1CTIME(3) Linux Programmer's Manual CTIME(3)
2
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
11 #include <time.h>
12 char *asctime(const struct tm *tm);
14 char *asctime_r(const struct tm *restrict tm, char *restrict buf);
15 char *ctime(const time_t *timep);
17 char *ctime_r(const time_t *restrict timep, char *restrict buf);
18 struct tm *gmtime(const time_t *timep);
20 struct tm *gmtime_r(const time_t *restrict timep,
21 struct tm *restrict result);
22 struct tm *localtime(const time_t *timep);
24 struct tm *localtime_r(const time_t *restrict timep,
25 struct tm *restrict result);
26 time_t mktime(struct tm *tm);
28 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
30 asctime_r(), ctime_r(), gmtime_r(), localtime_r():
32 _POSIX_C_SOURCE
33 || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
34
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 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 Broken-down time is stored in the structure tm, which is defined in
46 <time.h> as follows:
47 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 The members of the tm structure are:
61 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 tm_min The number of minutes after the hour, in the range 0 to 59.
66 tm_hour The number of hours past midnight, in the range 0 to 23.
68 tm_mday The day of the month, in the range 1 to 31.
70 tm_mon The number of months since January, in the range 0 to 11.
72 tm_year The number of years since 1900.
74 tm_wday The number of days since Sunday, in the range 0 to 6.
76 tm_yday The number of days since January 1, in the range 0 to 365.
78 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 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 "Wed Jun 30 21:49:08 1993\n"
88 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 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 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 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 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 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 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
152 On success, gmtime() and localtime() return a pointer to a struct tm.
153 On success, gmtime_r() and localtime_r() return the address of the
155 structure pointed to by result.
156 On success, asctime() and ctime() return a pointer to a string.
158 On success, asctime_r() and ctime_r() return a pointer to the string
160 pointed to by buf.
161 On success, mktime() returns the calendar time (seconds since the
163 Epoch), expressed as a value of type time_t.
164 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
170 EOVERFLOW
171 The result cannot be represented.
172
174 For an explanation of the terms used in this section, see at‐
175 tributes(7).
176 ┌───────────────┬───────────────┬──────────────────────────────────────┐
178 │Interface │ Attribute │ Value │
179 ├───────────────┼───────────────┼──────────────────────────────────────┤
180 │asctime() │ Thread safety │ MT-Unsafe race:asctime locale │
181 ├───────────────┼───────────────┼──────────────────────────────────────┤
182 │asctime_r() │ Thread safety │ MT-Safe locale │
183 ├───────────────┼───────────────┼──────────────────────────────────────┤
184 │ctime() │ Thread safety │ MT-Unsafe race:tmbuf race:asctime │
185 │ │ │ env locale │
186 ├───────────────┼───────────────┼──────────────────────────────────────┤
187 │ctime_r(), │ Thread safety │ MT-Safe env locale │
188 │gmtime_r(), │ │ │
189 │localtime_r(), │ │ │
190 │mktime() │ │ │
191 ├───────────────┼───────────────┼──────────────────────────────────────┤
192 │gmtime(), │ Thread safety │ MT-Unsafe race:tmbuf env locale │
193 │localtime() │ │ │
194 └───────────────┴───────────────┴──────────────────────────────────────┘
195
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 POSIX doesn't specify the parameters of ctime_r() to be restrict; that
203 is specific to glibc.
204
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 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 In many implementations, including glibc, a 0 in tm_mday is interpreted
219 as meaning the last day of the preceding month.
220 The glibc version of struct tm has additional fields
222 long tm_gmtoff; /* Seconds east of UTC */
224 const char *tm_zone; /* Timezone abbreviation */
225 defined when _BSD_SOURCE was set before including <time.h>. This is a
227 BSD extension, present in 4.3BSD-Reno.
228 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
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
239 This page is part of release 5.12 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 2021-03-22 CTIME(3)