;
     
       time_taken = (end.tv_sec - start.tv_sec) * 1e6;
       time_taken = (time_taken + (end.tv_usec - 
                                 start.tv_usec)) * 1e-6;
     
       cout << "Time taken by program is : " << fixed
            << time_taken << setprecision(6);
       cout << " sec" << endl;
       return 0;
   }
   

   输出：

   Time taken by program is : 0.000029 sec
   

4. 在C和C++中使用clock_gettime()函数。

   clock_gettime() : The clock_gettime() function gets the current time of the clock specified by clock_id, and puts it into the buffer pointed to by tp.
   Header File : “time.h”.
   Prototype / Syntax : int clock_gettime( clockid_t clock_id, struct timespec *tp );
   tp parameter points to a structure containing atleast the following members :
   
   struct timespec {
   time_t tv_sec; //seconds
   long tv_nsec; //nanoseconds
   };
   
   Return Value : return 0 for success, or -1 for failure.
   clock_id : clock id = CLOCK_REALTIME,
   CLOCK_PROCESS_CPUTIME_ID, CLOCK_MONOTONIC … etc.
   CLOCK_REALTIME : clock that measures real i.e., wall-clock) time.
   CLOCK_PROCESS_CPUTIME_ID : High-resolution per-process timer from the CPU.
   CLOCK_MONOTONIC : High resolution timer that is unaffected by system date changes (e.g. NTP daemons).

   下面的程序演示如何使用clock_gettime()函数测量执行时间。

   #include 
   #include 
   using namespace std;
     
   // A sample function whose time taken to
   // be measured
   void fun()
   {
       for (int i=0; i<10; i++)
       {
       }
   }
     
   int main()
   {
       /* int clock_gettime( clockid_t clock_id, struct 
        timespec *tp ); The clock_gettime() function gets
        the current time of the clock specified by clock_id, 
        and puts it into the buffer  pointed to by tp.tp 
        parameter points to a structure containing 
        atleast the following members:    
        struct timespec {
                  time_t   tv_sec;        // seconds 
                  long     tv_nsec;       // nanoseconds
              };
       clock id = CLOCK_REALTIME, CLOCK_PROCESS_CPUTIME_ID, 
                  CLOCK_MONOTONIC ...etc
       CLOCK_REALTIME : clock  that  measures real (i.e., wall-clock) time.
       CLOCK_PROCESS_CPUTIME_ID : High-resolution per-process timer 
                                  from the CPU.
       CLOCK_MONOTONIC : High resolution timer that is unaffected
                         by system date changes (e.g. NTP daemons).  */
       struct timespec start, end;
     
       // start timer.
       // clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
       // clock_gettime(CLOCK_REALTIME, &start);
       clock_gettime(CLOCK_MONOTONIC, &start);
     
       // unsync the I/O of C and C++.
       ios_base::sync_with_stdio(false);
     
       fun();
     
       // stop timer.
       // clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
       // clock_gettime(CLOCK_REALTIME, &end);
       clock_gettime(CLOCK_MONOTONIC, &end);
     
       // Calculating total time taken by the program.
       double time_taken;
       time_taken = (end.tv_sec - start.tv_sec) * 1e9;
       time_taken = (time_taken + (end.tv_nsec - start.tv_nsec)) * 1e-9;
     
       cout << "Time taken by program is : " << fixed
            << time_taken << setprecision(9);
       cout << " sec" << endl;
       return 0;
   }
   

   输出：

   Time taken by program is : 0.000028 sec
   

5. 在C++中使用chrono::high_resolution_clock 。

   chrono : Chrono library is used to deal with date and time. This library was designed to deal with the fact that timers and clocks might be different on different systems and thus to improve over time in terms of precision.chrono is the name of a header, but also of a sub-namespace, All the elements in this header are not defined directly under the std namespace (like most of the standard library) but under the std::chrono namespace.

   下面的程序演示如何使用high_resolution_clock函数测量执行时间。有关chrono库的详细信息，请参阅此

   #include 
   #include 
   using namespace std;
     
     
   // A sample function whose time taken to
   // be measured
   void fun()
   {
       for (int i=0; i<10; i++)
       {
       }
   }
     
   int main()
   {
       auto start = chrono::high_resolution_clock::now();
     
       // unsync the I/O of C and C++.
       ios_base::sync_with_stdio(false);
     
       fun();
     
       auto end = chrono::high_resolution_clock::now();
     
       // Calculating total time taken by the program.
       double time_taken = 
         chrono::duration_cast(end - start).count();
     
       time_taken *= 1e-9;
     
       cout << "Time taken by program is : " << fixed 
            << time_taken << setprecision(9);
       cout << " sec" << endl;
       return 0;
   }
   

   输出：

   Time taken by program is : 0.000024 sec
   

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