Does Linux have a function to calculate time?

Linux has functions for calculating time, such as: the function time() that can obtain the second-level time difference, the functions gettimeofday() and settimeofday() that can obtain the microsecond-level time difference, and the function that can obtain the nanosecond-level time difference. clock_gettime() and so on.

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

The following explains the time-related functions and time-conversion-related functions in Linux

1. Obtain time-related functions

1.1 Get the second-level time difference function

#include <time.h>
time_t time(time_t *timer);//通过函数返回值或者timer 变量均可以获取到当前时间

time_t is actually a long integer, representing UTC time (0:00:00 on January 1, 1970, Linux system The second-level time difference from Epoch time) to the current system time

1.2 Get the microsecond-level time difference function

#include <sys/time.h>
#include <unistd.h>
struct timeval {
    time_t       tv_sec;     /* seconds */
    suseconds_t tv_usec;    /* microseconds */
};

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

//函数执行成功返回0，失败返回-1. 其中timezone 是时区相关的结构体
int gettimeofday(struct timeval *tv, struct timezone *tz);

//用来设置制定的时间和时区信息
int settimeofday(const struct timeval *tv, const struct timezone *gz);

1.3 Get the nanosecond-level time difference function

#include <time.h>

/*
其中clk_id 用来制定对应的时钟类型，不同的类型可以用来获取不同的时间值，具体有四种：
CLOCK_REALTIME: 			系统实时时间，从UTC开始计时，若时间被用户更改计数时间相应改变；
CLOCK_MONOTONIC:			从系统启动开始计时，即使用户更改时间也没有影响；
CLOCK_PROCESS_CPUTIME_ID:	本进程开始到执行到当前程序系统CPU花费的时间；
CLOCK_THREAD_CPUTIME_ID:	本线程开始到执行到当前程序系统CPU花费的时间

*/

struct timespec{
    time_t tv_sec;    //s
    long tv_nsec;    //ns
};

int clock_gettime(clockid_t clk_id, struct timespec* tp);

2. Convert time-related functions

2.1 Convert the time parameter time_t obtained by the above acquisition time function into a structure Body

struct tm, this structure contains very detailed fields such as year, month, day, etc. As shown below:

#include <time.h>
 
struct tm{
    int tm_sec;   //秒
    int tm_min;   //分
    int tm_hour;  //时；取值区间为[0, 23]
    int tm_mday;  //日；取值区间为[1, 31]
    int tm_mon;   //月份；取值区间为[0, 11]; 0表示1月份依次递增到12月份
    int tm_year;   //年份；其值为1900年至今年数
    int tm_wday;  //星期；0代表星期天，1代表星期1，以此类推
    int tm_yday;   //日期；0代表1月1日
    int tm_isdst;   //夏令时标识符；使用夏令时为正，不使用t为0，不确定时为负*/
};

The commonly used functions to convert time_t into struct tm structure are as follows:

#include <time.h> 
struct tm* gmtime(const time_t* timep);
struct tm*localtime(const time_t* timep);

gmtime() and localtime() functions can convert time_t data format into tm format type. The difference is that the result of the conversion by gmtime() is the information corresponding to GMT (central time zone), while the result of the conversion by the localtime() function is the information of the current time zone.

2.2 Convert time_t into the time and date strings we habitually use

The corresponding conversion function is as follows:

#include <time.h>
char* ctime(time_t* timep);

2.3 The corresponding function to convert struct tm into time_t

is as follows:

#include <time.h>
time_t mktime(struct tm *p_tm);

2.4 Convert struct tm into the time and date strings we habitually use

The corresponding functions are as follows:

#include <time.h>
char *asctime(const struct tm *p_tm); //习惯性字符串 Thu Dec  9 07:13:35 2021

2.5 Convert the time string into struct tm format

/**************************************
** description： 将struct tm 按照指定的format格式转化成字符串
** parameter：
		** *s : 需要被转换的时间字符串
		** *format：时间字符串的格式
		** *tm：转换后的tm时间
**************************************/
char *strptime(const char *s, const char *format, struct tm *tm);

2.6 Convert struct tm into a string according to the specified format

/**************************************
** description： 将struct tm 按照指定的format格式转化成字符串
** parameter：
		** *s : 生成的时间字符串
		** max： 字符串最大字符数（即最大可生成的字符数量）
		** *format：生成的字符串格式
		** *tm：需要被转换的tm时间
**************************************/
size_t strftime(char *s, size_t max, const char *format,const struct tm *tm);

