Unix Time Functions CNS 3210

The basic Linux (and Unix) time service counts the number of seconds that have passed since January 1, 1970 (UTC). These seconds are normally stored in a variable whose type is time_t. This is called calendar time.

We can get calendar time by using the call time_t time (time_t *tptr); returns the time, and stores the value here if the argument is non-null.

time_t struct tm string formatted string kernel time calendar time broken down time gmtime localtime mktime ctime asctime strftime takes time zone into account

struct tm { int tm_sec;// seconds after the minute [0-61] int tm_min;// minutes after the hour [0-59] int tm_hour;// hours after midnight [0-23] int tm_mday;// day of the month [1-31] int tm_mon;// month of the year [0-11] int tm_year;// years since 1900 int tm_wday;// day of the week [0-6] int tm_yday;// days since Jan 1 [0-365] int tm_isdst// daylight savings flag [pos, 0, neg] }; allows leap seconds dststtnot avail

time_t to tm #include struct tm *gmtime (const time_t *tptr); struct tm *localtime (const time_t *tptr); broken down time is in local time, given time zone and daylight savings broken down time is in UTC

tm to time_t time_t mktime (struct tm *tptr);

generating time strings char *asctime (const struct tm *tmptr); char (ctime (const time_t *tptr); both generate a 26 character string of the form Tue Sep 26 16:49: \n\0

Generating a formatted string size_t strftime (char *tbuf, size_t maxsize, const char *format, const struct tm *tptr); returns the size of the formatted string if successful 0 if fails a broken down time the buffer to store the string in, and its max size the format string – works like printf

FormatDescriptionExample %aabbreviated weekday nameTue %Afull weekday nameTuesday %babbreviated month nameJan %Bfull weekday nameJanuary %cdate and timeTue Jan 14 19:40: %dday of the month [00-31]14 %Hhour of 24 hour day [00-23]19 %Ihour of 24 hour day [00-12]07 %jday of the year [ ]014 %mmonth [01-12]01 %Mminute [00-59]40 %pam or pmPM %Ssecond [00-61]05 %USunday week number [00-53]02 %wweekday [0=Sunday – 6]5 %WMonday week number [00-53]07 %xdate01/14/02 %Xtime19:40:04 %yyear without century [00-99]02 %Yyear with century2002 %Ztime zone nameMST