1CTIME(3)                   Linux Programmer's 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

SYNOPSIS

11       #include <time.h>
12
13       char *asctime(const struct tm *tm);
14       char *asctime_r(const struct tm *restrict tm, char *restrict buf);
15
16       char *ctime(const time_t *timep);
17       char *ctime_r(const time_t *restrict timep, char *restrict buf);
18
19       struct tm *gmtime(const time_t *timep);
20       struct tm *gmtime_r(const time_t *restrict timep,
21                           struct tm *restrict result);
22
23       struct tm *localtime(const time_t *timep);
24       struct tm *localtime_r(const time_t *restrict timep,
25                           struct tm *restrict result);
26
27       time_t mktime(struct tm *tm);
28
29   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
30
31       asctime_r(), ctime_r(), gmtime_r(), localtime_r():
32           _POSIX_C_SOURCE
33               || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
34

DESCRIPTION

36       The  ctime(),  gmtime(), and localtime() functions all take an argument
37       of data type time_t, which represents calendar time.  When  interpreted
38       as  an absolute time value, it represents the number of seconds elapsed
39       since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
40
41       The asctime() and mktime() functions both take an argument representing
42       broken-down time, which is a representation separated into year, month,
43       day, and so on.
44
45       Broken-down time is stored in the structure tm,  which  is  defined  in
46       <time.h> as follows:
47
48           struct tm {
49               int tm_sec;    /* Seconds (0-60) */
50               int tm_min;    /* Minutes (0-59) */
51               int tm_hour;   /* Hours (0-23) */
52               int tm_mday;   /* Day of the month (1-31) */
53               int tm_mon;    /* Month (0-11) */
54               int tm_year;   /* Year - 1900 */
55               int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
56               int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
57               int tm_isdst;  /* Daylight saving time */
58           };
59
60       The members of the tm structure are:
61
62       tm_sec    The number of seconds after the minute, normally in the range
63                 0 to 59, but can be up to 60 to allow for leap seconds.
64
65       tm_min    The number of minutes after the hour, in the range 0 to 59.
66
67       tm_hour   The number of hours past midnight, in the range 0 to 23.
68
69       tm_mday   The day of the month, in the range 1 to 31.
70
71       tm_mon    The number of months since January, in the range 0 to 11.
72
73       tm_year   The number of years since 1900.
74
75       tm_wday   The number of days since Sunday, in the range 0 to 6.
76
77       tm_yday   The number of days since January 1, in the range 0 to 365.
78
79       tm_isdst  A flag that indicates whether daylight saving time is in  ef‐
80                 fect  at  the  time described.  The value is positive if day‐
81                 light saving time is in effect, zero if it is not, and  nega‐
82                 tive if the information is not available.
83
84       The  call ctime(t) is equivalent to asctime(localtime(t)).  It converts
85       the calendar time t into a null-terminated string of the form
86
87           "Wed Jun 30 21:49:08 1993\n"
88
89       The abbreviations for the days of the week  are  "Sun",  "Mon",  "Tue",
90       "Wed",  "Thu",  "Fri", and "Sat".  The abbreviations for the months are
91       "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug",  "Sep",  "Oct",
92       "Nov",  and  "Dec".   The return value points to a statically allocated
93       string which might be overwritten by subsequent calls  to  any  of  the
94       date and time functions.  The function also sets the external variables
95       tzname, timezone, and daylight (see tzset(3))  with  information  about
96       the  current  timezone.  The reentrant version ctime_r() does the same,
97       but stores the string in a user-supplied buffer which should have  room
98       for at least 26 bytes.  It need not set tzname, timezone, and daylight.
99
100       The  gmtime()  function converts the calendar time timep to broken-down
101       time representation, expressed in Coordinated Universal Time (UTC).  It
102       may return NULL when the year does not fit into an integer.  The return
103       value points to a statically allocated struct which might be  overwrit‐
104       ten by subsequent calls to any of the date and time functions.  The gm‐
105       time_r() function does the same, but stores the data in a user-supplied
106       struct.
107
108       The  localtime()  function  converts the calendar time timep to broken-
109       down time representation, expressed relative to  the  user's  specified
110       timezone.   The function acts as if it called tzset(3) and sets the ex‐
111       ternal variables tzname with information about  the  current  timezone,
112       timezone  with  the difference between Coordinated Universal Time (UTC)
113       and local standard time in seconds, and daylight to a nonzero value  if
114       daylight  savings  time  rules apply during some part of the year.  The
115       return value points to a statically allocated  struct  which  might  be
116       overwritten  by subsequent calls to any of the date and time functions.
117       The localtime_r() function does the same, but  stores  the  data  in  a
118       user-supplied struct.  It need not set tzname, timezone, and daylight.
119
120       The  asctime()  function  converts the broken-down time value tm into a
121       null-terminated string with the same format  as  ctime().   The  return
122       value  points to a statically allocated string which might be overwrit‐
123       ten by subsequent calls to any of the date and time functions.  The as‐
124       ctime_r()  function does the same, but stores the string in a user-sup‐
125       plied buffer which should have room for at least 26 bytes.
126
127       The mktime() function converts a broken-down time structure,  expressed
128       as  local  time, to calendar time representation.  The function ignores
129       the values supplied by the caller in the tm_wday  and  tm_yday  fields.
130       The  value  specified in the tm_isdst field informs mktime() whether or
131       not daylight saving time (DST) is in effect for the  time  supplied  in
132       the  tm  structure: a positive value means DST is in effect; zero means
133       that DST is not in effect; and a negative  value  means  that  mktime()
134       should  (use  timezone  information and system databases to) attempt to
135       determine whether DST is in effect at the specified time.
136
137       The mktime() function modifies the fields of the tm structure  as  fol‐
138       lows:  tm_wday  and  tm_yday are set to values determined from the con‐
139       tents of the other fields; if structure members are outside their valid
140       interval,  they will be normalized (so that, for example, 40 October is
141       changed into 9 November); tm_isdst is set (regardless  of  its  initial
142       value)  to  a positive value or to 0, respectively, to indicate whether
143       DST is or is not in effect at the  specified  time.   Calling  mktime()
144       also  sets the external variable tzname with information about the cur‐
145       rent timezone.
146
147       If the specified broken-down time cannot  be  represented  as  calendar
148       time  (seconds  since the Epoch), mktime() returns (time_t) -1 and does
149       not alter the members of the broken-down time structure.
150

