1CTIME(3) Linux Programmer's Manual CTIME(3)
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6 asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r,
7 gmtime_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 *tm, char *buf);
15 char *ctime(const time_t *timep);
17 char *ctime_r(const time_t *timep, char *buf);
18 struct tm *gmtime(const time_t *timep);
20 struct tm *gmtime_r(const time_t *timep, struct tm *result);
21 struct tm *localtime(const time_t *timep);
23 struct tm *localtime_r(const time_t *timep, struct tm *result);
24 time_t mktime(struct tm *tm);
26 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
28 asctime_r(), ctime_r(), gmtime_r(), localtime_r():
30 _POSIX_C_SOURCE
31 || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
32
34 The ctime(), gmtime() and localtime() functions all take an argument of
35 data type time_t, which represents calendar time. When interpreted as
36 an absolute time value, it represents the number of seconds elapsed
37 since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
38 The asctime() and mktime() functions both take an argument representing
40 broken-down time, which is a representation separated into year, month,
41 day, and so on.
42 Broken-down time is stored in the structure tm, which is defined in
44 <time.h> as follows:
45 struct tm {
47 int tm_sec; /* Seconds (0-60) */
48 int tm_min; /* Minutes (0-59) */
49 int tm_hour; /* Hours (0-23) */
50 int tm_mday; /* Day of the month (1-31) */
51 int tm_mon; /* Month (0-11) */
52 int tm_year; /* Year - 1900 */
53 int tm_wday; /* Day of the week (0-6, Sunday = 0) */
54 int tm_yday; /* Day in the year (0-365, 1 Jan = 0) */
55 int tm_isdst; /* Daylight saving time */
56 };
57 The members of the tm structure are:
59 tm_sec The number of seconds after the minute, normally in the range
61 0 to 59, but can be up to 60 to allow for leap seconds.
62 tm_min The number of minutes after the hour, in the range 0 to 59.
64 tm_hour The number of hours past midnight, in the range 0 to 23.
66 tm_mday The day of the month, in the range 1 to 31.
68 tm_mon The number of months since January, in the range 0 to 11.
70 tm_year The number of years since 1900.
72 tm_wday The number of days since Sunday, in the range 0 to 6.
74 tm_yday The number of days since January 1, in the range 0 to 365.
76 tm_isdst A flag that indicates whether daylight saving time is in
78 effect at the time described. The value is positive if day‐
79 light saving time is in effect, zero if it is not, and nega‐
80 tive if the information is not available.
81 The call ctime(t) is equivalent to asctime(localtime(t)). It converts
83 the calendar time t into a null-terminated string of the form
84 "Wed Jun 30 21:49:08 1993\n"
86 ,in
87 The abbreviations for the days of the week are "Sun", "Mon", "Tue",
89 "Wed", "Thu", "Fri", and "Sat". The abbreviations for the months are
90 "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct",
91 "Nov", and "Dec". The return value points to a statically allocated
92 string which might be overwritten by subsequent calls to any of the
93 date and time functions. The function also sets the external variables
94 tzname, timezone, and daylight (see tzset(3)) with information about
95 the current timezone. The reentrant version ctime_r() does the same,
96 but stores the string in a user-supplied buffer which should have room
97 for at least 26 bytes. It need not set tzname, timezone, and daylight.
98 The gmtime() function converts the calendar time timep to broken-down
100 time representation, expressed in Coordinated Universal Time (UTC). It
101 may return NULL when the year does not fit into an integer. The return
102 value points to a statically allocated struct which might be overwrit‐
103 ten by subsequent calls to any of the date and time functions. The
104 gmtime_r() function does the same, but stores the data in a user-sup‐
105 plied struct.
106 The localtime() function converts the calendar time timep to broken-
108 down time representation, expressed relative to the user's specified
109 timezone. The function acts as if it called tzset(3) and sets the
110 external variables tzname with information about the current timezone,
111 timezone with the difference between Coordinated Universal Time (UTC)
112 and local standard time in seconds, and daylight to a nonzero value if
113 daylight savings time rules apply during some part of the year. The
114 return value points to a statically allocated struct which might be
115 overwritten by subsequent calls to any of the date and time functions.
116 The localtime_r() function does the same, but stores the data in a
117 user-supplied struct. It need not set tzname, timezone, and daylight.
118 The asctime() function converts the broken-down time value tm into a
120 null-terminated string with the same format as ctime(). The return
121 value points to a statically allocated string which might be overwrit‐
122 ten by subsequent calls to any of the date and time functions. The
123 asctime_r() function does the same, but stores the string in a user-
124 supplied buffer which should have room for at least 26 bytes.
125 The mktime() function converts a broken-down time structure, expressed
127 as local time, to calendar time representation. The function ignores
128 the values supplied by the caller in the tm_wday and tm_yday fields.
