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' */*.h real 0.38 user 0.00 sys 0.05 real >　(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.h> /* 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 <stdio.h> #include <unistd.h> 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<int> v; for(int a;in>>a;v.push_back(a)); cout<<v.size()<<endl; for(int i=0;i<v.size();i++) cout<<v[i]<<" "; cout<<endl; clock_t t2=clock(); cout<<"TotalTime:"<<t2-t1<<"ms"<<endl; } /*运行结果*/ Time to do 100000000 empty loops is 0.290000 seconds time ./test real 0m0.915s user 0m0.031s sys 0m0.266s user+sys=0.297,正好等于clock的时间

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

<sys/times.h> /* 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 <time.h> /* 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 <sys/time.h> /* 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 <time.h> 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。

<time.h> /* 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 <xlocale.h> 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_GNU extern 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

转载于:https://www.cnblogs.com/liuwanpeng/p/8119318.html