RETURN VALUE

152       On success, gmtime() and localtime() return a pointer to a struct tm.
153
154       On success, gmtime_r() and localtime_r()  return  the  address  of  the
155       structure pointed to by result.
156
157       On success, asctime() and ctime() return a pointer to a string.
158
159       On  success,  asctime_r()  and ctime_r() return a pointer to the string
160       pointed to by buf.
161
162       On success, mktime() returns  the  calendar  time  (seconds  since  the
163       Epoch), expressed as a value of type time_t.
164
165       On  error, mktime() returns the value (time_t) -1.  The remaining func‐
166       tions return NULL on error.  On error, errno is set to indicate the er‐
167       ror.
168

ERRORS

170       EOVERFLOW
171              The result cannot be represented.
172

ATTRIBUTES

174       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
175       tributes(7).
176
177       ┌───────────────┬───────────────┬──────────────────────────────────────┐
178Interface      Attribute     Value                                
179       ├───────────────┼───────────────┼──────────────────────────────────────┤
180asctime()      │ Thread safety │ MT-Unsafe race:asctime locale        │
181       ├───────────────┼───────────────┼──────────────────────────────────────┤
182asctime_r()    │ Thread safety │ MT-Safe locale                       │
183       ├───────────────┼───────────────┼──────────────────────────────────────┤
184ctime()        │ Thread safety │ MT-Unsafe race:tmbuf race:asctime    │
185       │               │               │ env locale                           │
186       ├───────────────┼───────────────┼──────────────────────────────────────┤
187ctime_r(),     │ Thread safety │ MT-Safe env locale                   │
188gmtime_r(),    │               │                                      │
189localtime_r(), │               │                                      │
190mktime()       │               │                                      │
191       ├───────────────┼───────────────┼──────────────────────────────────────┤
192gmtime(),      │ Thread safety │ MT-Unsafe race:tmbuf env locale      │
193localtime()    │               │                                      │
194       └───────────────┴───────────────┴──────────────────────────────────────┘
195

CONFORMING TO

197       POSIX.1-2001.  C89 and C99 specify asctime(), ctime(), gmtime(), local‐
198       time(),  and  mktime().   POSIX.1-2008  marks  asctime(),  asctime_r(),
199       ctime(), and ctime_r() as obsolete, recommending the use of strftime(3)
200       instead.
201
202       POSIX doesn't specify the parameters of ctime_r() to be restrict;  that
203       is specific to glibc.
204

NOTES

206       The four functions asctime(), ctime(), gmtime(), and localtime() return
207       a pointer to static data and hence are not  thread-safe.   The  thread-
208       safe  versions,  asctime_r(), ctime_r(), gmtime_r(), and localtime_r(),
209       are specified by SUSv2.
210
211       POSIX.1-2001 says: "The asctime(), ctime(), gmtime(),  and  localtime()
212       functions  shall  return values in one of two static objects: a broken-
213       down time structure and an array of type char.  Execution of any of the
214       functions may overwrite the information returned in either of these ob‐
215       jects by any of the other functions."  This can occur in the glibc  im‐
216       plementation.
217
218       In many implementations, including glibc, a 0 in tm_mday is interpreted
219       as meaning the last day of the preceding month.
220
221       The glibc version of struct tm has additional fields
222
223           long tm_gmtoff;           /* Seconds east of UTC */
224           const char *tm_zone;      /* Timezone abbreviation */
225
226       defined when _BSD_SOURCE was set before including <time.h>.  This is  a
227       BSD extension, present in 4.3BSD-Reno.
228
229       According  to POSIX.1-2001, localtime() is required to behave as though
230       tzset(3) was called, while localtime_r() does not  have  this  require‐
231       ment.   For  portable  code,  tzset(3)  should  be called before local‐
232       time_r().
233

SEE ALSO

235       date(1), gettimeofday(2),  time(2),  utime(2),  clock(3),  difftime(3),
236       strftime(3), strptime(3), timegm(3), tzset(3), time(7)
237

COLOPHON

239       This  page  is  part of release 5.13 of the Linux man-pages project.  A
240       description of the project, information about reporting bugs,  and  the
241       latest     version     of     this    page,    can    be    found    at
242       https://www.kernel.org/doc/man-pages/.
243
244
245
246                                  2021-03-22                          CTIME(3)
Impressum