ctime(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | 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 *restrict tm, char *restrict buf);

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

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

       struct tm *localtime(const time_t *timep);
       struct tm *localtime_r(const time_t *restrict timep,
                           struct tm *restrict 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
               || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_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

       On success, gmtime() and localtime() return a pointer to a
       struct tm.

       On success, gmtime_r() and localtime_r() return the address of
       the structure pointed to by result.

       On success, asctime() and ctime() return a pointer to a string.

       On success, asctime_r() and ctime_r() return a pointer to the
       string pointed to by buf.

       On success, mktime() returns the calendar time (seconds since the
       Epoch), expressed as a value of type time_t.

       On error, mktime() returns the value (time_t) -1.  The remaining
       functions return NULL on error.  On error, errno is set to
       indicate the error.

ERRORS         top

       EOVERFLOW
              The result cannot be represented.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌───────────────┬───────────────┬────────────────────────────────┐
       │Interface      Attribute     Value                          │
       ├───────────────┼───────────────┼────────────────────────────────┤
       │asctime()      │ Thread safety │ MT-Unsafe race:asctime locale  │
       ├───────────────┼───────────────┼────────────────────────────────┤
       │asctime_r()    │ Thread safety │ MT-Safe locale                 │
       ├───────────────┼───────────────┼────────────────────────────────┤
       │ctime()        │ Thread safety │ MT-Unsafe race:tmbuf           │
       │               │               │ race:asctime env locale        │
       ├───────────────┼───────────────┼────────────────────────────────┤
       │ctime_r(),     │ Thread safety │ MT-Safe env locale             │
       │gmtime_r(),    │               │                                │
       │localtime_r(), │               │                                │
       │mktime()       │               │                                │
       ├───────────────┼───────────────┼────────────────────────────────┤
       │gmtime(),      │ Thread safety │ MT-Unsafe race:tmbuf env       │
       │localtime()    │               │ locale                         │
       └───────────────┴───────────────┴────────────────────────────────┘

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.

       POSIX doesn't specify the parameters of ctime_r() to be restrict;
       that is specific to glibc.

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.1-2001, 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 5.13 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
       https://www.kernel.org/doc/man-pages/.

                               2021-03-22                       CTIME(3)

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