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1 .\" $NetBSD: ctime.3,v 1.37 2010/12/17 01:30:50 wiz Exp $
2 .Dd December 14, 2010
3 .Dt CTIME 3
4 .Os
5 .Sh NAME
6 .Nm asctime ,
7 .Nm asctime_r ,
8 .Nm ctime ,
9 .Nm ctime_r ,
10 .Nm ctime_rz ,
11 .Nm difftime ,
12 .Nm gmtime ,
13 .Nm gmtime_r ,
14 .Nm localtime ,
15 .Nm localtime_r ,
16 .Nm localtime_rz ,
17 .Nm mktime ,
18 .Nm mktime_z ,
19 .Nm tzalloc ,
20 .Nm tzgetname ,
21 .Nm tzfree ,
22 .Nd convert date and time to ASCII
23 .Sh LIBRARY
24 .Lb libc
25 .Sh SYNOPSIS
26 .In time.h
27 .Dv extern char *tzname[2];
28 .Ft char *
29 .Fn ctime "const time_t *clock"
30 .Ft char *
31 .Fn ctime_r "const time_t *clock" "char *buf"
32 .Ft char *
33 .Fn ctime_rz "const timezone_t tz" "const time_t *clock" "char *buf"
34 .Ft double
35 .Fn difftime "time_t time1" "time_t time0"
36 .Ft char *
37 .Fn asctime "const struct tm *tm"
38 .Ft char *
39 .Fn asctime_r "const struct tm restrict tm" "char * restrict buf"
40 .Ft struct tm *
41 .Fn localtime "const time_t *clock"
42 .Ft struct tm *
43 .Fn localtime_r "const time_t * restrict clock" "struct tm * restrict result"
44 .Ft struct tm *
45 .Fn localtime_rz "const timezone_t tz" "const time_t * restrict clock" "struct tm * restrict result"
46 .Ft struct tm *
47 .Fn gmtime "const time_t *clock"
48 .Ft struct tm *
49 .Fn gmtime_r "const time_t * restrict clock" "struct tm * restrict result"
50 .Ft time_t
51 .Fn mktime "struct tm *tm"
52 .Ft time_t
53 .Fn mktime_z "const timezone_t tz" "struct tm *tm"
54 .Ft timezone_t
55 .Fn tzalloc "const char *zone"
56 .Ft const char *
57 .Fn tzgetname "const timezone_t tz" "int isdst"
58 .Ft void
59 .Fn tzfree "const timezone_t tz"
60 .Sh DESCRIPTION
61 .Fn ctime
62 converts a
63 .Tp time_t ,
64 pointed to by
65 .Fa clock ,
66 representing the time in seconds since
67 00:00:00 UTC, 1970-01-01,
68 and returns a pointer to a
69 string of the form
70 .D1 Thu Nov 24 18:22:48 1986\en\e0
71 Years requiring fewer than four characters are padded with leading zeroes.
72 For years longer than four characters, the string is of the form
73 .D1 Thu Nov 24 18:22:48 81986\en\e0
74 with five spaces before the year.
75 These unusual formats are designed to make it less likely that older
76 software that expects exactly 26 bytes of output will mistakenly output
77 misleading values for out-of-range years.
78 .Pp
79 .Fn ctime_r
80 is similar to
81 .Fn ctime ,
82 except it places the result of the conversion on the
83 .Fa buf
84 argument which should be 26 or more bytes long, instead of using a global
85 static buffer.
86 .Pp
87 .Fn ctime_rz
88 is similar to
89 .Fn ctime_r ,
90 but it also takes a
91 .Ft "const timezone_t"
92 argument, returned by a previous call to
93 .Fn tzalloc .
94 .Pp
95 .Fn localtime
96 and
97 .Fn gmtime
98 return pointers to
99 .Va tm
100 structures, described below.
101 .Fn localtime
102 corrects for the time zone and any time zone adjustments
103 (such as Daylight Saving Time in the U.S.A.).
104 After filling in the
105 .Va tm
106 structure,
107 .Fn localtime
108 sets the
109 .Fa tm_isdst Ns 'th
110 element of
111 .Fa tzname
112 to a pointer to an
113 ASCII string that's the time zone abbreviation to be used with
114 .Fn localtime Ns 's
115 return value.
116 .Pp
117 .Fn gmtime
118 converts to Coordinated Universal Time.
119 .Pp
120 The
121 .Fn gmtime_r
122 and
123 .Fn localtime_r
124 functions provide the same functionality as
125 .Fn gmtime
126 and
127 .Fn localtime
128 differing in that the caller must supply a buffer area
129 .Fa result
130 in which the result is stored; also,
131 .Fn localtime_r
132 does not imply initialization of the local time conversion information;
133 the application may need to do so by calling
134 .Xr tzset 3 .