3. Example

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

int main(int argc, char *argv[])
{	
	char *pstr = NULL;
	struct tm *ptm = NULL;

	printf("************** 使用ctime获取时间time_t **************\n");
	time_t times = 0;
	time(&times);
	printf("time_t:%ld\n\n", times);
	
	printf("************** 使用ctime转换time_t成我们习惯性使用的时间和日期字符串 **************\n");
	pstr = ctime(&times);
	printf("ctime:%s\n", pstr);
	
	printf("************** 使用gmtime转换time_t成struct tm 时间和日期**************\n");
	ptm = gmtime(&times);
	printf("time : %d:%d:%d\n", ptm->tm_hour,  ptm->tm_min,  ptm->tm_sec);
	printf("date: %d:%d:%d\n", ptm->tm_year+1900, ptm->tm_mon+1, ptm->tm_mday);
	printf("year: wday:%d yday:%d isdst:%d\n\n", ptm->tm_wday, ptm->tm_yday, ptm->tm_isdst);
	
	printf("************** 使用asctime转换struct tm成我们习惯性使用的时间和日期字符串**************\n");
	pstr = asctime(ptm);
	printf("asctime:%s\n\n", pstr);
	
	printf("************** 使用localtime转换time_t成struct tm 时间和日期**************\n");
	ptm = localtime(&times);
	printf("time : %d:%d:%d\n", ptm->tm_hour,  ptm->tm_min,  ptm->tm_sec);
	printf("date: %d:%d:%d\n", ptm->tm_year+1900, ptm->tm_mon+1, ptm->tm_mday);
	printf("year: wday:%d yday:%d isdst:%d\n", ptm->tm_wday, ptm->tm_yday, ptm->tm_isdst);

	pstr = asctime(ptm);
	printf("asctime:%s\n\n", pstr);

	printf("************** 使用gettimeofday获取微秒级的时间**************\n");
	struct timeval tv;
	struct timezone tz;
	gettimeofday(&tv, &tz);
	ptm = localtime(&tv.tv_sec);
	printf("time : %d:%d:%d\n", ptm->tm_hour,  ptm->tm_min,  ptm->tm_sec);
	printf("date: %d:%d:%d\n", ptm->tm_year+1900, ptm->tm_mon+1, ptm->tm_mday);
	printf("year: wday:%d yday:%d isdst:%d\n", ptm->tm_wday, ptm->tm_yday, ptm->tm_isdst);
	printf("tv_usec:%ld\n\n", tv.tv_usec);
	
	printf("************** 使用clock_gettime获取纳秒级的时间**************\n");
	struct timespec tp;
	clock_gettime(CLOCK_REALTIME, &tp);
	ptm = localtime(&tp.tv_sec);
	printf("time : %d:%d:%d\n", ptm->tm_hour,  ptm->tm_min,  ptm->tm_sec);
	printf("date: %d:%d:%d\n", ptm->tm_year+1900, ptm->tm_mon+1, ptm->tm_mday);
	printf("year: wday:%d yday:%d isdst:%d\n", ptm->tm_wday, ptm->tm_yday, ptm->tm_isdst);	
	printf("tp.tv_nsec:%ld\n\n", tp.tv_nsec);
	return 0;
}

Specific time character conversion

int str_to_time(void)
{
	char pstr[128] = {0};
    struct tm t;
    strptime("2021-04-23 12:34:56", "%Y-%m-%d %H:%M:%S", &t);
	
    printf("**** tm_isdst: %d, tm_yday:%d, tm_wday%d,\n %d-%d-%d \n %d:%d:%d\n", 
    t.tm_isdst, t.tm_yday, t.tm_wday, t.tm_year+1900, t.tm_mon+1, t.tm_mday,
    t.tm_hour, t.tm_min, t.tm_sec);
    printf("mktime ts:%ld\n", mktime(&t));
	printf("asctime:%s\n", asctime(&t));
	
	strftime(pstr, sizeof(pstr), "%Y-%m-%d %H:%M:%S", &t);
    printf("pstr:%s\n", pstr);
}

int time_to_str(void)
{
	char pstr[128] = {0};
    struct tm t = {
               .tm_sec = 56,    /* Seconds (0-60) */
               .tm_min = 34,    /* Minutes (0-59) */
               .tm_hour = 12,   /* Hours (0-23) */
               .tm_mday = 23,   /* Day of the month (1-31) */
               .tm_mon = 4-1,     /* Month (0-11) */
               .tm_year = 2021-1900,   /* Year - 1900 */
               .tm_wday = 5,     /* Day of the week (0-6, Sunday = 0) */
               .tm_yday = 113,   /* Day in the year (0-365, 1 Jan = 0) */
               .tm_isdst = 0,    /* Daylight saving time */
           };
    strftime(pstr, sizeof(pstr), "%Y-%m-%d %H:%M:%S", &t);
    printf("pstr:%s\n", pstr);
}

If you want to Get the time since system startup, even if the user changes the time, there will be no impact, in milliseconds, for example:

unsigned long long clock_systick_get(void)
{
    int ret = -1;
    unsigned long long time;
    int cnt = 0;
    struct timespec  now = {0, 0};

    while (ret < 0 && cnt < 3)
    {
        ret = clock_gettime(CLOCK_MONOTONIC, &now);	//获取失败重试，最大执行3次
        cnt++;
    }
    time = now.tv_sec * 1000 + now.tv_nsec / (1000000);
    return time;
}

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