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Section: C Library Functions (3)
Index Return to Main Contents


asctime, ctime, difftime, gmtime, localtime, mktime - convert date and time  


ds - -#include <time.h>

extern char *tzname[2];

char *ctime(time_t const *clock);

char *ctime_r(time_t const *clock, char *buf);

double difftime(time_t time1, time_t time0);

char *asctime(struct tm const *tm);

char *asctime_r(struct tm const *restrict tm,
    char *restrict result);

struct tm *localtime(time_t const *clock);

struct tm *localtime_r(time_t const *restrict clock,
    struct tm *restrict result);

struct tm *localtime_rz(timezone_t restrict zone,
    time_t const *restrict clock,
    struct tm *restrict result);

struct tm *gmtime(time_t const *clock);

struct tm *gmtime_r(time_t const *restrict clock,
    struct tm *restrict result);

time_t mktime(struct tm *tm);

time_t mktime_z(timezone_t restrict zone,
    struct tm *restrict tm);

cc ... ltz


Ctime converts a long integer, pointed to by clock, and returns a pointer to a string of the form

Thu Nov 24 18:22:48 1986\n\0
Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form

Thu Nov 24 18:22:48 81986\n\0
with five spaces before the year. These unusual formats are designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years.

The *clock timestamp represents the time in seconds since 1970-01-01 00:00:00 Coordinated Universal Time (UTC). The POSIX standard says that timestamps must be nonnegative and must ignore leap seconds. Many implementations extend POSIX by allowing negative timestamps, and can therefore represent timestamps that predate the introduction of UTC and are some other flavor of Universal Time (UT). Some implementations support leap seconds, in contradiction to POSIX.

Localtime and gmtime return pointers to structures, described below. Localtime corrects for the time zone and any time zone adjustments (such as Daylight Saving Time in the United States). After filling in the structure, localtime sets the tm_isdst'th element of tzname to a pointer to a string that's the time zone abbreviation to be used with localtime's return value.

Gmtime converts to Coordinated Universal Time.

Asctime converts a time value contained in a structure to a string, as shown in the above example, and returns a pointer to the string.

Mktime converts the broken-down time, expressed as local time, in the structure pointed to by tm into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to their normal ranges. (A positive or zero value for tm_isdst causes mktime to presume initially that summer time (for example, Daylight Saving Time in the U.S.A.) respectively, is or is not in effect for the specified time. A negative value for tm_isdst causes the mktime function to attempt to divine whether summer time is in effect for the specified time; in this case it does not use a consistent rule and may give a different answer when later presented with the same argument.) On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to their normal ranges; the final value of tm_mday is not set until tm_mon and tm_year are determined. Mktime returns the specified calendar time; If the calendar time cannot be represented, it returns -1.

Difftime returns the difference between two calendar times, (time1 - time0), expressed in seconds.

Ctime_r, localtime_r, gmtime_r, and asctime_r are like their unsuffixed counterparts, except that they accept an additional argument specifying where to store the result if successful.

Localtime_rz and mktime_z are like their unsuffixed counterparts, except that they accept an extra initial zone argument specifying the time zone to be used for conversion. If zone is null, UTC is used; otherwise, zone should be have been allocated by tzalloc and should not be freed until after all uses (e.g., by calls to strftime) of the filled-in tm_zone fields.

Declarations of all the functions and externals, and the structure, are in the <time.h> header file. The structure (of type) struct tm includes the following fields:

     int tm_sec;      /* seconds (0-60) */
     int tm_min;      /* minutes (0-59) */
     int tm_hour;     /* hours (0-23) */
     int tm_mday;     /* day of month (1-31) */
     int tm_mon;      /* month of year (0-11) */
     int tm_year;     /* year - 1900 */
     int tm_wday;     /* day of week (Sunday = 0) */
     int tm_yday;     /* day of year (0-365) */
     int tm_isdst;    /* is summer time in effect? */
     char *tm_zone;   /* abbreviation of time zone name */
     long tm_gmtoff;  /* offset from UT in seconds */

The tm_zone and tm_gmtoff fields exist, and are filled in, only if arrangements to do so were made when the library containing these functions was created. There is no guarantee that these fields will continue to exist in this form in future releases of this code.

Tm_isdst is non-zero if summer time is in effect.

Tm_gmtoff is the offset (in seconds) of the time represented from UT, with positive values indicating east of the Prime Meridian. The field's name is derived from Greenwich Mean Time, a precursor of UT.  


/usr/local/etc/zoneinfo             time zone information directory

/usr/local/etc/zoneinfo/localtime  local time zone file

/usr/local/etc/zoneinfo/posixrules used with POSIX-style TZ's

/usr/local/etc/zoneinfo/GMT        for UTC leap seconds

If /usr/local/etc/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/local/etc/zoneinfo/posixrules.  


getenv(3), newstrftime(3), newtzset(3), time(2), tzfile(5)  


The return values of asctime, ctime, gmtime, and localtime point to static data overwritten by each call. The tm_zone field of a returned struct tm points to a static array of characters, which can be overwritten by later calls to tzset. The remaining functions and data are thread-safe.

Asctime, asctime_r, ctime, and ctime_r behave strangely for years before 1000 or after 9999. The 1989 and 1999 editions of the C Standard say that years from -99 through 999 are converted without extra spaces, but this conflicts with longstanding tradition and with this implementation. The 2011 edition says that the behavior is undefined if the year is before 1000 or after 9999. Traditional implementations of these two functions are restricted to years in the range 1900 through 2099. To avoid this portability mess, new programs should use strftime instead.




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