Linux get system Current time __linux

Recently, I'm looking at the problem of time programming in Linux:

1. Time-related structures under Linux
struct Timeval
{
int tv_sec;
int tv_usec;
};
The tv_sec is the number of seconds from the beginning of the morning, and the Tv_usec is microsecond (10E-6 second).

struct TIMEZONE
{
int tv_minuteswest;
int tv_dsttime;
};
Tv_minuteswest is the time difference between GMT and the west, and Tv_dsttime is the way to fix it.

struct TIMESPEC
{
long int tv_sec;
long int tv_nsec;
};
Tv_nsec is the 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;
};
Tm_sec table "Seconds", between [0,61], the extra two seconds is used to handle the skip-second problem.
Tm_min the number of "minutes" in the table, between [0,59].
Tm_hour the "time" number of the table, between [0,23].
Tm_mday table "Day of the Month", between [1,31].
Tm_mon table "The first few months of the year", between [0,11].
Tm_year to add 1900 to that year.
Tm_wday table "in the first few days", between [0,6].
Tm_yday table "The first few days of the year", between [0,365], leap years have 366 days.
Tm_isdst Whether the table is "Daylight saving Time".

struct Itimerval
{
struct Timeval it_interval;
struct Timeval it_value;
};
The It_interval member represents the initial value of the interval counter, while the It_value member represents the current value of the interval counter.

2. Get current time
In all UNIX, there is a function of time ()
time_t time (time_t *t);
This function returns the number of seconds from the start of the epoch, and if T is non-null, it fills the time value into T.

Linux provides gettimeofday () for certain requirements that require higher accuracy.
int gettimeofday (struct timeval * tv,struct timezone *tz);
int settimeofday (const struct Timeval * tv,const struct timezone);

struct TM format time function

struct TM * GMTIME (const time_t * t);
Converted to Greenwich. Sometimes called GMT or UTC.

struct TM * localtime (const time_t *t);
Convert cost to time. It can be used to modify TZ environment variables in a machine, different users to represent different times.

time_t mktime (struct TM *TP);
Convert TM into time_t format, using local time.

tme_t TIMEGM (Strut TM *TP);
Convert TM becomes time_t format, using UTC time.

Double Difftime (time_t t2,time_t t1);
Calculates the second difference.


3. Text Time Format function

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

The CTime rate uses local time, and Asctime is represented by 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, whose format is specified by FMT.

%a: Name of the day, abbreviation.
%A: Name of this day.
%b: Month name, abbreviation.
%B: Month names, full name.
%c: Is the same as the ctime/asctime format.
%d: The name of the day of the month, from 0.
%H: The day of the first few hours, 24-hour system, from 0.
%I: The day of the first few hours, 12-hour system, from 0.
%j: The first days of the year, from 0.
%m: The first few months of the year, from 0.
%m: The first minute of the hour, calculated from 0.
%p:am or PM.
%s: The first few seconds of the minute, calculated from 0.
%u: At the beginning of the year, the first day is calculated.
%w: At the beginning of the year, the first one is calculated.
%w: When the first few days, from 0.
%x: local date.
%x: local time.
%y: two-digit years.
%Y: four-digit years.
%Z: Abbreviation for time zone name.
%%:% symbol.

char * strptime (char *s,char *fmt,struct TM *TP);
Like scanf, the decoded string becomes the TM format.

%h: Same with%b and%b.
%c: Reads the%x and%x formats.
%c: Read%c format.
%e: Same as%d.
%d: Read%m/%d/%y format.
%k: Same with%h.
%l: Same with%i.
%r: Read the "%i:%m:%s%p" format.
%R: Read the "%h:%m" format.
%T: Read the "%h:%m:%s" format.
%y: Reads two-digit years.
%Y: Reads four-digit years.

Here's a small example of how to get the system's current time:
C codetime_tNowstructTM*timenow;CharSTRTEMP[255];    Time (&now);    TimeNow = LocalTime (&now); printf ("Recent time is:%s/n", Asctime (TimeNow));