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1 NEWCTIME(3) Library Functions Manual NEWCTIME(3)
4 asctime, ctime, difftime, gmtime, localtime, mktime - convert date and
8 #include <time.h>
10 extern char *tzname; /* (optional) */
12 char *ctime(time_t const *clock);
14 char *ctime_r(time_t const *clock, char *buf);
16 double difftime(time_t time1, time_t time0);
18 char *asctime(struct tm const *tm);
20 char *asctime_r(struct tm const *restrict tm,
21 char *restrict result);
23 struct tm *localtime(time_t const *clock);
25 struct tm *localtime_r(time_t const *restrict clock,
26 struct tm *restrict result);
28 struct tm *localtime_rz(timezone_t restrict zone,
29 time_t const *restrict clock,
30 struct tm *restrict result);
32 struct tm *gmtime(time_t const *clock);
34 struct tm *gmtime_r(time_t const *restrict clock,
35 struct tm *restrict result);
37 time_t mktime(struct tm *tm);
39 time_t mktime_z(timezone_t restrict zone,
40 struct tm *restrict tm);
42 cc ... -ltz
45 Ctime converts a long integer, pointed to by clock, and returns a
46 pointer to a string of the form
47 Thu Nov 24 18:22:48 1986\n\0
48 Years requiring fewer than four characters are padded with leading
49 zeroes. For years longer than four characters, the string is of the
51 Thu Nov 24 18:22:48 81986\n\0
52 with five spaces before the year. These unusual formats are designed
53 to make it less likely that older software that expects exactly 26
54 bytes of output will mistakenly output misleading values for out-of-
55 range years.
57 The *clock timestamp represents the time in seconds since 1970-01-01
58 00:00:00 Coordinated Universal Time (UTC). The POSIX standard says
59 that timestamps must be nonnegative and must ignore leap seconds. Many
60 implementations extend POSIX by allowing negative timestamps, and can
61 therefore represent timestamps that predate the introduction of UTC and
62 are some other flavor of Universal Time (UT). Some implementations
63 support leap seconds, in contradiction to POSIX.
65 Localtime and gmtime return pointers to "tm" structures, described
66 below. Localtime corrects for the time zone and any time zone
67 adjustments (such as Daylight Saving Time in the United States). After
68 filling in the "tm" structure, localtime sets the tm_isdst'th element
69 of tzname to a pointer to a string that's the time zone abbreviation to
70 be used with localtime's return value.
72 Gmtime converts to Coordinated Universal Time.
74 Asctime converts a time value contained in a "tm" structure to a
75 string, as shown in the above example, and returns a pointer to the
78 Mktime converts the broken-down time, expressed as local time, in the
79 structure pointed to by tm into a calendar time value with the same
80 encoding as that of the values returned by the time function. The
81 original values of the tm_wday and tm_yday components of the structure
82 are ignored, and the original values of the other components are not
83 restricted to their normal ranges. (A positive or zero value for
84 tm_isdst causes mktime to presume initially that daylight saving time
85 respectively, is or is not in effect for the specified time. A
86 negative value for tm_isdst causes the mktime function to attempt to
87 divine whether daylight saving time is in effect for the specified
88 time; in this case it does not use a consistent rule and may give a
89 different answer when later presented with the same argument.) On
90 successful completion, the values of the tm_wday and tm_yday components
91 of the structure are set appropriately, and the other components are
92 set to represent the specified calendar time, but with their values
93 forced to their normal ranges; the final value of tm_mday is not set
94 until tm_mon and tm_year are determined. Mktime returns the specified
95 calendar time; If the calendar time cannot be represented, it returns
98 Difftime returns the difference between two calendar times, (time1 -
99 time0), expressed in seconds.
101 Ctime_r, localtime_r, gmtime_r, and asctime_r are like their unsuffixed
102 counterparts, except that they accept an additional argument specifying
103 where to store the result if successful.
105 Localtime_rz and mktime_z are like their unsuffixed counterparts,
106 except that they accept an extra initial zone argument specifying the
107 timezone to be used for conversion. If zone is null, UT is used;
108 otherwise, zone should be have been allocated by tzalloc and should not
109 be freed until after all uses (e.g., by calls to strftime) of the
110 filled-in tm_zone fields.
112 Declarations of all the functions and externals, and the "tm"
113 structure, are in the <time.h> header file. The structure (of type)
114 struct tm includes the following fields:
116 int tm_sec; /* seconds (0-60) */
117 int tm_min; /* minutes (0-59) */
118 int tm_hour; /* hours (0-23) */
119 int tm_mday; /* day of month (1-31) */
120 int tm_mon; /* month of year (0-11) */
121 int tm_year; /* year - 1900 */
122 int tm_wday; /* day of week (Sunday = 0) */
123 int tm_yday; /* day of year (0-365) */
124 int tm_isdst; /* is daylight saving time in effect? */
125 char *tm_zone; /* time zone abbreviation (optional) */
126 long tm_gmtoff; /* offset from UT in seconds (optional) */
128 Tm_isdst is non-zero if daylight saving time is in effect.
130 Tm_gmtoff is the offset (in seconds) of the time represented from UT,
131 with positive values indicating east of the Prime Meridian. The
132 field's name is derived from Greenwich Mean Time, a precursor of UT.
134 In struct tm the tm_zone and tm_gmtoff fields exist, and are filled in,
135 only if arrangements to do so were made when the library containing
136 these functions was created. Similarly, the tzname variable is
137 optional. There is no guarantee that these fields and this variable
138 will continue to exist in this form in future releases of this code.
141 /usr/share/zoneinfo timezone information directory
142 /usr/share/zoneinfo/localtime local timezone file
143 /usr/share/zoneinfo/posixrules used with POSIX-style TZ's
144 /usr/share/zoneinfo/GMT for UTC leap seconds
146 If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
149 SEE ALSO
150 getenv(3), newstrftime(3), newtzset(3), time(2), tzfile(5)
153 The return values of asctime, ctime, gmtime, and localtime point to
154 static data overwritten by each call. The tzname variable (once set)
155 and the tm_zone field of a returned struct tm both point to an array of
156 characters that can be freed or overwritten by later calls to the
157 functions localtime, tzfree, and tzset, if these functions affect the
158 timezone information that specifies the abbreviation in question. The
159 remaining functions and data are thread-safe.
161 Asctime, asctime_r, ctime, and ctime_r behave strangely for years
162 before 1000 or after 9999. The 1989 and 1999 editions of the C
163 Standard say that years from -99 through 999 are converted without
164 extra spaces, but this conflicts with longstanding tradition and with
165 this implementation. The 2011 edition says that the behavior is
166 undefined if the year is before 1000 or after 9999. Traditional
167 implementations of these two functions are restricted to years in the
168 range 1900 through 2099. To avoid this portability mess, new programs
169 should use strftime instead.