linux下时间的处理

在linux下获取时间 ，首先需要获取时刻，再通过时刻来获取具体的年/月/日/时/分/秒等信息。

1 采用time方法获取当前时间的方法如下：

    函数说明
#include <time.h>

//返回当前时间相对于1970年1月1日0时0分0秒的秒数
//其中time_t 是long int类型，表示某一时刻；如果参数为NULL值，则表示当前时间的时刻，否则参数和返回值都会给值。

time_t time (time_t *__timer)

//通过时刻获取具体的时间。

struct tm *localtime (__const time_t *__timer)

两者结合，获取系统当前时间的代码如下：
    time_t tim=time(NULL);

    tm* now=localtime(&tim);

    printf("\n@Time: %04d:%02d:%02d  %02d:%02d:%02d \n"
           ,(now->tm_year+1900),(now->tm_mon+1),now->tm_mday
           ,now->tm_hour,now->tm_min,now->tm_sec);

2 通过clock_gettime()获取当前时间的方法

     函数说明

int clock_gettime (clockid_t __clock_id, struct timespec *__tp)

其中 clockid_t 的值主要如下：
//绝对时间，表示系统当前时间

#   define CLOCK_REALTIME        0
//相对时间，它不受系统时间修改的影响，相对某个时刻的时间
#   define CLOCK_MONOTONIC        1
/* High-resolution timer from the CPU.  */
#   define CLOCK_PROCESS_CPUTIME_ID    2
/* Thread-specific CPU-time clock.  */
#   define CLOCK_THREAD_CPUTIME_ID    3

支持至纳秒

struct timespec
  {
    __time_t tv_sec;        /* Seconds.  */
    long int tv_nsec;        /* Nanoseconds.  */

  };

代码如下：
    struct timespec ts;

    clock_gettime (CLOCK_REALTIME, &ts); //获取时刻

    tm* now=localtime(&ts.tv_sec);

    printf("\n@Time: %04d:%02d:%02d  %02d:%02d:%02d \n"
           ,(now->tm_year+1900),(now->tm_mon+1),now->tm_mday
           ,now->tm_hour,now->tm_min,now->tm_sec);

3 通过gettimeofday()获取当前时间的方法：

//此方法只支持至微秒

  struct timeval
  {
    __time_t tv_sec;        /* Seconds.  */
    __suseconds_t tv_usec;    /* Microseconds.  */

  };
代码如下:

  struct timeval tv;

  gettimeofday(&tv, NULL);
  tm* now=localtime(&ts.tv_sec);
    printf("\n@Time: %04d:%02d:%02d  %02d:%02d:%02d \n"
           ,(now->tm_year+1900),(now->tm_mon+1),now->tm_mday
           ,now->tm_hour,now->tm_min,now->tm_sec);

4 时间如何转为时刻,如：

    tm* now;

    time_t tick=mktime(now);

5 时刻转为UTC时间用gmtime

相关函数的说明

1	time_t time(time_t *time); This returns the current calendar time of the system in number of seconds elapsed since January 1, 1970. If the system has no time, .1 is returned.
2	char *ctime(const time_t *time); This returns a pointer to a string of the form day month year hours:minutes:seconds year\n\0.
3	struct tm *localtime(const time_t *time); This returns a pointer to the tm structure representing local time.
4	clock_t clock(void); This returns a value that approximates the amount of time the calling program has been running. A value of .1 is returned if the time is not available.
5	char * asctime ( const struct tm * time ); This returns a pointer to a string that contains the information stored in the structure pointed to by time converted into the form: day month date hours:minutes:seconds year\n\0
6	struct tm *gmtime(const time_t *time); This returns a pointer to the time in the form of a tm structure. The time is represented in Coordinated Universal Time (UTC), which is essentially Greenwich Mean Time (GMT).
7	time_t mktime(struct tm *time); This returns the calendar-time equivalent of the time found in the structure pointed to by time.
8	double difftime ( time_t time2, time_t time1 ); This function calculates the difference in seconds between time1 and time2.
9	size_t strftime(); This function can be used to format date and time a specific format.