.\" Copyright (c) 1989, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" This code is derived from software contributed to Berkeley by .\" Arthur Olson. .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" From: @(#)ctime.3 8.1 (Berkeley) 6/4/93 .\" $FreeBSD: src/lib/libc/stdtime/ctime.3,v 1.24 2007/01/09 00:28:11 imp Exp $ .\" .Dd January 2, 1999 .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 .Nd transform binary date and time values .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In time.h .Vt extern char *tzname ; .Ft char * .Fn asctime "const struct tm *timeptr" .Ft char * .Fn asctime_r "const struct tm *restrict timeptr" "char *restrict buf" .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 struct tm * .Fn gmtime "const time_t *clock" .Ft struct tm * .Fn gmtime_r "const time_t *clock" "struct tm *result" .Ft struct tm * .Fn localtime "const time_t *clock" .Ft struct tm * .Fn localtime_r "const time_t *clock" "struct tm *result" .Ft time_t .Fn mktime "struct tm *timeptr" .Ft time_t .Fn timegm "struct tm *timeptr" .Sh DESCRIPTION The functions .Fn ctime , .Fn gmtime , and .Fn localtime all take as an argument a time value representing the time in seconds since the Epoch (00:00:00 .Tn UTC , January 1, 1970; see .Xr time 3 ) . .Pp The function .Fn localtime converts the time value pointed at by .Fa clock . It returns a pointer to a .Dq Fa struct tm (described below), which contains the broken-out time information for the value after adjusting for the current time zone (and any other factors such as Daylight Saving Time). Time zone adjustments are performed as specified by the .Ev TZ environment variable (see .Xr tzset 3 ) . The function .Fn localtime uses .Xr tzset 3 to initialize time conversion information, if .Xr tzset 3 has not already been called by the process. .Pp After filling in the tm structure, .Fn localtime sets the .Fa tm_isdst Ns 'th element of .Fa tzname to a pointer to an .Tn ASCII string containing the time zone abbreviation to be used with .Fn localtime Ns 's return value. .Pp The function .Fn gmtime also converts the time value, but makes no time zone adjustment. It returns a pointer to a tm structure (described below). .Pp The .Fn ctime function adjusts the time value for the current time zone, in the same manner as .Fn localtime . It returns a pointer to a 26-character string of the form: .Bd -literal -offset indent Thu Nov 24 18:22:48 1986\en\e0 .Ed .Pp All of the fields have constant width. .Pp The .Fn ctime_r function provides the same functionality as .Fn ctime , except that the caller must provide the output buffer .Fa buf (which must be at least 26 characters long) to store the result. The .Fn localtime_r and .Fn gmtime_r functions provide the same functionality as .Fn localtime and .Fn gmtime , respectively, except the caller must provide the output buffer .Fa result . .Pp The .Fn asctime function converts the broken-out time in the structure .Fa tm (pointed at by .Fa *timeptr ) to the form shown in the example above. .Pp The .Fn asctime_r function provides the same functionality as .Fn asctime , except that the caller provides the output buffer .Fa buf (which must be at least 26 characters long) to store the result. .Pp The functions .Fn mktime and .Fn timegm convert the broken-out time (in the structure pointed to by .Fa *timeptr ) into a time value with the same encoding as that of the values returned by the .Xr time 3 function (that is, seconds from the Epoch, .Tn UTC ) . The .Fn mktime function interprets the input structure according to the current timezone setting (see .Xr tzset 3 ) . The .Fn timegm function interprets the input structure as representing Universal Coordinated Time .Pq Tn UTC . .Pp The original values of the .Fa tm_wday and .Fa tm_yday components of the structure are ignored. The original values of the other components are not restricted to their normal ranges and will be normalized, if need be. For example, October 40 is changed into November 9, a .Fa tm_hour of \-1 means 1 hour before midnight, .Fa tm_mday of 0 means the day preceding the current month, and .Fa tm_mon of \-2 means 2 months before January of .Fa tm_year . (A positive or zero value for .Fa tm_isdst causes .Fn mktime to presume initially that summer time (for example, Daylight Saving Time) is or is not (respectively) 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. The .Fa tm_isdst and .Fa tm_gmtoff members are forced to zero by .Fn timegm . ) .Pp 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. The .Fn mktime function returns the specified calendar time; if the calendar time cannot be represented, it returns \-1; .Pp The .Fn difftime function returns the difference between two calendar times, .Pf ( Fa time1 - .Fa time0 ) , expressed in seconds. .Pp External declarations, as well as the tm structure definition, are contained in the .In time.h include file. The tm structure includes at least 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? \(**/ char \(**tm_zone; /\(** abbreviation of timezone name \(**/ long tm_gmtoff; /\(** offset from UTC in seconds \(**/ .Ed .Pp The field .Fa tm_isdst is non-zero if summer (i.e., Daylight Saving) time is in effect. .Pp The field .Fa tm_gmtoff is the offset (in seconds) of the time represented from .Tn UTC , with positive values indicating locations east of the Prime Meridian. .Sh SEE ALSO .Xr date 1 , .Xr gettimeofday 2 , .Xr getenv 3 , .Xr time 3 , .Xr tzset 3 , .Xr tzfile 5 .Sh STANDARDS The .Fn asctime , .Fn ctime , .Fn difftime , .Fn gmtime , .Fn localtime , and .Fn mktime functions conform to .St -isoC , and conform to .St -p1003.1-96 provided the selected local timezone does not contain a leap-second table (see .Xr zic 8 ) . .Pp The .Fn asctime_r , .Fn ctime_r , .Fn gmtime_r , and .Fn localtime_r functions are expected to conform to .St -p1003.1-96 (again provided the selected local timezone does not contain a leap-second table). .Pp The .Fn timegm function is not specified by any standard; its function cannot be completely emulated using the standard functions described above. .Sh HISTORY This manual page is derived from the time package contributed to Berkeley by .An Arthur Olson and which appeared in .Bx 4.3 . .Sh BUGS Except for .Fn difftime , .Fn mktime , and the .Fn \&_r variants of the other functions, these functions leaves their result in an internal static object and return a pointer to that object. Subsequent calls to these function will modify the same object. .Pp The C Standard provides no mechanism for a program to modify its current local timezone setting, and the .Tn POSIX Ns No \&-standard method is not reentrant. (However, thread-safe implementations are provided in the .Tn POSIX threaded environment.) .Pp The .Va tm_zone field of a returned .Vt tm structure points to a static array of characters, which will also be overwritten by any subsequent calls (as well as by subsequent calls to .Xr tzset 3 and .Xr tzsetwall 3 ) . .Pp Use of the external variable .Fa tzname is discouraged; the .Fa tm_zone entry in the tm structure is preferred.