# ctime

## NAME



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


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#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



           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.  See timegm(3) for a UTC
           equivalent of this function.
    
           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



           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_ , and leaves the
           _tm- >tm_wday_ member unmodified.  The remaining functions return
           NULL on error.  On error, _errno_ is set to indicate the error.
    


## ERRORS



           **EOVERFLOW**
                  The result cannot be represented.
    


## ATTRIBUTES



           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



           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



           **asctime**()
           **ctime**()
           **gmtime**()
           **localtime**()
           **mktime**()
                  C23, POSIX.1-2024.
    
           **gmtime_r**()
           **localtime_r**()
                  POSIX.1-2024.
    
           **asctime_r**()
           **ctime_r**()
                  None.
    


## CAVEATS



       **Thread safety**
           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.
    
       **mktime()**
           _(time_t) -1_ can represent a valid time (one second before the
           Epoch).  To determine whether **mktime**() failed, one must use the
           _tm- >tm_wday_ field.  See the example program in EXAMPLES.
    
           The handling of a non-negative _tm_isdst_ in **mktime**() is poorly
           specified, and passing a value that is incorrect for the time
           specified yields unspecified results.  Since **mktime**() is one of
           the few functions that knows when DST is in effect, providing a
           correct value may be difficult.  One workaround for this is to
           call **mktime**() twice, once with _tm_isdst_ set to zero, and once with
           _tm_isdst_ set to a positive value, and discarding the results from
           the call that changes it.  If neither call changes _tm_isdst_ then
           the time specified probably happens during a fall-back period
           where DST begins or ends, and both results are valid but represent
           two different times.  If both calls change it, that could indicate
           a fall-forward transition, or some other reason why the time
           specified does not exist.
    
           The specification of time zones and daylight saving time are up to
           regional governments, change often, and may include
           discontinuities beyond _mktime_ 's ability to document a result.  For
           example, a change in the timezone definition may cause a clock
           time to be repeated or skipped without a corresponding DST change.
    


## EXAMPLES



           The program below defines a wrapper that allows detecting invalid
           and ambiguous times, with **EINVAL** and **ENOTUNIQ** , respectively.
    
           The following shell session shows sample runs of the program:
    
               $ **TZ=UTC ./a.out 1969 12 31 23 59 59 0** ;
               -1
               $
               $ **export TZ=Europe/Madrid** ;
               $
               $ **./a.out 2147483647 2147483647 00 00 00 00 -1** ;
               a.out: mktime: Value too large for defined data type
               $
               $ **./a.out 2024 08 23 00 17 53 -1** ;
               1724365073
               $ **./a.out 2024 08 23 00 17 53 0** ;
               a.out: my_mktime: Invalid argument
               1724368673
               $ **./a.out 2024 08 23 00 17 53 1** ;
               1724365073
               $
               $ **./a.out 2024 02 23 00 17 53 -1** ;
               1708643873
               $ **./a.out 2024 02 23 00 17 53 0** ;
               1708643873
               $ **./a.out 2024 02 23 00 17 53 1** ;
               a.out: my_mktime: Invalid argument
               1708640273
               $
               $ **./a.out 2023 03 26 02 17 53 -1** ;
               a.out: my_mktime: Invalid argument
               1679793473
               $
               $ **./a.out 2023 10 29 02 17 53 -1** ;
               a.out: my_mktime: Name not unique on network
               1698542273
               $ **./a.out 2023 10 29 02 17 53 0** ;
               1698542273
               $ **./a.out 2023 10 29 02 17 53 1** ;
               1698538673
               $
               $ **./a.out 2023 02 29 12 00 00 -1** ;
               a.out: my_mktime: Invalid argument
               1677668400
    
       **Program source: mktime.c**
    
           #include <err.h>
           #include <errno.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <time.h>
    
           #define is_signed(T)  ((T) -1 < 1)
    
           static time_t my_mktime(struct tm *tp);
    
           int
           main(int argc, char *argv[])
           {
               char       **p;
               time_t     t;
               struct tm  tm;
    
               if (argc != 8) {
                   fprintf(stderr, "Usage: %s yyyy mm dd HH MM SS isdst\n", argv[0]);
                   exit(EXIT_FAILURE);
               }
    
               p = &argv[1];
               tm.tm_year  = atoi(*p++) - 1900;
               tm.tm_mon   = atoi(*p++) - 1;
               tm.tm_mday  = atoi(*p++);
               tm.tm_hour  = atoi(*p++);
               tm.tm_min   = atoi(*p++);
               tm.tm_sec   = atoi(*p++);
               tm.tm_isdst = atoi(*p++);
    
               errno = 0;
               tm.tm_wday = -1;
               t = my_mktime(&tm);
               if (tm.tm_wday == -1)
                   err(EXIT_FAILURE, "mktime");
               if (errno == EINVAL || errno == ENOTUNIQ)
                   warn("my_mktime");
    
               if (is_signed(time_t))
                   printf("%jd\n", (intmax_t) t);
               else
                   printf("%ju\n", (uintmax_t) t);
    
               exit(EXIT_SUCCESS);
           }
    
           static time_t
           my_mktime(struct tm *tp)
           {
               int            e, isdst;
               time_t         t;
               struct tm      tm;
               unsigned char  wday[sizeof(tp->tm_wday)];
    
               e = errno;
    
               tm = *tp;
               isdst = tp->tm_isdst;
    
               memcpy(wday, &tp->tm_wday, sizeof(wday));
               tp->tm_wday = -1;
               t = mktime(tp);
               if (tp->tm_wday == -1) {
                   memcpy(&tp->tm_wday, wday, sizeof(wday));
                   return -1;
               }
    
               if (isdst == -1)
                   tm.tm_isdst = tp->tm_isdst;
    
               if (   tm.tm_sec   != tp->tm_sec
                   || tm.tm_min   != tp->tm_min
                   || tm.tm_hour  != tp->tm_hour
                   || tm.tm_mday  != tp->tm_mday
                   || tm.tm_mon   != tp->tm_mon
                   || tm.tm_year  != tp->tm_year
                   || tm.tm_isdst != tp->tm_isdst)
               {
                   errno = EINVAL;
                   return t;
               }
    
               if (isdst != -1)
                   goto out;
    
               tm = *tp;
               tm.tm_isdst = !tm.tm_isdst;
    
               tm.tm_wday = -1;
               mktime(&tm);
               if (tm.tm_wday == -1)
                   goto out;
    
               if (tm.tm_isdst != tp->tm_isdst) {
                   errno = ENOTUNIQ;
                   return t;
               }
           out:
               errno = e;
               return t;
           }
    


* * *