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CTIME(3)                 BSD Library Functions Manual                 CTIME(3)

NAME
     asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r, localtime, localtime_r, mktime, timegm
     -- transform binary date and time values

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <time.h>

     extern char *tzname[2];

     char *
     asctime(const struct tm *timeptr);

     char *
     asctime_r(const struct tm *restrict timeptr, char *restrict buf);

     char *
     ctime(const time_t *clock);

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

     double
     difftime(time_t time1, time_t time_);

     struct tm *
     gmtime(const time_t *clock);

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

     struct tm *
     localtime(const time_t *clock);

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

     time_t
     mktime(struct tm *timeptr);

     time_t
     timegm(struct tm *timeptr);

DESCRIPTION
     The functions ctime(), gmtime(), and localtime() all take as an argument a time value representing the
     time in seconds since the Epoch (00:00:00 UTC, January 1, 1970; see time(3)).  When encountering an
     error, these functions return NULL and set errno to an appropriate value.

     The function localtime() converts the time value pointed at by clock.  It returns a pointer to a
     ``struct tm'' (described below), which contains the broken-out time information for the value after
     adjusting for the current time zone (and any other factors such as Daylight Saving Time).  Time zone
     adjustments are performed as specified by the TZ environment variable (see tzset(3)).  The function
     localtime() uses tzset(3) to initialize time conversion information, if tzset(3) has not already been
     called by the process.

     After filling in the tm structure, localtime() sets the tm_isdst'th element of tzname to a pointer to
     an ASCII string containing the time zone abbreviation to be used with localtime()'s return value.

     The function gmtime() also converts the time value, but makes no time zone adjustment.  It returns a
     pointer to a tm structure (described below).

     The ctime() function adjusts the time value for the current time zone, in the same manner as
     localtime().  It returns a pointer to a 26-character string of the form:

           Thu Nov 24 18:22:48 1986\n\0

     All of the fields have constant width.

     The ctime_r() function provides the same functionality as ctime(), except that the caller must provide
     the output buffer buf (which must be at least 26 characters long) to store the result.  The
     localtime_r() and gmtime_r() functions provide the same functionality as localtime() and gmtime(),
     respectively, except the caller must provide the output buffer result.

     The asctime() function converts the broken-out time in the structure tm (pointed at by *timeptr) to the
     form shown in the example above.

     The asctime_r() function provides the same functionality as asctime(), except that the caller provides
     the output buffer buf (which must be at least 26 characters long) to store the result.

     The functions mktime() and timegm() convert the broken-out time (in the structure pointed to by
     *timeptr) into a time value with the same encoding as that of the values returned by the time(3) func-tion function
     tion (that is, seconds from the Epoch, UTC).  The mktime() function interprets the input structure
     according to the current timezone setting (see tzset(3)).  The timegm() function interprets the input
     structure as representing Universal Coordinated Time (UTC).

     The original values of the tm_wday and tm_yday components of the structure are ignored. The original
     values of the other components are not restricted to their normal ranges and will be normalized, if
     need be.  For example, October 40 is changed into November 9, a tm_hour of -1 means 1 hour before mid-night, midnight,
     night, tm_mday of 0 means the day preceding the current month, and tm_mon of -2 means 2 months before
     January of tm_year.  (A positive or zero value for tm_isdst causes mktime() to presume initially that
     summer time (for example, Daylight Saving Time) is or is not (respectively) 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.  The tm_isdst and tm_gmtoff members are forced to zero by
     timegm().)

     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.  The mktime() function returns the specified calendar time; if the calendar
     time cannot be represented, it returns -1;

     The difftime() function returns the difference between two calendar times, (time1 - time_), expressed
     in seconds.

     External declarations, as well as the tm structure definition, are contained in the <time.h> include
     file.  The tm structure includes at least 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 timezone name */
           long tm_gmtoff; /* offset from UTC in seconds */

     The field tm_isdst is non-zero if summer (i.e., Daylight Saving) time is in effect.

     The field tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values
     indicating locations east of the Prime Meridian.

SEE ALSO
     date(1), gettimeofday(2), getenv(3), time(3), tzset(3), tzfile(5)

STANDARDS
     The asctime(), ctime(), difftime(), gmtime(), localtime(), and mktime() functions conform to ISO/IEC
     9899:1990 (``ISO C90''), and conform to ISO/IEC 9945-1:1996 (``POSIX.1'') provided the selected local
     timezone does not contain a leap-second table (see zic(8)).

     The asctime_r(), ctime_r(), gmtime_r(), and localtime_r() functions are expected to conform to ISO/IEC
     9945-1:1996 (``POSIX.1'') (again provided the selected local timezone does not contain a leap-second
     table).

     The timegm() function is not specified by any standard; its function cannot be completely emulated
     using the standard functions described above.

HISTORY
     This manual page is derived from the time package contributed to Berkeley by Arthur Olson and which
     appeared in 4.3BSD.

BUGS
     Except for difftime(), mktime(), and the _r() variants of the other functions, these functions leaves
     their result in an internal static object and return a pointer to that object.  Subsequent calls to
     these function will modify the same object.

     The C Standard provides no mechanism for a program to modify its current local timezone setting, and
     the POSIX-standard method is not reentrant.  (However, thread-safe implementations are provided in the
     POSIX threaded environment.)

     The tm_zone field of a returned tm structure points to a static array of characters, which will also be
     overwritten by any subsequent calls (as well as by subsequent calls to tzset(3) and tzsetwall(3)).

     Use of the external variable tzname is discouraged; the tm_zone entry in the tm structure is preferred.

BSD                             January 2, 1999                            BSD

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