ctime(3) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON

ctime(3)                Library Functions 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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <time.h>

       char *asctime(const struct tm *tm);
       char *asctime_r(const struct tm *restrict tm,
                           char buf[restrict 26]);

       char *ctime(const time_t *timep);
       char *ctime_r(const time_t *restrict timep,
                           char buf[restrict 26]);

       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, described in
       tm(3type).

       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 as if it called tzset(3).  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 also sets the external
       variables tzname, timezone, and daylight as if it called
       tzset(3).  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.  The function also sets the external
       variables tzname, timezone, and daylight as if it called
       tzset(3).

       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                       │
       └────────────────┴───────────────┴──────────────────────────────┘

VERSIONS         top

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

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

       According to POSIX.1, 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().

STANDARDS         top

       asctime()
       ctime()
       gmtime()
       localtime()
       mktime()
              C23, POSIX.1-2024.

       gmtime_r()
       localtime_r()
              POSIX.1-2024.

       asctime_r()
       ctime_r()
              None.

HISTORY         top

       gmtime()
       localtime()
       mktime()
              C89, POSIX.1-1988.

       asctime()
       ctime()
              C89, POSIX.1-1988.  Marked obsolescent in C23 and in
              POSIX.1-2008 (recommending strftime(3)).

       gmtime_r()
       localtime_r()
              POSIX.1-1996.

       asctime_r()
       ctime_r()
              POSIX.1-1996.  Marked obsolescent in POSIX.1-2008.
              Removed in POSIX.1-2024 (recommending strftime(3)).

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 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 that return a pointer to one of these
       object types may overwrite the information in any object of the
       same type pointed to by the value returned from any previous call
       to any of them."  This can occur in the glibc implementation.

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 the man-pages (Linux kernel and C library
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       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
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       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
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       man-pages@man7.org

Linux man-pages 6.9.1          2024-06-15                       ctime(3)

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