1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
6 .\" preserved on all copies.
8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" References consulted:
26 .\" Linux libc source code
27 .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
29 .\" Modified Sat Jul 24 19:49:27 1993 by Rik Faith (faith@cs.unc.edu)
30 .\" Modified Fri Apr 26 12:38:55 MET DST 1996 by Martin Schulze (joey@linux.de)
31 .\" Modified 2001-11-13, aeb
32 .\" Modified 2001-12-13, joey, aeb
33 .\" Modified 2004-11-16, mtk
35 .TH CTIME 3 2014-08-19 "" "Linux Programmer's Manual"
37 asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
38 localtime_r \- transform date and time to broken-down time or ASCII
43 .BI "char *asctime(const struct tm *" tm );
45 .BI "char *asctime_r(const struct tm *" tm ", char *" buf );
47 .BI "char *ctime(const time_t *" timep );
49 .BI "char *ctime_r(const time_t *" timep ", char *" buf );
51 .BI "struct tm *gmtime(const time_t *" timep );
53 .BI "struct tm *gmtime_r(const time_t *" timep ", struct tm *" result );
55 .BI "struct tm *localtime(const time_t *" timep );
57 .BI "struct tm *localtime_r(const time_t *" timep ", struct tm *" result );
59 .BI "time_t mktime(struct tm *" tm );
63 Feature Test Macro Requirements for glibc (see
64 .BR feature_test_macros (7)):
73 _POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
74 _SVID_SOURCE || _POSIX_SOURCE
84 an argument of data type \fItime_t\fP, which represents calendar time.
85 When interpreted as an absolute time value, it represents the number of
86 seconds elapsed since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
92 functions both take an argument
93 representing broken-down time, which is a representation
94 separated into year, month, day, and so on.
96 Broken-down time is stored
97 in the structure \fItm\fP, which is defined in \fI<time.h>\fP as follows:
102 int tm_sec; /* Seconds (0-60) */
103 int tm_min; /* Minutes (0-59) */
104 int tm_hour; /* Hours (0-23) */
105 int tm_mday; /* Day of the month (1-31) */
106 int tm_mon; /* Month (0-11) */
107 int tm_year; /* Year - 1900 */
108 int tm_wday; /* Day of the week (0-6, Sunday = 0) */
109 int tm_yday; /* Day in the year (0-365, 1 Jan = 0) */
110 int tm_isdst; /* Daylight saving time */
115 The members of the \fItm\fP structure are:
118 The number of seconds after the minute, normally in the range 0 to 59,
119 but can be up to 60 to allow for leap seconds.
122 The number of minutes after the hour, in the range 0 to 59.
125 The number of hours past midnight, in the range 0 to 23.
128 The day of the month, in the range 1 to 31.
131 The number of months since January, in the range 0 to 11.
134 The number of years since 1900.
137 The number of days since Sunday, in the range 0 to 6.
140 The number of days since January 1, in the range 0 to 365.
143 A flag that indicates whether daylight saving time is in effect at the
145 The value is positive if daylight saving time is in
146 effect, zero if it is not, and negative if the information is not
152 .BI asctime(localtime( t )) \fR.
153 It converts the calendar time \fIt\fP into a
154 null-terminated string of the form
157 "Wed Jun 30 21:49:08 1993\\n"
160 The abbreviations for the days of the week are "Sun", "Mon", "Tue", "Wed",
161 "Thu", "Fri", and "Sat".
162 The abbreviations for the months are "Jan",
163 "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", and
165 The return value points to a statically allocated string which
166 might be overwritten by subsequent calls to any of the date and time
168 The function also sets the external
169 variables \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP (see
171 with information about the current timezone.
172 The reentrant version
174 does the same, but stores the
175 string in a user-supplied buffer
176 which should have room for at least 26 bytes.
178 set \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP.
182 function converts the calendar time \fItimep\fP to
183 broken-down time representation, expressed in Coordinated Universal Time
185 It may return NULL when the year does not fit into an integer.
186 The return value points to a statically allocated struct which might be
187 overwritten by subsequent calls to any of the date and time functions.
190 function does the same, but stores the data in a
191 user-supplied struct.
195 function converts the calendar time \fItimep\fP to
196 broken-down time representation,
197 expressed relative to the user's specified timezone.
198 The function acts as if it called
200 and sets the external variables \fItzname\fP with
201 information about the current timezone, \fItimezone\fP with the difference
202 between Coordinated Universal Time (UTC) and local standard time in
203 seconds, and \fIdaylight\fP to a nonzero value if daylight savings
204 time rules apply during some part of the year.
205 The return value points to a statically allocated struct which might be
206 overwritten by subsequent calls to any of the date and time functions.
209 function does the same, but stores the data in a
210 user-supplied struct.
211 It need not set \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP.
215 function converts the broken-down time value
216 \fItm\fP into a null-terminated string with the same format as
218 The return value points to a statically allocated string which might be
219 overwritten by subsequent calls to any of the date and time functions.
222 function does the same, but stores the string in
223 a user-supplied buffer which should have room for at least 26 bytes.
227 function converts a broken-down time structure, expressed
228 as local time, to calendar time representation.
230 the values supplied by the caller in the
235 The value specified in the
239 whether or not daylight saving time (DST)
240 is in effect for the time supplied in the
243 a positive value means DST is in effect;
244 zero means that DST is not in effect;
245 and a negative value means that
247 should (use timezone information and system databases to)
248 attempt to determine whether DST is in effect at the specified time.
252 function modifies the fields of the
254 structure as follows:
258 are set to values determined from the contents of the other fields;
259 if structure members are outside their valid interval, they will be
260 normalized (so that, for example, 40 October is changed into 9 November);
262 is set (regardless of its initial value)
263 to a positive value or to 0, respectively,
264 to indicate whether DST is or is not in effect at the specified time.
267 also sets the external variable \fItzname\fP with
268 information about the current timezone.
270 If the specified broken-down
271 time cannot be represented as calendar time (seconds since the Epoch),
275 and does not alter the
276 members of the broken-down time structure.
278 Each of these functions returns the value described, or NULL
281 in case an error was detected.
283 For an explanation of the terms used in this section, see
289 Interface Attribute Value
292 T} Thread safety MT-Unsafe race:asctime locale
295 T} Thread safety MT-Safe locale
301 race:asctime env locale
308 T} Thread safety MT-Safe env locale
312 T} Thread safety MT-Unsafe race:tmbuf env locale
331 recommending the use of
341 return a pointer to static data and hence are not thread-safe.
342 The thread-safe versions,
348 are specified by SUSv2.
357 functions shall return values in one of two static objects:
358 a broken-down time structure and an array of type
360 Execution of any of the functions may overwrite the information returned
361 in either of these objects by any of the other functions."
362 This can occur in the glibc implementation.
364 In many implementations, including glibc, a 0 in
366 is interpreted as meaning the last day of the preceding month.
368 The glibc version of \fIstruct tm\fP has additional fields
372 long tm_gmtoff; /* Seconds east of UTC */
373 const char *tm_zone; /* Timezone abbreviation */
379 was set before including
381 This is a BSD extension, present in 4.3BSD-Reno.
383 According to POSIX.1-2004,
385 is required to behave as though
389 does not have this requirement.
390 .\" See http://thread.gmane.org/gmane.comp.time.tz/2034/
393 should be called before
397 .BR gettimeofday (2),