129 The value specified in the tm_isdst field informs mktime() whether or
130 not daylight saving time (DST) is in effect for the time supplied in
131 the tm structure: a positive value means DST is in effect; zero means
132 that DST is not in effect; and a negative value means that mktime()
133 should (use timezone information and system databases to) attempt to
134 determine whether DST is in effect at the specified time.
135 The mktime() function modifies the fields of the tm structure as fol‐
137 lows: tm_wday and tm_yday are set to values determined from the con‐
138 tents of the other fields; if structure members are outside their valid
139 interval, they will be normalized (so that, for example, 40 October is
140 changed into 9 November); tm_isdst is set (regardless of its initial
141 value) to a positive value or to 0, respectively, to indicate whether
142 DST is or is not in effect at the specified time. Calling mktime()
143 also sets the external variable tzname with information about the cur‐
144 rent timezone.
145 If the specified broken-down time cannot be represented as calendar
147 time (seconds since the Epoch), mktime() returns (time_t) -1 and does
148 not alter the members of the broken-down time structure.
149
151 On success, gmtime() and localtime() return a pointer to a struct tm.
152 On success, gmtime_r() and localtime_r() return the address of the
154 structure pointed to by result.
155 On success, asctime() and ctime() return a pointer to a string.
157 On success, asctime_r() and ctime_r() return a pointer to the string
159 pointed to by buf.
160 On success, mktime() returns the calendar time (seconds since the
162 Epoch), expressed as a value of type time_t.
163 On error, mktime() returns the value (time_t) -1. The remaining func‐
165 tions return NULL on error. On error, errno is set to indicate the
166 cause of the error.
167
169 EOVERFLOW
170 The result cannot be represented.
171
173 For an explanation of the terms used in this section, see
174 attributes(7).
175 ┌───────────────┬───────────────┬─────────────────────────────────┐
177 │Interface │ Attribute │ Value │
178 ├───────────────┼───────────────┼─────────────────────────────────┤
179 │asctime() │ Thread safety │ MT-Unsafe race:asctime locale │
180 ├───────────────┼───────────────┼─────────────────────────────────┤
181 │asctime_r() │ Thread safety │ MT-Safe locale │
182 ├───────────────┼───────────────┼─────────────────────────────────┤
183 │ctime() │ Thread safety │ MT-Unsafe race:tmbuf │
184 │ │ │ race:asctime env locale │
185 ├───────────────┼───────────────┼─────────────────────────────────┤
186 │ctime_r(), │ Thread safety │ MT-Safe env locale │
187 │gmtime_r(), │ │ │
188 │localtime_r(), │ │ │
189 │mktime() │ │ │
190 ├───────────────┼───────────────┼─────────────────────────────────┤
191 │gmtime(), │ Thread safety │ MT-Unsafe race:tmbuf env locale │
192 │localtime() │ │ │
193 └───────────────┴───────────────┴─────────────────────────────────┘
195 POSIX.1-2001. C89 and C99 specify asctime(), ctime(), gmtime(), local‐
196 time(), and mktime(). POSIX.1-2008 marks asctime(), asctime_r(),
197 ctime(), and ctime_r() as obsolete, recommending the use of strftime(3)
198 instead.
199
201 The four functions asctime(), ctime(), gmtime() and localtime() return
202 a pointer to static data and hence are not thread-safe. The thread-
203 safe versions, asctime_r(), ctime_r(), gmtime_r() and localtime_r(),
204 are specified by SUSv2.
205 POSIX.1-2001 says: "The asctime(), ctime(), gmtime(), and localtime()
207 functions shall return values in one of two static objects: a broken-
208 down time structure and an array of type char. Execution of any of the
209 functions may overwrite the information returned in either of these
210 objects by any of the other functions." This can occur in the glibc
211 implementation.
212 In many implementations, including glibc, a 0 in tm_mday is interpreted
214 as meaning the last day of the preceding month.
215 The glibc version of struct tm has additional fields
217 const char *tm_zone; /* Timezone abbreviation */
219 defined when _BSD_SOURCE was set before including <time.h>. This is a
221 BSD extension, present in 4.3BSD-Reno.
222 According to POSIX.1-2004, localtime() is required to behave as though
224 tzset(3) was called, while localtime_r() does not have this require‐
225 ment. For portable code, tzset(3) should be called before local‐
226 time_r().
227
229 date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3),
230 strftime(3), strptime(3), timegm(3), tzset(3), time(7)
231
233 This page is part of release 4.15 of the Linux man-pages project. A
234 description of the project, information about reporting bugs, and the
235 latest version of this page, can be found at
236 https://www.kernel.org/doc/man-pages/.
237 2017-09-15 CTIME(3)