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1.时间值

1.1 日历时间（UTC）

　该值是自1 9 7 0年1月1日0 0 : 0 0 : 0 0以来国际标准时间（ U T C）所经过的秒数累计值（早期的手册称 U T C为格林尼治标准时间） ,系统中用time_t类型表示。

1.2 进程时间

　　也叫CPU时间，用来度量占用的CPU资源。

　　单位：时钟滴答，系统每秒的滴答数可以通过sysconf获取，曾经取值50/60/100等，用clock_t类型表示；就算1000，精度才1ms，不适用高精度测量

　　三个进程时间值：

• 时钟时间real，也叫墙上时间，时钟时间是整个进程的实际执行时间，从进程开始到进程结束，包含期间进程切换，其他进程的执行时间。此时间跟进程的数量有关，例如同样一个进程，如果系统中有3个和10个其他进程和的情况下，时钟时间是不一样的，即跟系统的负荷有关系。
• 用户时间user，该进程处于用户态时运行的CPU时间
• 系统时间sys，系统时间是该进程进入内核态以后的CPU运行时间

一般情况：read >= (user+sys)，多线程并行执行时是例外

:/work/platform-zynq/linux-xlnx-repo-smp$ time -p grep 'xilinx'  */*.hreal 0.38user 0.00sys 0.05real >　(user＋sys)

2.时间相关函数

2.1 clock()

• clock函数计算user time + system time
• 单位时钟滴答
• CLOCKS_PER_SECOND是1秒的时钟滴答数，可用sysconf函数获取，实际是个宏定义，不过不通系统定义可能是不同的

   
    /* Time used by the program so far (user time + system time).   The result / CLOCKS_PER_SECOND is program time in seconds.  */extern clock_t clock (void) __THROW;
   

获取CLOCKS_PER_SECOND的值：

#include 
   
    #include 
    
     int main(int argc, char *argv[]){	printf("CLOCKS_PER_SECOND=%d\r\n",sysconf(_SC_CLK_TCK));	return 0;}在MPSOC开发板上中执行的结果为：CLOCKS_PER_SECOND=100
    
   

精度太低了，10ms

例子：

int main(int argc, char **argv){        clock_t t1=clock();    ifstream in("data.txt");    vector
   
     v;    for(int a;in>>a;v.push_back(a));    cout<
    
     <
    
   

2.2 times()

clock()函数的增强版,可获取real、usr、sys time,跟命令time差不多。

返回real time

   
    /* Structure describing CPU time used by a process and its children.  */struct tms  {    clock_t tms_utime;		/* User CPU time.  */    clock_t tms_stime;		/* System CPU time.  */    clock_t tms_cutime;		/* User CPU time of dead children.  */    clock_t tms_cstime;		/* System CPU time of dead children.  */  };/* Store the CPU time used by this process and all its   dead children (and their dead children) in BUFFER.   Return the elapsed real time, or (clock_t) -1 for errors.   All times are in CLK_TCKths of a second.  */extern clock_t times (struct tms *__buffer) __THROW;
   

2.3 time()

• 获取系统时间，1 9 7 0年1月1日0 0 : 0 0 : 0 0以来国际标准时间（ U T C）所经过的秒数累计值
• 对应系统时间，若用此函数作为start和end，则计算出来的时间对应real墙上时间

   
    /* Return the current time and put it in *TIMER if TIMER is not NULL.  */extern time_t time (time_t *__timer) __THROW;
   

2.4 gettimeofday()

• 跟time()差不多，但比time()精度高，可达到us级
• 返回的时间是 从今日凌晨到现在的时间差，绝对值
• 用start,end方式得到的差值对应real time

