.\" $OpenBSD: ctime.3,v 1.34 2007/05/31 19:19:34 jmc Exp $ .\" .\" .Dd $Mdocdate: May 31 2007 $ .Dt CTIME 3 .Os .Sh NAME .Nm asctime , .Nm asctime_r , .Nm ctime , .Nm ctime_r , .Nm difftime , .Nm gmtime , .Nm gmtime_r , .Nm localtime , .Nm localtime_r , .Nm mktime , .Nm timegm , .Nm timelocal .Nd convert date and time to ASCII .Sh SYNOPSIS .Fd #include .Fd #include .Pp .Fd extern char *tzname[2]; .Ft "char *" .Fn ctime "const time_t *clock" .Ft "char *" .Fn ctime_r "const time_t *clock" "char *buf" .Ft double .Fn difftime "time_t time1" "time_t time0" .Ft "char *" .Fn asctime "const struct tm *tm" .Ft "char *" .Fn asctime_r "const struct tm *tm" "char *buf" .Ft "struct tm *" .Fn localtime "const time_t *clock" .Ft "struct tm *" .Fn localtime_r "const time_t *clock" "struct tm *result" .Ft "struct tm *" .Fn gmtime "const time_t *clock" .Ft "struct tm *" .Fn gmtime_r "const time_t *clock" "struct tm *result" .Ft time_t .Fn mktime "struct tm *tm" .Ft time_t .Fn timegm "struct tm *tm" .Ft time_t .Fn timelocal "struct tm *tm" .Sh DESCRIPTION The .Fn ctime function converts a .Li time_t , pointed to by .Fa clock , representing the time in seconds since 00:00:00 UTC, 1970-01-01, and returns a pointer to a string of the form .Pp .Dl Thu Nov 24 18:22:48 1986\en .Pp Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form .Pp .Dl Thu Nov 24 18:22:48\ \ \ \ \ 81986\en .Pp with five spaces before the year. These unusual formats are designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years. .Pp The .Fn ctime_r function converts the calendar time pointed to by .Fa clock to local time in exactly the same way as .Fn ctime and puts the string into the array pointed to by .Fa buf (which contains at least 26 bytes) and returns .Fa buf . Unlike .Fn ctime , the thread-safe version .Fn ctime_r is not required to set .Fa tzname . .Pp The .Fn localtime and .Fn gmtime functions return pointers to .Li tm structures, described below. .Fn localtime corrects for the time zone and any time zone adjustments (such as Daylight Saving Time in the United States). After filling in the .Li tm structure, .Fn localtime sets the .Fa tm_isdst Ns 'th element of .Fa tzname to a pointer to an ASCII string that's the time zone abbreviation to be used with the return value of .Fn localtime . .Pp .Fn gmtime converts to Coordinated Universal Time. .Pp The .Fn localtime_r and .Fn gmtime_r functions convert the calendar time pointed to by .Fa clock into a broken-down time in exactly the same way as their non-reentrant counterparts, .Fn localtime and .Fn gmtime , but instead store the result directly into the structure pointed to by .Fa result . Unlike .Fn localtime , the reentrant version is not required to set .Fa tzname . .Pp .Fn asctime converts a time value contained in a .Li tm structure to a string, as shown in the above example, and returns a pointer to the string. .Fn asctime_r uses the buffer pointed to by .Fa buf (which should contain at least 26 bytes) and then returns .Fa buf . .Pp .Fn mktime converts the broken-down time, expressed as local time, in the structure pointed to by .Fa tm into a calendar time value with the same encoding as that of the values returned by the .Fn time function. The original values of the .Fa tm_wday and .Fa 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 .Fa tm_isdst causes .Fn mktime to presume initially that summer time (for example, Daylight Saving Time in the U.S.A.)\& respectively, is or is not in effect for the specified time. A negative value for .Fa tm_isdst causes the .Fn mktime function to attempt to divine whether summer time is in effect for the specified time.) On successful completion, the values of the .Fa tm_wday and .Fa 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 .Fa tm_mday is not set until .Fa tm_mon and .Fa tm_year are determined. .Fn mktime returns the specified calendar time; if the calendar time cannot be represented, it returns \-1. .Pp .Fn timelocal is a deprecated interface that is equivalent to calling .Fn mktime with a negative value for .Fa tm_isdst . .Pp .Fn timegm is a deprecated interface that converts the broken-down time, as returned by .Fn gmtime , into a calendar time value with the same encoding as that of the values returned by the .Fn time function. .Pp .Fn difftime returns the difference between two calendar times, .Pf ( Fa time1 No \- Fa time0 ) , expressed in seconds. .Pp Declarations of all the functions and externals, and the .Li tm structure, are in the .Aq Pa time.h header file. The structure (of type) .Li struct tm includes the following fields: .Bd -literal -offset indent 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 summer time in effect? */ long tm_gmtoff; /* offset from UTC in seconds */ char *tm_zone; /* abbreviation of timezone name */ .Ed .Pp The .Fa tm_zone and .Fa tm_gmtoff fields exist, and are filled in, only if arrangements to do so were made when the library containing these functions was created. There is no guarantee that these fields will continue to exist in this form in future releases of this code. .Pp .Fa tm_isdst is non-zero if summer time is in effect. .Pp .Fa tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating east of the Prime Meridian. .Sh FILES .Bl -tag -width "/usr/share/zoneinfo/posixrules" -compact .It Pa /usr/share/zoneinfo time zone information directory .It Pa /etc/localtime local time zone file .It Pa /usr/share/zoneinfo/posixrules used with POSIX-style TZ's .It Pa /usr/share/zoneinfo/GMT for UTC leap seconds .El .Pp If .Pa /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from .Pa /usr/share/zoneinfo/posixrules . .Sh SEE ALSO .Xr getenv 3 , .Xr strftime 3 , .Xr time 3 , .Xr tzset 3 , .Xr tzfile 5 , .Xr zic 8 .Sh NOTES The return values of the non re-entrant functions point to static data; the data is overwritten by each call. The .Fa tm_zone field of a returned .Li struct tm points to a static array of characters, which will also be overwritten at the next call (and by calls to .Xr tzset 3 ) . .Pp .Fn asctime and .Fn ctime behave strangely for years before 1000 or after 9999. The 1989 and 1999 editions of the C Standard say that years from \-99 through 999 are converted without extra spaces, but this conflicts with longstanding tradition and with this implementation. Traditional implementations of these two functions are restricted to years in the range 1900 through 2099. To avoid this portability mess, new programs should use .Fn strftime instead. .Pp The default system time zone may be set by running .Dq Li zic -l timezone as the superuser. .Pp Avoid using out-of-range values with .Fn mktime when setting up lunch with promptness sticklers in Riyadh. .\" based on @(#)newctime.3 8.1 .\" This file is in the public domain, so clarified as of .\" 2003-02-14 by Arthur David Olson (arthur_david_olson@nih.gov).