1ctime(3)                   Library Functions Manual                   ctime(3)
2
3
4

NAME

6       asctime,  ctime,  gmtime,  localtime,  mktime,  asctime_r, ctime_r, gm‐
7       time_r, localtime_r - transform date and time to  broken-down  time  or
8       ASCII
9

LIBRARY

11       Standard C library (libc, -lc)
12

SYNOPSIS

14       #include <time.h>
15
16       char *asctime(const struct tm *tm);
17       char *asctime_r(const struct tm *restrict tm,
18                           char buf[restrict 26]);
19
20       char *ctime(const time_t *timep);
21       char *ctime_r(const time_t *restrict timep,
22                           char buf[restrict 26]);
23
24       struct tm *gmtime(const time_t *timep);
25       struct tm *gmtime_r(const time_t *restrict timep,
26                           struct tm *restrict result);
27
28       struct tm *localtime(const time_t *timep);
29       struct tm *localtime_r(const time_t *restrict timep,
30                           struct tm *restrict result);
31
32       time_t mktime(struct tm *tm);
33
34   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
35
36       asctime_r(), ctime_r(), gmtime_r(), localtime_r():
37           _POSIX_C_SOURCE
38               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
39

DESCRIPTION

41       The  ctime(),  gmtime(), and localtime() functions all take an argument
42       of data type time_t, which represents calendar time.  When  interpreted
43       as  an absolute time value, it represents the number of seconds elapsed
44       since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
45
46       The asctime() and mktime() functions both take an argument representing
47       broken-down time, which is a representation separated into year, month,
48       day, and so on.
49
50       Broken-down time is stored in the structure tm, described in tm(3type).
51
52       The call ctime(t) is equivalent to asctime(localtime(t)).  It  converts
53       the calendar time t into a null-terminated string of the form
54
55           "Wed Jun 30 21:49:08 1993\n"
56
57       The  abbreviations  for  the  days of the week are "Sun", "Mon", "Tue",
58       "Wed", "Thu", "Fri", and "Sat".  The abbreviations for the  months  are
59       "Jan",  "Feb",  "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct",
60       "Nov", and "Dec".  The return value points to  a  statically  allocated
61       string  which  might  be  overwritten by subsequent calls to any of the
62       date and time functions.  The function also sets the external variables
63       tzname,  timezone,  and  daylight (see tzset(3)) with information about
64       the current timezone.  The reentrant version ctime_r() does  the  same,
65       but  stores the string in a user-supplied buffer which should have room
66       for at least 26 bytes.  It need not set tzname, timezone, and daylight.
67
68       The gmtime() function converts the calendar time timep  to  broken-down
69       time representation, expressed in Coordinated Universal Time (UTC).  It
70       may return NULL when the year does not fit into an integer.  The return
71       value  points to a statically allocated struct which might be overwrit‐
72       ten by subsequent calls to any of the date and time functions.  The gm‐
73       time_r() function does the same, but stores the data in a user-supplied
74       struct.
75
76       The localtime() function converts the calendar time  timep  to  broken-
77       down  time  representation,  expressed relative to the user's specified
78       timezone.  The function acts as if it called tzset(3) and sets the  ex‐
79       ternal  variables  tzname  with information about the current timezone,
80       timezone with the difference between Coordinated Universal  Time  (UTC)
81       and  local standard time in seconds, and daylight to a nonzero value if
82       daylight savings time rules apply during some part of  the  year.   The
83       return  value  points  to  a statically allocated struct which might be
84       overwritten by subsequent calls to any of the date and time  functions.
85       The  localtime_r()  function  does  the  same, but stores the data in a
86       user-supplied struct.  It need not set tzname, timezone, and daylight.
87
88       The asctime() function converts the broken-down time value  tm  into  a
89       null-terminated  string  with  the  same format as ctime().  The return
90       value points to a statically allocated string which might be  overwrit‐
91       ten by subsequent calls to any of the date and time functions.  The as‐
92       ctime_r() function does the same, but stores the string in a  user-sup‐
93       plied buffer which should have room for at least 26 bytes.
94
95       The  mktime() function converts a broken-down time structure, expressed
96       as local time, to calendar time representation.  The  function  ignores
97       the  values  supplied  by the caller in the tm_wday and tm_yday fields.
98       The value specified in the tm_isdst field informs mktime()  whether  or
99       not  daylight  saving  time (DST) is in effect for the time supplied in
100       the tm structure: a positive value means DST is in effect;  zero  means
101       that  DST  is  not  in effect; and a negative value means that mktime()
102       should (use timezone information and system databases  to)  attempt  to
103       determine whether DST is in effect at the specified time.
104
105       The  mktime()  function modifies the fields of the tm structure as fol‐
106       lows: tm_wday and tm_yday are set to values determined  from  the  con‐
107       tents of the other fields; if structure members are outside their valid
108       interval, they will be normalized (so that, for example, 40 October  is
109       changed  into  9  November); tm_isdst is set (regardless of its initial
110       value) to a positive value or to 0, respectively, to  indicate  whether
111       DST  is  or  is  not in effect at the specified time.  Calling mktime()
112       also sets the external variable tzname with information about the  cur‐
113       rent timezone.
114
115       If  the  specified  broken-down  time cannot be represented as calendar
116       time (seconds since the Epoch), mktime() returns (time_t) -1  and  does
117       not alter the members of the broken-down time structure.
118

