1CTIME(3) Library Functions Manual CTIME(3)
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6 ctime, localtime, gmtime, asctime, timezone, tzset - convert date and
7 time to ASCII
8
10 void tzset()
11 char *ctime(clock)
13 time_t *clock;
14 #include <time.h>
16 char *asctime(tm)
18 struct tm *tm;
19 struct tm *localtime(clock)
21 time_t *clock;
22 struct tm *gmtime(clock)
24 time_t *clock;
25 char *timezone(zone, dst)
27
29 Tzset uses the value of the environment variable TZ to set up the time
30 conversion information used by localtime.
31 If TZ does not appear in the environment, the TZDEFAULT file (as
33 defined in tzfile.h) is used by localtime. If this file fails for any
34 reason, the GMT offset as provided by the kernel is used. In this
35 case, DST is ignored, resulting in the time being incorrect by some
36 amount if DST is currently in effect. If this fails for any reason,
37 GMT is used.
38 If TZ appears in the environment but its value is a null string, Green‐
40 wich Mean Time is used; if TZ appears and begins with a slash, it is
41 used as the absolute pathname of the tzfile(5)-format file from which
42 to read the time conversion information; if TZ appears and begins with
43 a character other than a slash, it's used as a pathname relative to the
44 system time conversion information directory, defined as TZDIR in the
45 include file tzfile.h. If this file fails for any reason, GMT is used.
46 Programs that always wish to use local wall clock time should explic‐
48 itly remove the environmental variable TZ with unsetenv(3).
49 Ctime converts a long integer, pointed to by clock, such as returned by
51 time(2) into ASCII and returns a pointer to a 26-character string in
52 the following form. All the fields have constant width.
53 Sun Sep 16 01:03:52 1973\n
55 Localtime and gmtime return pointers to structures containing the bro‐
57 ken-down time. Localtime corrects for the time zone and possible day‐
58 light savings time; gmtime converts directly to GMT, which is the time
59 UNIX uses. Asctime converts a broken-down time to ASCII and returns a
60 pointer to a 26-character string.
61 The structure declaration from the include file is:
63 struct tm {
65 int tm_sec; /* 0-59 seconds */
66 int tm_min; /* 0-59 minutes */
67 int tm_hour; /* 0-23 hour */
68 int tm_mday; /* 1-31 day of month */
69 int tm_mon; /* 0-11 month */
70 int tm_year; /* 0- year - 1900 */
71 int tm_wday; /* 0-6 day of week (Sunday = 0) */
72 int tm_yday; /* 0-365 day of year */
73 int tm_isdst; /* flag: daylight savings time in effect */
74 char **tm_zone; /* abbreviation of timezone name */
75 long tm_gmtoff; /* offset from GMT in seconds */
76 };
77 Tm_isdst is non-zero if a time zone adjustment such as Daylight Savings
79 time is in effect.
80 Tm_gmtoff is the offset (in seconds) of the time represented from GMT,
82 with positive values indicating East of Greenwich.
83 Timezone remains for compatibility reasons only; it's impossible to
85 reliably map timezone's arguments (zone, a "minutes west of GMT" value
86 and dst, a "daylight saving time in effect" flag) to a time zone abbre‐
87 viation.
88 If the environmental string TZNAME exists, timezone returns its value,
90 unless it consists of two comma separated strings, in which case the
91 second string is returned if dst is non-zero, else the first string.
92 If TZNAME doesn't exist, zone is checked for equality with a built-in
93 table of values, in which case timezone returns the time zone or day‐
94 light time zone abbreviation associated with that value. If the
95 requested zone does not appear in the table, the difference from GMT is
96 returned; e.g. in Afghanistan, timezone(-(60*4+30), 0) is appropriate
97 because it is 4:30 ahead of GMT, and the string GMT+4:30 is returned.
98 Programs that in the past used the timezone function should return the
99 zone name as set by localtime to assure correctness.
100
102 /usr/share/zoneinfotime zone information directory
103 /etc/localtime local time zone file
104
106 gettimeofday(2), getenv(3), time(3), tzfile(5), environ(7)
107
109 The return values point to static data whose content is overwritten by
110 each call. The tm_zone field of a returned struct tm points to a
111 static array of characters, which will also be overwritten at the next
112 call (and by calls to tzset).
1134th Berkeley Distribution November 27, 1996 CTIME(3)