135 .Pp
136 .Fn localtime_rz
137 is similar to
138 .Fn localtime_r ,
139 but it also takes a
140 .Ft "const timezone_t"
141 argument, returned by a previous call to
142 .Fn tzalloc .
143 .Pp
144 .Fn asctime
145 converts a time value contained in a
146 .Dq tm
147 structure to a string,
148 as shown in the above example,
149 and returns a pointer to the string.
150 .Pp
151 .Fn mktime
152 converts the broken-down time,
153 expressed as local time,
154 in the structure pointed to by
155 .Fa tm
156 into a calendar time value with the same encoding as that of the values
157 returned by the
158 .Xr time 3
159 function.
160 The original values of the
161 .Fa tm_wday
162 and
163 .Fa tm_yday
164 components of the structure are ignored,
165 and the original values of the other components are not restricted
166 to their normal ranges.
167 (A positive or zero value for
168 .Fa tm_isdst
169 causes
170 .Fn mktime
171 to presume initially that summer time (for example, Daylight Saving Time
172 in the U.S.A.) respectively,
173 is or is not in effect for the specified time.
174 A negative value for
175 .Fa tm_isdst
176 causes the
177 .Fn mktime
178 function to attempt to divine whether summer time is in effect
179 for the specified time; in this case it does not use a consistent
180 rule and may give a different answer when later
181 presented with the same argument.)
182 On successful completion, the values of the
183 .Fa tm_wday
184 and
185 .Fa tm_yday
186 components of the structure are set appropriately,
187 and the other components are set to represent the specified calendar time,
188 but with their values forced to their normal ranges; the final value of
189 .Fa tm_mday
190 is not set until
191 .Fa tm_mon
192 and
193 .Fa tm_year
194 are determined.
195 .Fn mktime
196 returns the specified calendar time; if the calendar time cannot be
197 represented, it returns
198 .Va "(time_t)-1" .
199 .Pp
200 .Fn mktime_z
201 is similar to
202 .Fn mktime
203 but it also takes a
204 .Ft "const timezone_t"
205 argument, returned by a previous call to
206 .Fn tzalloc .
207 .Pp
208 .Fn difftime
209 returns the difference between two calendar times,
210 .Fa ( time1 No - Fa time0 ) ,
211 expressed in seconds.
212 .Pp
213 .Fn tzalloc
214 takes as an argument a timezone name and returns a
215 .Ft timezone_t
216 object suitable to be used in
217 .Fn ctime_rz ,
218 .Fn localtime_rz ,
219 and
220 .Fn mktime_z .
221 Instead of setting the environment variable
222 .Va TZ ,
223 and globally changing the behavior of the calling program, one can use
224 multiple timezones at the same time by using separate
225 .Ft timezone_t
226 objects allocated by
227 .Fn tzalloc
228 and calling the
229 .Dq z
230 variants of the functions.
231 .Pp
232 .Fn tzgetname
233 returns the name for the given
234 .Fa tz .
235 If
236 .Fa isdst
237 is
238 .Va 0 ,
239 the call is equivalent to
240 .Va tzname[0] .
241 If
242 .Fa isdst
243 is set to
244 .Va 1
245 the call is equivalent to
246 .Va tzname[1] .
247 .Pp
248 .Fn tzfree
249 frees the
250 .Fa tz
251 argument previously returned by
252 .Fa tzalloc .
253 .Pp
254 The structure (of type)
255 .Va "struct tm"
256 includes the following fields:
257 .Bd -literal -offset indent
258 int tm_sec; /* seconds after the minute [0,61] */
259 int tm_min; /* minutes after the hour [0,59] */
260 int tm_hour; /* hours since midnight [0,23] */
261 int tm_mday; /* day of the month [1,31] */
262 int tm_mon; /* months since January [0,11] */
263 int tm_year; /* years since 1900 */
264 int tm_wday; /* day of week [0,6] (Sunday = 0) */
265 int tm_yday; /* day of year [0,365] (Jan 1 = 0) */
266 int tm_isdst; /* daylight savings flag */
267 long tm_gmtoff; /* offset from UTC in seconds */
268 char *tm_zone; /* abbreviation of timezone name */
269 .Ed
270 .Pp
271 The
272 .Fa tm_zone
273 and
274 .Fa tm_gmtoff
275 fields exist, and are filled in, only if arrangements to do
276 so were made when the library containing these functions was
277 created.
278 There is no guarantee that these fields will continue to exist
279 in this form in future releases of this code.
280 The
281 .Fa tm_zone
282 field will become invalid and point to freed storage if the corresponding
283 .Va "struct tm"
284 was returned by
285 .Fn localtime_rz
286 and the
287 .Ft "const timezone_t"
288 .Fa tz
289 argument has been freed by
290 .Fn tzfree .
291 .Pp
292 .Fa tm_isdst
293 is non-zero if summer time is in effect.
