嵌入式Linux获取系统时间

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NoName51C_T_O 2022-01-05 11:32:31 博主文章分类：Linux时间子系统 ©著作权

文章标签 linux 时间时间子系统 2d #include 文章分类 后端开发

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1 概要

时区TZ环境变量，对时间相关函数的影响，关于TZ的介绍查看

2 gettimeofday

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

这个函数获取的是UTC时间，与TZ环境变量无关。

struct timezone {
int tz_minuteswest; /* minutes west of Greenwich */
int tz_dsttime; /* type of DST correction */
};

在gettimeofday中tz_minuteswest和 tz_dsttime一直返回0.

3 gmtime

struct tm *gmtime(const time_t *timep);
struct tm *gmtime_r(const time_t *timep, struct tm *result);

gmtime是将timep(utc时间)秒数转化为tm结构时区为零的年月日时分秒时间，与系统时区没有关系，

改变TZ环境变量对这个函数没有影响。

比如，当前北京时间是2021-02-08 09:16:03，那么utc时间应该是1612746963(2021-02-08 01:16:03),

如果time = 1612746963，那么gmtime(&time)，返回的应该是2021-02-08 01:16:03

4 localtime

struct tm *localtime(const time_t *timep);
struct tm *localtime_r(const time_t *timep, struct tm *result);

localtime是将timep(utc时间)秒数转化为tm结构时区为本地的年月日时分秒时间，与系统时区有关系，

改变TZ环境变量对这个函数有影响。

比如，当前北京时间是2021-02-08 09:16:03，那么utc时间应该是1612746963(2021-02-08 01:16:03),

当time = 1612746963。

例1：如果TZ=CST-08:00为东8区(中国时区)，那么localtime(&time)，返回的应该是2021-02-08 09:16:03。

例2：如果TZ=UTC-00:00为零时区(本初子午线地区)，那么localtime(&time)，返回的应该是2021-02-08 01:16:03。

5 mktime

time_t mktime(struct tm *tm);

将当前本地时间转换为UTC时间，和时区有关系。

比如当前本地时间为2021-02-08 09:16:03。

当前时区为东八区，那么调用mktime得到的utc时间1612746963。

当前时区是0时区，那么待用mktime得到的utc时间应该是 1612775763(1612746963+8*60*60)。

6 tzset

void tzset (void);
extern char *tzname[2];
extern long timezone;

如果TZ=CST-8:00,调用tzset ()函数后，

tzname[0] = CST
tzname[1] = CST
timezone = -28800

7 测试代码

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

#if 0

struct tm *gmtime(const time_t *timep);
struct tm *gmtime_r(const time_t *timep, struct tm *result);

struct tm *localtime(const time_t *timep);
struct tm *localtime_r(const time_t *timep, struct tm *result);

time_t mktime(struct tm *tm);


struct tm {
   int tm_sec;    /* Seconds (0-60) */
   int tm_min;    /* Minutes (0-59) */
   int tm_hour;   /* Hours (0-23) */
   int tm_mday;   /* Day of the month (1-31) */
   int tm_mon;    /* Month (0-11) */
   int tm_year;   /* Year - 1900 */
   int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
   int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
   int tm_isdst;  /* Daylight saving time */
};

struct timeval {
   time_t      tv_sec;     /* seconds */
   suseconds_t tv_usec;    /* microseconds */
};

struct timezone {
   int tz_minuteswest;     /* minutes west of Greenwich */
   int tz_dsttime;         /* type of DST correction */
};

void tzset (void);

extern char *tzname[2];
extern long timezone;
extern int daylight;

#endif

void print_tm(struct tm *tm)
{
   printf("tm_year = %04d\n", tm->tm_year);
   printf("tm_mon = %02d\n", tm->tm_mon);
   printf("tm_mday = %02d\n", tm->tm_mday);
   printf("tm_hour = %02d\n", tm->tm_hour);
   printf("tm_min = %02d\n", tm->tm_min);
   printf("tm_sec = %02d\n", tm->tm_sec);
   printf("tm_wday = %d\n", tm->tm_wday);
   printf("tm_yday = %03d\n", tm->tm_yday);
   printf("tm_isdst = %d\n", tm->tm_isdst);

