Summary: This issue mainly introduces the common time expression structures in C/C++ and the conversion relationship between them.

一、FILETIME

The FILETIME structure preserves the file'sunsigned 64-bit date and time value. This value represents fromThe number of 100-nanosecond units since the beginning of January 1, 1601.
The specific structure is as follows:

typedef struct _FILETIME
{
    
    
  DWORD dwLowDateTime;
  DWORD dwHighDateTime;
} FILETIME, FAR *LPFILETIME;

The DWORD type is unsigned long , and dwLowDateTime represents the low-order 32 bits of the file time value.
dwHighDateTime represents the high-order 32 bits of the file time value.

Two, time_t

The utc time seen in the computer isCounting seconds from (January 01, 1970 0:00:00). The UTC time you see is the total number of seconds from the time point in 1970 to the specific time. time_t is actually a long long integer.
The specific structure is as follows:

typedef long  time_t;

Three, timeval

timeval is also used to represent UTC time, that is, the time starting from 0:00:00 on January 1, 1970. But the timeval structure can be accurate to microseconds.
The specific structure is as follows:

struct timeval
{
    
    
    long tv_sec; /*秒*/
    long tv_usec; /*微秒*/
};

four, tm

The tm structure is used to store UTC time, and is structured according to the year, month, day, hour, minute, and second.
The specific structure is as follows:

struct tm
{
    
    
    int tm_sec;  /*秒，正常范围0-59， 但允许至61*/
    int tm_min;  /*分钟，0-59*/
    int tm_hour; /*小时， 0-23*/
    int tm_mday; /*日，即一个月中的第几天，1-31*/
    int tm_mon;  /*月， 从一月算起，0-11  1+p->tm_mon;*/
    int tm_year;  /*年， 从1900至今已经多少年  1900＋ p->tm_year;*/
    int tm_wday; /*星期，一周中的第几天， 从星期日算起，0-6*/
    int tm_yday; /*从今年1月1日到目前的天数，范围0-365*/
    int tm_isdst; /*日光节约时间的旗标*/
};

5. Conversion between time structures

Introduce the header file #include<time.h>

1.FILETIME --> time_t

time_t FILETIMEToTime_t(FILETIME *pTime)
{
    
    
	ULARGE_INTEGER ull;//ULARGE_INTEGER 是64位无符号整型结构
	ull.LowPart = pTime->dwLowDateTime;
	ull.HighPart = pTime->dwHighDateTime;
	return ull.QuadPart / 10000000ULL - 11644473600ULL;
}

2.time_t --> tm

struct tm* gmtime(const time_t *timep);
/*将time_t表示的时间转换为没有经过时区转换的UTC时间，是一个struct tm结构指针*/
stuct tm* localtime(const time_t *timep);
/*和gmtime类似，但是它是经过时区转换的时间。*/

3.tm --> time_t

time_t mktime(struct tm* timeptr);
/*将struct tm 结构的时间转换为从1970年至今的秒数*/

6. The time structure is displayed as the current common time

1.tm structure

char *asctime(const struct tm* timeptr);
/*将结构中的信息转换为真实世界的时间，以字符串的形式显示*/

/*或者通过自定义输出*/
printf("time: %04d-%02d-%02d %02d:%02d:%02d\n", 1900 + p->tm_year,1+p->tm_mon,p->tm_mday,p->tm_hour,p->tm_min,p->tm_sec );

2. time_t structure

char *ctime(const time_t *timep);
/*将timep转换为真是世界的时间，以字符串显示，它和asctime不同就在于传入的参数形式不一样*/

end

This is the end of this issue for the analysis of the time structure in C/C++, see you next time:)

[Conversion between time structures in C/C++]

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