Obtain the current time in Linux

Linux obtains the current time-general Linux technology-Linux programming and kernel information. The following is a detailed description. Recently, I have been querying about time programming in Linux:

1. Time-related struct in Linux
Struct timeval
{
Int TV _sec;
Int TV _usec;
};
TV _sec is the number of seconds from the early morning, And TV _usec is the microsecond (10E-6 second ).

Struct timezone
{
Int TV _minuteswest;
Int TV _dsttime;
};
TV _minuteswest is the time difference from Greenwich Mean Time to the west, while TV _dsttime is the time correction method.

Struct timespec
{
Long int TV _sec;
Long int TV _nsec;
};
TV _nsec is nano second (10E-9 second ).

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;
};
The number of seconds in the tm_sec table. In the range of [0, 61], two seconds are used to handle the second hop problem.
The number of "points" in the tm_min table, which is between [0, 59.
The "Hour" number in the tm_hour table, which is between [0, 23.
The tm_mday table "day of the month" is in the range.
The month of the current year in the tm_mon table, between [0, 11.
Tm_year adds 1900 to indicate that year.
The date of the tm_wday table, which is in the range of [0, 6.
The tm_yday table "the day of the current year". In the range of [0,365], the leap year has 366 days.
Whether the tm_isdst table is daylight saving time 」.

Struct itimerval
{
Struct timeval it_interval;
Struct timeval it_value;
};
The it_interval Member indicates the initial value of the interval counter, while the it_value Member indicates the current value of the interval counter.

2. Obtain the current time
In all UNIX systems, there is a time () function.
Time_t time (time_t * t );
This function returns the number of seconds since epoch. If t is non-null, it will enter the time value in t.

For some requirements that require high accuracy, Linux provides gettimeofday ().
Int gettimeofday (struct timeval * TV, struct timezone * tz );
Int settimeofday (const struct timeval * TV, const struct timezone * tz );

Struct tm format Time Function

Struct tm * gmtime (const time_t * t );
Converts it to Greenwich Mean Time. It is sometimes called GMT or UTC.

Struct tm * localtime (const time_t * t );
Time of conversion cost. It can be used to modify the TZ environment variables on a single machine. Different users indicate different times.

Time_t mktime (struct tm * tp );
Convert tm to time_t format and use local time.

Tme_t timegm (strut tm * tp );
Convert tm to time_t format and use UTC time.

Double difftime (time_t t2, time_t t1 );
Calculate the second difference.


3. Text time format functions

Char * asctime (struct tm * tp );
Char * ctime (struct tm * tp );
Both functions convert the time format to the standard UNIX time format.
Mon May 3 08:23:35 1999

Ctime uses the local time, while asctime uses the timezone information in the tm structure.

Size_t strftime (char * str, size_t max, char * fmt, struct tm * tp );
Strftime is a bit like sprintf, and its format is specified by fmt.

% A: the name of the day.
% A: The full name of the day.
% B: name of the month.
% B: The full name of the month.
% C: The format is the same as that of ctime/asctime.
% D: name of the day of the month, counted from zero.
% H: the hour of the day, in the 24-hour format, counted from zero.
% I: the hour of the day, in 12-hour format, counted from zero.
% J: the day of the current year, counted from zero.
% M: The month of the current year, counted from zero.
% M: the fraction of the hour, counted from zero.
% P: AM or PM.
% S: the second of the minute, counted from zero.
% U: the number of the current year, which is calculated from the first day.
% W: the number of the current year, which is calculated from the first one.
% W: the number of days counted from zero.
% X: Local date.
% X: local time.
% Y: two-digit year.
% Y: the four-digit year.
% Z: abbreviation of the time zone name.
%: % Symbol.

Char * strptime (char * s, char * fmt, struct tm * tp );
Like scanf, the interpretation string is in the tm format.

% H: Same as % B and % B.
% C: Read % x and % X format.
% C: Read % C format.
% E: Same as % d.
% D: Read % m/% d/% y format.
% K: Same as % H.
% L: Same as % I.
% R: Read the format "% I: % M: % S % p.
% R: Read the format "% H: % M.
% T: Read "% H: % M: % S" format.
% Y: read two-digit year.
% Y: Read the four-digit year.

The following example shows how to obtain the current system time:
Time_t now;
Struct tm * timenow;
Char strtemp [255];

Time (& now );
Timenow = localtime (& now );
Printf ("recent time is: % s \ n", asctime (timenow ));