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

void print_tv(struct timeval *tv)
{
   printf("tv->tv_sec = %ld\n", tv->tv_sec);
   printf("tv->tv_usec = %ld\n", tv->tv_usec);
}

void print_tz(struct timezone *tz)
{
   printf("tz->tz_minuteswest = %d\n", tz->tz_minuteswest);
   printf("tz->tz_dsttime = %d\n", tz->tz_dsttime);
}


void print_tz_info(void)
{
   printf("tzname[0] = %s\n", tzname[0]);
   printf("tzname[1] = %s\n", tzname[1]);
   printf("timezone = %ld\n", timezone);
}


int main()
{
   struct tm *ptm;
   struct tm tm;
   struct timeval tv;
   struct timezone tz;
   time_t time;

   int ret;

   ret = gettimeofday(&tv, &tz);

   if(ret) {
      printf("------------gettimeofday Err\n");
      return ret;
   }

   printf("+++++++++++++++++++++gettimeofday+++++++++++++++++++++\n");
   print_tv(&tv);
   print_tz(&tz);

   time = tv.tv_sec;

   printf("+++++++++++++++++++++localtime+++++++++++++++++++++\n");
   ptm = localtime(&time);
   print_tm(ptm);

   printf("+++++++++++++++++++++localtime_r+++++++++++++++++++++\n");
   localtime_r(&time, &tm);
   print_tm(&tm);

   printf("+++++++++++++++++++++localtime_r -> mktime+++++++++++++++++++++\n");
   printf("mktime = %ld\n", mktime(&tm));

   printf("+++++++++++++++++++++gmtime+++++++++++++++++++++\n");
   ptm = gmtime(&time);
   print_tm(ptm);

   printf("+++++++++++++++++++++gmtime_r+++++++++++++++++++++\n");
   gmtime_r(&time, &tm);
   print_tm(&tm);


   printf("+++++++++++++++++++++print_tz_info+++++++++++++++++++++\n");
   //tzset();
   print_tz_info();

   return 0;
}

打印信息：

# date
Mon Feb  8 10:36:43 CST 2021
# ./test 
+++++++++++++++++++++gettimeofday+++++++++++++++++++++
tv->tv_sec = 1612751804
tv->tv_usec = 419185
tz->tz_minuteswest = 0
tz->tz_dsttime = 0
+++++++++++++++++++++localtime+++++++++++++++++++++
tm_year = 0121
tm_mon = 01
tm_mday = 08
tm_hour = 10
tm_min = 36
tm_sec = 44
tm_wday = 1
tm_yday = 038
tm_isdst = 0
2021-02-08 10:36:44
+++++++++++++++++++++localtime_r+++++++++++++++++++++
tm_year = 0121
tm_mon = 01
tm_mday = 08
tm_hour = 10
tm_min = 36
tm_sec = 44
tm_wday = 1
tm_yday = 038
tm_isdst = 0
2021-02-08 10:36:44
+++++++++++++++++++++localtime_r -> mktime+++++++++++++++++++++
mktime = 1612751804
+++++++++++++++++++++gmtime+++++++++++++++++++++
tm_year = 0121
tm_mon = 01
tm_mday = 08
tm_hour = 02
tm_min = 36
tm_sec = 44
tm_wday = 1
tm_yday = 038
tm_isdst = 0
2021-02-08 02:36:44
+++++++++++++++++++++gmtime_r+++++++++++++++++++++
tm_year = 0121
tm_mon = 01
tm_mday = 08
tm_hour = 02
tm_min = 36
tm_sec = 44
tm_wday = 1
tm_yday = 038
tm_isdst = 0
2021-02-08 02:36:44
+++++++++++++++++++++print_tz_info+++++++++++++++++++++
tzname[0] = CST
tzname[1] = CST
timezone = -28800
#