NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

CTIME(3)                  Linux Programmer's Manual                 CTIME(3)

NAME         top

       asctime,   ctime,  gmtime,  localtime,  mktime,  asctime_r,  ctime_r,
       gmtime_r, localtime_r - transform date and time to  broken-down  time
       or ASCII

SYNOPSIS         top

       #include <time.h>

       char *asctime(const struct tm *tm);
       char *asctime_r(const struct tm *tm, char *buf);

       char *ctime(const time_t *timep);
       char *ctime_r(const time_t *timep, char *buf);

       struct tm *gmtime(const time_t *timep);
       struct tm *gmtime_r(const time_t *timep, struct tm *result);

       struct tm *localtime(const time_t *timep);
       struct tm *localtime_r(const time_t *timep, struct tm *result);

       time_t mktime(struct tm *tm);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       asctime_r(), ctime_r(), gmtime_r(), localtime_r():
              _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
              _SVID_SOURCE || _POSIX_SOURCE

DESCRIPTION         top

       The ctime(), gmtime() and localtime() functions all take an argument
       of data type time_t, which represents calendar time.  When
       interpreted as an absolute time value, it represents the number of
       seconds elapsed since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

       The asctime() and mktime() functions both take an argument
       representing broken-down time, which is a representation separated
       into year, month, day, and so on.

       Broken-down time is stored in the structure tm, which is defined in
       <time.h> as follows:

           struct tm {
               int tm_sec;    /* Seconds (0-60) */
               int tm_min;    /* Minutes (0-59) */
               int tm_hour;   /* Hours (0-23) */
               int tm_mday;   /* Day of the month (1-31) */
               int tm_mon;    /* Month (0-11) */
               int tm_year;   /* Year - 1900 */
               int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
               int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
               int tm_isdst;  /* Daylight saving time */
           };

       The members of the tm structure are:

       tm_sec    The number of seconds after the minute, normally in the
                 range 0 to 59, but can be up to 60 to allow for leap
                 seconds.

       tm_min    The number of minutes after the hour, in the range 0 to 59.

       tm_hour   The number of hours past midnight, in the range 0 to 23.

       tm_mday   The day of the month, in the range 1 to 31.

       tm_mon    The number of months since January, in the range 0 to 11.

       tm_year   The number of years since 1900.

       tm_wday   The number of days since Sunday, in the range 0 to 6.

       tm_yday   The number of days since January 1, in the range 0 to 365.

       tm_isdst  A flag that indicates whether daylight saving time is in
                 effect at the time described.  The value is positive if
                 daylight saving time is in effect, zero if it is not, and
                 negative if the information is not available.

       The call ctime(t) is equivalent to asctime(localtime(t)).  It
       converts the calendar time t into a null-terminated string of the
       form

              "Wed Jun 30 21:49:08 1993\n"

       The abbreviations for the days of the week are "Sun", "Mon", "Tue",
       "Wed", "Thu", "Fri", and "Sat".  The abbreviations for the months are
       "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct",
       "Nov", and "Dec".  The return value points to a statically allocated
       string which might be overwritten by subsequent calls to any of the
       date and time functions.  The function also sets the external
       variables tzname, timezone, and daylight (see tzset(3)) with
       information about the current timezone.  The reentrant version
       ctime_r() does the same, but stores the string in a user-supplied
       buffer which should have room for at least 26 bytes.  It need not set
       tzname, timezone, and daylight.

       The gmtime() function converts the calendar time timep to broken-down
       time representation, expressed in Coordinated Universal Time (UTC).
       It may return NULL when the year does not fit into an integer.  The
       return value points to a statically allocated struct which might be
       overwritten by subsequent calls to any of the date and time
       functions.  The gmtime_r() function does the same, but stores the
       data in a user-supplied struct.

       The localtime() function converts the calendar time timep to broken-
       down time representation, expressed relative to the user's specified
       timezone.  The function acts as if it called tzset(3) and sets the
       external variables tzname with information about the current
       timezone, timezone with the difference between Coordinated Universal
       Time (UTC) and local standard time in seconds, and daylight to a
       nonzero value if daylight savings time rules apply during some part
       of the year.  The return value points to a statically allocated
       struct which might be overwritten by subsequent calls to any of the
       date and time functions.  The localtime_r() function does the same,
       but stores the data in a user-supplied struct.  It need not set
       tzname, timezone, and daylight.

       The asctime() function converts the broken-down time value tm into a
       null-terminated string with the same format as ctime().  The return
       value points to a statically allocated string which might be
       overwritten by subsequent calls to any of the date and time
       functions.  The asctime_r() function does the same, but stores the
       string in a user-supplied buffer which should have room for at least
       26 bytes.

       The mktime() function converts a broken-down time structure,
       expressed as local time, to calendar time representation.  The
       function ignores the values supplied by the caller in the tm_wday and
       tm_yday fields.  The value specified in the tm_isdst field informs
       mktime() whether or not daylight saving time (DST) is in effect for
       the time supplied in the tm structure: a positive value means DST is
       in effect; zero means that DST is not in effect; and a negative value
       means that mktime() should (use timezone information and system
       databases to) attempt to determine whether DST is in effect at the
       specified time.

       The mktime() function modifies the fields of the tm structure as
       follows: tm_wday and tm_yday are set to values determined from the
       contents of the other fields; if structure members are outside their
       valid interval, they will be normalized (so that, for example, 40
       October is changed into 9 November); tm_isdst is set (regardless of
       its initial value) to a positive value or to 0, respectively, to
       indicate whether DST is or is not in effect at the specified time.
       Calling mktime() also sets the external variable tzname with
       information about the current timezone.

       If the specified broken-down time cannot be represented as calendar
       time (seconds since the Epoch), mktime() returns (time_t) -1 and does
       not alter the members of the broken-down time structure.

RETURN VALUE         top

       Each of these functions returns the value described, or NULL (-1 in
       case of mktime()) in case an error was detected.

CONFORMING TO         top

       POSIX.1-2001.  C89 and C99 specify asctime(), ctime(), gmtime(),
       localtime(), and mktime().  POSIX.1-2008 marks asctime(),
       asctime_r(), ctime(), and ctime_r() as obsolete, recommending the use
       of strftime(3) instead.

NOTES         top

       The four functions asctime(), ctime(), gmtime() and localtime()
       return a pointer to static data and hence are not thread-safe.  The
       thread-safe versions, asctime_r(), ctime_r(), gmtime_r() and
       localtime_r(), are specified by SUSv2.

       POSIX.1-2001 says: "The asctime(), ctime(), gmtime(), and localtime()
       functions shall return values in one of two static objects: a broken-
       down time structure and an array of type char.  Execution of any of
       the functions may overwrite the information returned in either of
       these objects by any of the other functions."  This can occur in the
       glibc implementation.

       In many implementations, including glibc, a 0 in tm_mday is
       interpreted as meaning the last day of the preceding month.

       The glibc version of struct tm has additional fields

              long tm_gmtoff;           /* Seconds east of UTC */
              const char *tm_zone;      /* Timezone abbreviation */

       defined when _BSD_SOURCE was set before including <time.h>.  This is
       a BSD extension, present in 4.3BSD-Reno.

       According to POSIX.1-2004, localtime() is required to behave as
       though tzset(3) was called, while localtime_r() does not have this
       requirement.  For portable code, tzset(3) should be called before
       localtime_r().

SEE ALSO         top

       date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3),
       strftime(3), strptime(3), timegm(3), tzset(3), time(7)

COLOPHON         top

       This page is part of release 3.75 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

                                 2014-08-19                         CTIME(3)