   
    /* Get the current time of day and timezone information,   putting it into *TV and *TZ.  If TZ is NULL, *TZ is not filled.   Returns 0 on success, -1 on errors.   NOTE: This form of timezone information is obsolete.   Use the functions and variables declared in 
    
      instead.  */extern int gettimeofday (struct timeval *__restrict __tv,			 __timezone_ptr_t __tz) __THROW __nonnull ((1));			 /* A time value that is accurate to the nearest   microsecond but also has a range of years.  */struct timeval  {    __time_t tv_sec;		/* Seconds.  */    __suseconds_t tv_usec;	/* Microseconds.  */  };/*__tz填入NULL即可*/
    
   

2.5 time()/gettimeofday()的时间转换

这两个函数获取的时间不易读，可通过若干函数转换成人们易读的方式。

   
    /* Return the `time_t' representation of TP and normalize TP.  */extern time_t mktime (struct tm *__tp) __THROW;/* Format TP into S according to FORMAT.   Write no more than MAXSIZE characters and return the number   of characters written, or 0 if it would exceed MAXSIZE.  */extern size_t strftime (char *__restrict __s, size_t __maxsize,			const char *__restrict __format,			const struct tm *__restrict __tp) __THROW;__END_NAMESPACE_STD# ifdef __USE_XOPEN/* Parse S according to FORMAT and store binary time information in TP.   The return value is a pointer to the first unparsed character in S.  */extern char *strptime (const char *__restrict __s,		       const char *__restrict __fmt, struct tm *__tp)     __THROW;# endif# ifdef __USE_XOPEN2K8/* Similar to the two functions above but take the information from   the provided locale and not the global locale.  */# include 
    
     extern size_t strftime_l (char *__restrict __s, size_t __maxsize,			  const char *__restrict __format,			  const struct tm *__restrict __tp,			  __locale_t __loc) __THROW;# endif# ifdef __USE_GNUextern char *strptime_l (const char *__restrict __s,			 const char *__restrict __fmt, struct tm *__tp,			 __locale_t __loc) __THROW;# endif__BEGIN_NAMESPACE_STD/* Return the `struct tm' representation of *TIMER   in Universal Coordinated Time (aka Greenwich Mean Time).  */extern struct tm *gmtime (const time_t *__timer) __THROW;/* Return the `struct tm' representation   of *TIMER in the local timezone.  */extern struct tm *localtime (const time_t *__timer) __THROW;__END_NAMESPACE_STD# ifdef __USE_POSIX/* Return the `struct tm' representation of *TIMER in UTC,   using *TP to store the result.  */extern struct tm *gmtime_r (const time_t *__restrict __timer,			    struct tm *__restrict __tp) __THROW;/* Return the `struct tm' representation of *TIMER in local time,   using *TP to store the result.  */extern struct tm *localtime_r (const time_t *__restrict __timer,			       struct tm *__restrict __tp) __THROW;# endif	/* POSIX */__BEGIN_NAMESPACE_STD/* Return a string of the form "Day Mon dd hh:mm:ss yyyy\n"   that is the representation of TP in this format.  */extern char *asctime (const struct tm *__tp) __THROW;/* Equivalent to `asctime (localtime (timer))'.  */extern char *ctime (const time_t *__timer) __THROW;__END_NAMESPACE_STD# ifdef __USE_POSIX/* Reentrant versions of the above functions.  *//* Return in BUF a string of the form "Day Mon dd hh:mm:ss yyyy\n"   that is the representation of TP in this format.  */extern char *asctime_r (const struct tm *__restrict __tp,			char *__restrict __buf) __THROW;/* Equivalent to `asctime_r (localtime_r (timer, *TMP*), buf)'.  */extern char *ctime_r (const time_t *__restrict __timer,		      char *__restrict __buf) __THROW;# endif	/* POSIX */
    
   

3.测量某段代码执行时间的函数选择

• us级函数执行时间测量，使用clock_t类型的函数肯定无法满足要求；
• 只能用gettimeofday()函数
  • 该函数对应墙上时间，进程切换也被统计进来了
  • 也没有太好的办法，只能多执行若干遍，求平均吧