ctime(3) ctime(3)NAME asctime, ctime, difftime, gmtime, localtime, mktime, tzset, tzsetwall - convert date and time to ASCII SYNOPSIS extern char *tzname[2]; void tzset() void tzsetwall() #include <sys/types.h> char *ctime(clock) time_t *clock; double difftime(time1,time0) time_t time1; time_t time0; #include <time.h> char *asctime(tm) struct tm *tm; struct tm *localtime(clock) long *clock; struct tm *gmtime(clock) long *clock; time_t mktime(tm) struct tm *tm; extern long timezone; extern int daylight; DESCRIPTION tzset uses the value of the environment variable TZ to set time conversion information used by localtime. If TZ does not appear in the environment, the best available approximation to local wall-clock time, as specified by the file localtime in the system time-conversion information directory, is used by the localtime function. If TZ appears in the environment but its value is a null string, Coordinated Universal Time (UTC) is used (without leap- second correction). If TZ appears in the environment and its value is not a null string, TZ is used in one of the following ways: If the value begins with a colon (:), it is used as a pathname of a file from which to read the time- January 1992 1
ctime(3) ctime(3)conversion information. If the value does not begin with a colon, it is first used as the pathname of a file from which to read the time conversion information, and if that file cannot be read, it is used directly as a specification of the time-conversion information. If it begins with a slash (/), it is used as an absolute pathname when TZ is used as a pathname; otherwise, it is used as a pathname relative to a system time-conversion information directory. The file must be in the format specified in tzfile(5). When TZ is used directly as a specification of the time- conversion information, it must have the following syntax (spaces are inserted for clarity): std offset[dst[offset][,rule]] The placeholders mean the following: std and dst Specify three or more bytes that are the designation for the standard (std) or summer (dst) time zone. Only std is required. If dst is missing, then summer time does not apply in this locale. Uppercase and lowercase letters are explicitly allowed. Any characters except a leading colon (:), digits, a comma (,), a minus sign (-), a plus sign (+), and ASCII NUL are allowed. offset Add the value one of offset to the local time to arrive at UTC. The format of offset is: hh[:mm[:ss]] The minutes (mm) and seconds (ss) are optional. The hour (hh) is required and may be a single digit. The value of offset following std is required. If no offset follows dst, summer time is assumed to be one hour ahead of standard time. One or more digits may be used; the value is always interpreted as a decimal number. The hour must be between 0 and 24, and the minutes and seconds, if present, between 0 and 59. If preceded by a - (minus sign), the time zone is east of the prime meridian; otherwise, it is west, which may be indicated by an optional preceding + (plus sign). 2 January 1992
ctime(3) ctime(3)rule Specify when to change to and back from summer time. The format of rule is: date/time,date/time where the first date describes when the change from standard to summer time occurs and the second date describes when the change back happens. Each time field describes when, in current local time, the change to the other time is made. The format of date is one of the following: Jn The Julian day n (1 < n < 365). Leap days are not counted so that in all years, including leap years, February 28 is day 59, and March 1 is day 60. It is impossible to explicitly refer to the occasional February 29. n The zero-based Julian day (0 < n < 365). Leap days are counted, and it is possible to refer to February 29. Mm.n.d The d'th day (0 < d < 6) of week n of month m of the year (1 < n < 5, 1 < m < 12, where week 5 means ``the last d day in month m'' which may occur in either the fourth or the fifth week). Week 1 is the first week in which the d'th day occurs. Day 0 is Sunday. The time has the same format as offset except that no leading - or + (minus or plus sign) is allowed. The default, if time is not given, is 02:00:00. If no rule is present in TZ, the rules specified by the tzfile(5)-format file posixrules in the system time- conversion information directory are used, with the standard and summer time offsets from UTC replaced by those specified by the values of offset in TZ. For compatibility with System V Release 3.1, a semicolon (;) may be used to separate rule from the rest of the specification. For compatibility with applications that expect the environment variable TZ to be in the format of System V Release 2, the value of TZ should not include rule. If the TZ environment variable does not specify a tzfile(5)-format and cannot be interpreted as a direct specification, UTC is used with the standard time abbreviation set to the value of the TZ environment variable January 1992 3
ctime(3) ctime(3)(or to the leading characters of its value if it is lengthy). In most installations TZ is set by default, when the user logs on, to the value in the file /etc/TIMEZONE. tzsetwall arranges for the system's best approximation to local wall-clock time to be delivered by subsequent calls to localtime. ctime converts a long integer, pointed to by clock, representing the time in seconds since 00:00:00 UTC, January 1, 1970, and returns a pointer to a 26-character string of the form Thu Nov 24 18:22:48 1986 All the fields have a constant width. localtime and gmtime return pointers to tm structures, described below. localtime corrects for the time zone and any time-zone adjustments, such as daylight-saving time (DST) in the United States. Before doing so, localtime calls tzset, if tzset has not been called in the current process. After filling in the tm structure, localtime sets the tm_isdst'th element of tzname to a pointer to an ASCII string that's the time-zone abbreviation to be used with the return value of localtime. gmtime converts to UTC. asctime converts the time value tm to a 26-character string, as shown in the above example, and returns a pointer to the string. mktime converts the broken-down time, expressed as local time, in the structure pointed to by tm into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to their normal ranges. (A positive or zero value for tm_isdst causes mktime to presume initially that summer time (for example, daylight-saving time in the United States) respectively, is or is not in effect for the specified time. A negative value for tm_isdst causes the mktime function to attempt to divine whether summer time is in effect for the specified time.) On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to their normal ranges. The final value of tm_mday is not set 4 January 1992
ctime(3) ctime(3)until tm_mon and tm_year are determined. Mktime returns the specified calendar time; if the calendar time cannot be represented, it returns -1. difftime returns the difference between two calendar times, time1 - time0, expressed in seconds. Declarations of all the functions and externals, and the tm structure, are in the <time.h> header file. The structure (of type) struct tm includes the following fields: int tm_sec; /* seconds (0 - 60) */ int tm_min; /* minutes (0 - 59) */ int tm_hour; /* hours (0 - 23) */ int tm_mday; /* day of month (1 - 31) */ int tm_mon; /* month of year (0 - 11) */ int tm_year; /* year - 1900 */ int tm_wday; /* day of week (Sunday = 0) */ int tm_yday; /* day of year (0 - 365) */ int tm_isdst; /* is DST (``summer'' time) in effect? */ Tm_isdst is nonzero if a timezone adjustment such as daylight-savings time or summer time is in effect. External Variables Two external variables can provide additional timezone- related values. They are timezone and daylight. However, neither one contains a valid value until after a call has been made to localtime(), tzset(), or tzsetwall(). When valid, the external variable timezone contains the difference, in seconds, between UTC and local standard time (in Pacific Standard Time, timezone is 5*60*60). When valid, the external variable daylight is nonzero if a timezone adjustment such as DST or summer time is in effect. NOTES The return values point to static data whose content is overwritten by each call. FILES /etc/zoneinfo Time-zone information directory /etc/zoneinfo/localtime Local time zone file /etc/zoneinfo/posixrules File used with POSIX-style TZ's /etc/zoneinfo/GMT File for UTC leap seconds January 1992 5
ctime(3) ctime(3)NOTES If /etc/zoneinfo/GMT is absent, UTC leap seconds are loaded from /etc/zoneinfo/posixrules. SEE ALSO time(2), getenv(3), tzfile(4), environ(5) 6 January 1992