Advanced Programming in the UNIX® Environment

ptg10805159

Section 6.10 Time and Date Routines 191

kernel

time_t

(calendar time) timespec

struct tm

(broken-down time)

string formatted string

timeval

gettimeofday time clock_gettime

tv_sec tv_sec

gmtime

localtime

mktime

strptime

strftimestrftime_l

Figure 6.9 Relationship of the various time functions

The two functionslocaltimeandgmtimeconvert a calendar time into what’s

called a broken-down time, atmstructure.

struct tm { /* a broken-down time */

int tm_sec; /* seconds after the minute: [0 - 60] */

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; /* months since January: [0 - 11] */

int tm_year; /* years since 1900 */

int tm_wday; /* days since Sunday: [0 - 6] */

int tm_yday; /* days since January 1: [0 - 365] */

int tm_isdst; /* daylight saving time flag: <0, 0, >0 */

};

The reason that the seconds can be greater than 59 is to allow for a leap second. Note

that all the fields except the day of the month are0-based. The daylight saving time flag

is positive if daylight saving time is in effect, 0 if it’s not in effect, and negative if the

information isn’t available.

In older versions of the Single UNIX Specification, double leap seconds wereallowed. Thus

the valid range of values for thetm_secmember was 0–61. The formal definition of UTC

doesn’t allow for double leap seconds, so the valid range for seconds is now 0–60.