RETURN VALUE

120       On success, gmtime() and localtime() return a pointer to a struct tm.
121
122       On  success,  gmtime_r()  and  localtime_r()  return the address of the
123       structure pointed to by result.
124
125       On success, asctime() and ctime() return a pointer to a string.
126
127       On success, asctime_r() and ctime_r() return a pointer  to  the  string
128       pointed to by buf.
129
130       On  success,  mktime()  returns  the  calendar  time (seconds since the
131       Epoch), expressed as a value of type time_t.
132
133       On error, mktime() returns the value (time_t) -1.  The remaining  func‐
134       tions return NULL on error.  On error, errno is set to indicate the er‐
135       ror.
136

ERRORS

138       EOVERFLOW
139              The result cannot be represented.
140

ATTRIBUTES

142       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
143       tributes(7).
144
145       ┌───────────────┬───────────────┬──────────────────────────────────────┐
146Interface      Attribute     Value                                
147       ├───────────────┼───────────────┼──────────────────────────────────────┤
148asctime()      │ Thread safety │ MT-Unsafe race:asctime locale        │
149       ├───────────────┼───────────────┼──────────────────────────────────────┤
150asctime_r()    │ Thread safety │ MT-Safe locale                       │
151       ├───────────────┼───────────────┼──────────────────────────────────────┤
152ctime()        │ Thread safety │ MT-Unsafe race:tmbuf race:asctime    │
153       │               │               │ env locale                           │
154       ├───────────────┼───────────────┼──────────────────────────────────────┤
155ctime_r(),     │ Thread safety │ MT-Safe env locale                   │
156gmtime_r(),    │               │                                      │
157localtime_r(), │               │                                      │
158mktime()       │               │                                      │
159       ├───────────────┼───────────────┼──────────────────────────────────────┤
160gmtime(),      │ Thread safety │ MT-Unsafe race:tmbuf env locale      │
161localtime()    │               │                                      │
162       └───────────────┴───────────────┴──────────────────────────────────────┘
163

VERSIONS

165       POSIX doesn't specify the parameters of ctime_r() to be restrict;  that
166       is specific to glibc.
167
168       In many implementations, including glibc, a 0 in tm_mday is interpreted
169       as meaning the last day of the preceding month.
170
171       According to POSIX.1-2001, localtime() is required to behave as  though
172       tzset(3)  was  called,  while localtime_r() does not have this require‐
173       ment.  For portable code,  tzset(3)  should  be  called  before  local‐
174       time_r().
175

STANDARDS

177       asctime()
178       ctime()
179       gmtime()
180       localtime()
181       mktime()
182              C11, POSIX.1-2008.
183
184       asctime_r()
185       ctime_r()
186       gmtime_r()
187       localtime_r()
188              POSIX.1-2008.
189

HISTORY

191       gmtime()
192       localtime()
193       mktime()
194              C89, POSIX.1-2001.
195
196       asctime()
197       ctime()
198              C89,  POSIX.1-2001.  Marked obsolete in POSIX.1-2008 (recommend‐
199              ing strftime(3)).
200
201       gmtime_r()
202       localtime_r()
203              POSIX.1-2001.
204
205       asctime_r()
206       ctime_r()
207              POSIX.1-2001.  Marked  obsolete  in  POSIX.1-2008  (recommending
208              strftime(3)).
209

NOTES

211       The four functions asctime(), ctime(), gmtime(), and localtime() return
212       a pointer to static data and hence are not  thread-safe.   The  thread-
213       safe  versions,  asctime_r(), ctime_r(), gmtime_r(), and localtime_r(),
214       are specified by SUSv2.
215
216       POSIX.1-2001 says: "The asctime(), ctime(), gmtime(),  and  localtime()
217       functions  shall  return values in one of two static objects: a broken-
218       down time structure and an array of type char.  Execution of any of the
219       functions may overwrite the information returned in either of these ob‐
220       jects by any of the other functions."  This can occur in the glibc  im‐
221       plementation.
222

SEE ALSO

224       date(1),  gettimeofday(2),  time(2),  utime(2),  clock(3), difftime(3),
225       strftime(3), strptime(3), timegm(3), tzset(3), time(7)
226
227
228
229Linux man-pages 6.04              2023-03-30                          ctime(3)
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