1CTIME(3) Linux Programmer's Manual CTIME(3)
2
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 >= 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
31 _SVID_SOURCE || _POSIX_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 */
48 int tm_min; /* minutes */
49 int tm_hour; /* hours */
50 int tm_mday; /* day of the month */
51 int tm_mon; /* month */
52 int tm_year; /* year */
53 int tm_wday; /* day of the week */
54 int tm_yday; /* day in the year */
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 The abbreviations for the days of the week are "Sun", "Mon", "Tue",
88 "Wed", "Thu", "Fri", and "Sat". The abbreviations for the months are
89 "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct",
90 "Nov", and "Dec". The return value points to a statically allocated
91 string which might be overwritten by subsequent calls to any of the
92 date and time functions. The function also sets the external variables
93 tzname, timezone, and daylight (see tzset(3)) with information about
94 the current timezone. The reentrant version ctime_r() does the same,
95 but stores the string in a user-supplied buffer which should have room
96 for at least 26 bytes. It need not set tzname, timezone, and daylight.
97 The gmtime() function converts the calendar time timep to broken-down
99 time representation, expressed in Coordinated Universal Time (UTC). It
100 may return NULL when the year does not fit into an integer. The return
101 value points to a statically allocated struct which might be overwrit‐
102 ten by subsequent calls to any of the date and time functions. The
103 gmtime_r() function does the same, but stores the data in a user-sup‐
104 plied struct.
105 The localtime() function converts the calendar time timep to broken-
107 down time representation, expressed relative to the user's specified
108 timezone. The function acts as if it called tzset(3) and sets the
109 external variables tzname with information about the current timezone,
110 timezone with the difference between Coordinated Universal Time (UTC)
111 and local standard time in seconds, and daylight to a nonzero value if
112 daylight savings time rules apply during some part of the year. The
113 return value points to a statically allocated struct which might be
114 overwritten by subsequent calls to any of the date and time functions.
115 The localtime_r() function does the same, but stores the data in a
116 user-supplied struct. It need not set tzname, timezone, and daylight.
117 The asctime() function converts the broken-down time value tm into a
119 null-terminated string with the same format as ctime(). The return
120 value points to a statically allocated string which might be overwrit‐
121 ten by subsequent calls to any of the date and time functions. The
122 asctime_r() function does the same, but stores the string in a user-
123 supplied buffer which should have room for at least 26 bytes.
124 The mktime() function converts a broken-down time structure, expressed
126 as local time, to calendar time representation. The function ignores
127 the values supplied by the caller in the tm_wday and tm_yday fields.
128 The value specified in the tm_isdst field informs mktime() whether or
129 not daylight saving time (DST) is in effect for the time supplied in
130 the tm structure: a positive value means DST is in effect; zero means
131 that DST is not in effect; and a negative value means that mktime()
132 should (use timezone information and system databases to) attempt to
133 determine whether DST is in effect at the specified time.
134 The mktime() function modifies the fields of the tm structure as fol‐
136 lows: tm_wday and tm_yday are set to values determined from the con‐
137 tents of the other fields; if structure members are outside their valid
138 interval, they will be normalized (so that, for example, 40 October is
139 changed into 9 November); tm_isdst is set (regardless of its initial
140 value) to a positive value or to 0, respectively, to indicate whether
141 DST is or is not in effect at the specified time. Calling mktime()
142 also sets the external variable tzname with information about the cur‐
143 rent timezone.
144 If the specified broken-down time cannot be represented as calendar
146 time (seconds since the Epoch), mktime() returns (time_t) -1 and does
147 not alter the members of the broken-down time structure.
148
150 Each of these functions returns the value described, or NULL (-1 in
151 case of mktime()) in case an error was detected.
152
154 POSIX.1-2001. C89 and C99 specify asctime(), ctime(), gmtime(), local‐
155 time(), and mktime(). POSIX.1-2008 marks asctime(), asctime_r(),
156 ctime(), and ctime_r() as obsolete, recommending the use of strftime(3)
157 instead.
158
160 The four functions asctime(), ctime(), gmtime() and localtime() return
161 a pointer to static data and hence are not thread-safe. Thread-safe
162 versions asctime_r(), ctime_r(), gmtime_r() and localtime_r() are spec‐
163 ified by SUSv2, and available since libc 5.2.5.
164 POSIX.1-2001 says: "The asctime(), ctime(), gmtime(), and localtime()
166 functions shall return values in one of two static objects: a broken-
167 down time structure and an array of type char. Execution of any of the
168 functions may overwrite the information returned in either of these
169 objects by any of the other functions." This can occur in the glibc
170 implementation.
171 In many implementations, including glibc, a 0 in tm_mday is interpreted
173 as meaning the last day of the preceding month.
174 The glibc version of struct tm has additional fields
176 long tm_gmtoff; /* Seconds east of UTC */
178 const char *tm_zone; /* Timezone abbreviation */
179 defined when _BSD_SOURCE was set before including <time.h>. This is a
181 BSD extension, present in 4.3BSD-Reno.
182 According to POSIX.1-2004, localtime() is required to behave as though
184 tzset(3) was called, while localtime_r() does not have this require‐
185 ment. For portable code tzset(3) should be called before local‐
186 time_r().
187
189 date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3),
190 strftime(3), strptime(3), timegm(3), tzset(3), time(7)
191
193 This page is part of release 3.53 of the Linux man-pages project. A
194 description of the project, and information about reporting bugs, can
195 be found at http://www.kernel.org/doc/man-pages/.
196 2010-02-25 CTIME(3)