294 .Pp
295 .Fa tm_gmtoff
296 is the offset (in seconds) of the time represented
297 from UTC, with positive values indicating east
298 of the Prime Meridian.
299 .Sh RETURN VALUES
300 On success the
301 .Fn asctime
302 and
303 .Fn ctime
304 functions return a pointer to a static character buffer, and the
305 .Fn asctime_r ,
306 .Fn ctime_r ,
307 and
308 .Fn ctime_rz
309 function return a pointer to the user-supplied buffer.
310 On failure they all return
311 .Dv NULL
312 and no errors are defined for them.
313 On success the
314 .Fn gmtime ,
315 and
316 .Fn localtime
317 functions return a pointer to a statically allocated
318 .Va "struct tm"
319 whereas the
320 .Fn gmtime_r ,
321 .Fn localtime_r ,
322 and
323 .Fn localtime_rz ,
324 functions return a pointer to the user-supplied
325 .Va "struct tm" .
326 On failure they all return
327 .Dv NULL
328 and the global variable
329 .Va errno
330 is set to indicate the error.
331 The
332 .Fn mktime
333 and
334 .Fn mktime_z
335 function returns the specified time since the Epoch as a
336 .Va time_t
337 type value.
338 If the time cannot be represented, then
339 .Fn mktime
340 and
341 .Fn mktime_z
342 return
343 .Va "(time_t)-1"
344 setting the global variable
345 .Va errno
346 to indicate the error.
347 The
348 .Fn tzalloc
349 function returns a pointer to a
350 .Ft timezone_t
351 object or
352 .Dv NULL
353 on failure, setting
354 .Va errno
355 to indicate the error.
356 .Fn tzgetzone
357 function returns string containing the name of the timezone given in
358 .Fa tz .
359 .Sh FILES
360 .Bl -tag -width /usr/share/zoneinfo/posixrules -compact
361 .It Pa /etc/localtime
362 local time zone file
363 .It Pa /usr/share/zoneinfo
364 time zone information directory
365 .It Pa /usr/share/zoneinfo/posixrules
366 used with POSIX-style TZ's
367 .It Pa /usr/share/zoneinfo/GMT
368 for UTC leap seconds
369 .El
370 .Pp
371 If
372 .Pa /usr/share/zoneinfo/GMT
373 is absent, UTC leap seconds are loaded from
374 .Pa /usr/share/zoneinfo/posixrules .
375 .Sh ERRORS
376 The
377 .Fn gmtime_r ,
378 .Fn localtime_r ,
379 .Fn localtime_rz ,
380 .Fn gmtime ,
381 .Fn localtime ,
382 and
383 .Fn mktime ,
384 and
385 .Fn mktime_z
386 will fail when:
387 .Bl -tag -width Er
388 .It Bq Er EINVAL
389 The result cannot be represented.
390 .It Bq Er EOVERFLOW
391 The result cannot be represented.
392 .El
393 .Pp
394 All functions that return values except their
395 .Dq z
396 variants, can also return the same errors as
397 .Xr open 2
398 and
399 .Xr malloc 3 .
400 .Sh SEE ALSO
401 .Xr getenv 3 ,
402 .Xr strftime 3 ,
403 .Xr time 3 ,
404 .Xr tzset 3 ,
405 .Xr tzfile 5
406 .Sh STANDARDS
407 The
408 .Fn ctime ,
409 .Fn difftime ,
410 .Fn asctime ,
411 .Fn localtime ,
412 .Fn gmtime
413 and
414 .Fn mktime
415 functions conform to
416 .St -ansiC
417 The
418 .Fn ctime_r ,
419 .Fn asctime_r ,
420 .Fn localtime_r
421 and
422 .Fn gmtime_r
423 functions conform to
424 .St -p1003.1c-95 .
425 .Sh NOTES
426 The return values point to static data; the data is overwritten by
427 each call.
428 The
429 .Fa tm_zone
430 field of a returned
431 .Va "struct tm"
432 points to a static array of characters, which
433 will also be overwritten at the next call
434 (and by calls to
435 .Xr tzset 3 ) .
436 .Pp
437 .Fn asctime
438 and
439 .Fn ctime
440 behave strangely for years before 1000 or after 9999.
441 The 1989 and 1999 editions of the C Standard say
442 that years from \-99 through 999 are converted without
443 extra spaces, but this conflicts with longstanding
444 tradition and with this implementation.
445 Traditional implementations of these two functions are
446 restricted to years in the range 1900 through 2099.
447 To avoid this portability mess, new programs should use
448 .Fn strftime
449 instead.
450 .Pp
451 Avoid using out-of-range values with
452 .Fn mktime
453 when setting up lunch with promptness sticklers in Riyadh.
454 .\" @(#)newctime.3 8.3
455 .\" This file is in the public domain, so clarified as of
456 .\" 2009-05-17 by Arthur David Olson.