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