time函数

time函数

• time

#include<time.h>

time_t time(time_t *t);

typdef long int time_t;

time() returns the time as the number of secs since 1970-01-01 00:00:00 +0000(UTC)

假如参数t不是NULL，返回值（秒数）(日历时间）也存储在t中。

失败返回-1.

• ctime

#include <time.h>

char *ctime(const time_t *time);

将秒数转化为具体日期（ASCII），长度固定26字节。

the time is 1449579811

the time ascii is Tue Dec  8 21:03:31 2015

等价：asctime(localtime(t));

• broken-down time

Broken-down time is stored in the structure tm whichi is deiined in time.h as follows:

struct tm {

    int tm_sec;

    int tm_min;

    int tm_hour;

    int tm_mday;

    int tm_mon;

    int tm_year;

    int tm_wday;

    int tm_yday;

    int tm_isdst;

};

tm_year指的是自1900年以来的年数。

• asctime

char *asctime(const struct tm *tm);

将分离时间装换为字符串时间。

• localtime

struct tm *localtime(const time_t *timep);

将秒数装换为分离时间。

• gmtime

struct tm *gmtime(const time_t *timep);

• mktime

time_t mktime(struct tm *tm);

将分离时间转换为秒数。

• difftime

double difftime(time_t time1, time_t time0);

The difftime() function returns the number of seconds elapsed between time time1 and time time0, represented as a double. Each of the times is specified in calendar time, which means its value is a measurement (in seconds) relative to the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

• clock

clock_t clock(void);

DESCRIPTION

   The  clock()  function returns an approximation of processor time used by the program.

RETURN VALUE

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.  
If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t) -1.

• gettimeofday

#include <sys/time.h>

struct timeval{

    long tv_sec;

    long tv_usec;

};

int gettimeofday(struct timeval *tv, struct timezone *tz);

gettimeofday将时间保存在tv中，tz一般使用NULL代替，不建议设置tz。返回日历时间即绝对时间（非本地时间）。

通过不同时间段运行函数可测量时间。

• clock_gettime/clock_settime

#include <time.h>
struct timespec {
               time_t   tv_sec;        /* seconds */
               long     tv_nsec;       /* nanoseconds */
};
int clock_gettime(clockid_t clk_id, struct timespec *tp);
int clock_settime(clockid_t clk_id, const struct timespec *tp);

The functions clock_gettime() and clock_settime() retrieve and set the time of the specified clock clk_id.

The clk_id argument is the identifier of the particular clock on which to act. A clock may be system-wide and hence visible for all processes, or per-process if it measures time only within a single process.m
CLOCK_REALTIME： real time，受adjtime和NTP影响和系统管理员调时的影响。
CLOCK_REALTIME_COARSE：快但少精度的CLOCK_REALTIME。
CLOCK_MONOTONIC：单调时钟，不受管理员调时影响，但受NTP和adjtime等调时影响。
CLOCK_MONOTONIC_RAW：和CLOCK_MONOTONIC相似，但基于硬件时间，不受NTP和aditime等任何调时影响。准确的启动时间应倚赖于此。
CLOCK_PROCESS_CPUTIME_ID：Pre-process CPU-time clock，测量进程内所有线程花费时间。
CLOCK_THREAD_CPUTIME_ID：Thread-specific CPU-time clock。

• setitimer&getitimer

#include <sys/time.h>

int getitimer(int which, struct itimerval *value);

int setitimer(int which, struct itimerval *newval, struct itimerval *oldval);

struct itimerval {

    struct timeval it_interval;

    struct timeval it_value;

};

itimerval 结构中的 it_value 是减少的时间,当这个值为 0 的时候就发出相应的信号了. 然后设置为 it_interval 值。

当it_interval为0时，timer为单触发时钟，singal-shot timer。

Linux 操作系统为每一个进程提供了 3 个内部间隔计时器.

ITIMER_REAL:减少实际时间.到时的时候发出 SIGALRM 信号.

ITIMER_VIRTUAL:减少有效时间(进程执行的时间).产生 SIGVTALRM 信号.

ITIMER_PROF:减少进程的有效时间和系统时间(为进程调度用的时间).这个经常和上面一 个使用用来计算系统内核时间和用户时间.产生 SIGPROF 信号.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include <signal.h>

struct itimerval it; 

void sighandler(int sig)
{
    printf("catch sig:[%d]...
", sig); 

    it.it_value.tv_sec = it.it_interval.tv_sec;
    it.it_value.tv_usec = it.it_interval.tv_usec;
}

int main(int argc, char *argv[])
{
    struct sigaction sa; 
    memset(&sa, 0, sizeof(struct sigaction));
    sa.sa_handler=sighandler;
    sa.sa_flags=0;
    sigaction(SIGALRM, &sa, NULL);

    //signal(SIGALRM, sighandler);
    //signal(SIGVTALRM, sighandler);

    it.it_interval.tv_sec = 2;
    it.it_interval.tv_usec = 100;
    it.it_value.tv_sec = it.it_interval.tv_sec;
    it.it_value.tv_usec = it.it_interval.tv_usec;
    setitimer(ITIMER_REAL, &it, NULL);
    //setitimer(ITIMER_VIRTUAL, &it, NULL);

    while(1);
    return 0;
}

参考：

linux应用time和timezone