changes from Arthur Olson -- if TZ wrong, just use UTC

[unix-history] / usr / src / lib / libc / gen / ctime.3

.\" Copyright (c) 1989 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 are permitted
.\" provided that the above copyright notice and this paragraph are
.\" duplicated in all such forms and that any documentation,
.\" advertising materials, and other materials related to such
.\" distribution and use acknowledge that the software was developed
.\" by the University of California, Berkeley.  The name of the
.\" University may not be used to endorse or promote products derived
.\" from this software without specific prior written permission.
.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
.\"
.\"	@(#)ctime.3	6.11 (Berkeley) %G%
.\"
.TH CTIME 3 ""
.SH NAME
asctime, ctime, difftime, gmtime, localtime, mktime, tzset, tzsetwall \- convert date and time to ASCII
.SH SYNOPSIS
.nf
.B extern char *tzname[2];
.PP
.B void tzset()
.PP
.B void tzsetwall()
.PP
.B #include <sys/types.h>
.PP
.B char *ctime(clock)
.B time_t *clock;
.PP
.B double difftime(time1, time0)
.B time_t time1;
.B time_t time0;
.PP
.B #include <time.h>
.PP
.B char *asctime(tm)
.B struct tm *tm;
.PP
.B struct tm *localtime(clock)
.B long *clock;
.PP
.B struct tm *gmtime(clock)
.B long *clock;
.PP
.B time_t mktime(tm)
.B struct tm *tm;
.fi
.SH DESCRIPTION
.I Tzset
uses the value of the environment variable
.B TZ
to set time conversion information used by
.IR localtime .
If
.B TZ
does not appear in the environment,
the best available approximation to local wall clock time, as specified
by the
.IR tzfile (5)-format
file
.B localtime
in the system time conversion information directory, is used by
.IR localtime .
If
.B TZ
appears in the environment but its value is a null string,
Coordinated Universal Time (UTC) is used (without leap second
correction).  If
.B TZ
appears in the environment and its value is not a null string:
.IP
if the value begins with a colon, it is used as a pathname of a file
from which to read the time conversion information;
.IP
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, is used directly as a specification of
the time conversion information.
.PP
When
.B TZ
is used as a pathname, if it begins with a slash,
it is used as an absolute 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
.IR tzfile (5).
.PP
When
.B TZ
is used directly as a specification of the time conversion information,
it must have the following syntax (spaces inserted for clarity):
.IP
\fIstd\|offset\fR[\fIdst\fR[\fIoffset\fR][\fB,\fIrule\fR]]
.PP
Where:
.RS
.TP 15
.IR std " and " dst
Three or more bytes that are the designation for the standard
.RI ( std )
or summer
.RI ( dst )
time zone.  Only
.I std
is required; if
.I dst
is missing, then summer time does not apply in this locale.
Upper- and lowercase letters are explicitly allowed.  Any characters
except a leading colon
.RB ( : ),
digits, comma
.RB ( , ),
minus
.RB ( \(mi ),
plus
.RB ( \(pl ),
and ASCII NUL are allowed.
.TP
.I offset
Indicates the value one must add to the local time to arrive at
Coordinated Universal Time.  The
.I offset
has the form:
.RS
.IP
\fIhh\fR[\fB:\fImm\fR[\fB:\fIss\fR]]
.RE
.IP
The minutes
.RI ( mm )
and seconds
.RI ( ss )
are optional.  The hour
.RI ( hh )
is required and may be a single digit.  The
.I offset
following
.I std
is required.  If no
.I offset
follows
.IR 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 zero and 24, and the minutes (and
seconds) \(em if present \(em between zero and 59.  If preceded by a
.RB `` \(mi '',
the time zone shall be east of the Prime Meridian; otherwise it shall be
west (which may be indicated by an optional preceding
.RB `` \(pl '').
.TP
.I rule
Indicates when to change to and back from summer time.  The
.I rule
has the form:
.RS
.IP
\fIdate\fB/\fItime\fB,\fIdate\fB/\fItime\fR
.RE
.IP
where the first
.I date
describes when the change from standard to summer time occurs and the
second
.I date
describes when the change back happens.  Each
.I time
field describes when, in current local time, the change to the other
time is made.
.IP
The format of
.I date
is one of the following:
.RS
.TP 10
.BI J n
The Julian day
.I n
.RI "(1\ \(<=" "\ n\ " "\(<=\ 365).
Leap days are not counted; that is, in all years \(em including leap
years \(em February 28 is day 59 and March 1 is day 60.  It is
impossible to explicitly refer to the occasional February 29.
.TP
.I n
The zero-based Julian day
.RI "(0\ \(<=" "\ n\ " "\(<=\ 365).
Leap days are counted, and it is possible to refer to February 29.
.TP
.BI M m . n . d
The
.IR d' th
day
.RI "(0\ \(<=" "\ d\ " "\(<=\ 6)
of week
.I n
of month
.I m
of the year
.RI "(1\ \(<=" "\ n\ " "\(<=\ 5,
.RI "1\ \(<=" "\ m\ " "\(<=\ 12,
where week 5 means ``the last
.I d
day in month
.IR m ''
which may occur in either the fourth or the fifth week).  Week 1 is the
first week in which the
.IR d' th
day occurs.  Day zero is Sunday.
.RE
.IP "" 15
The
.I time
has the same format as
.I offset
except that no leading sign
.RB (`` \(mi ''
or
.RB `` \(pl '')
is allowed.  The default, if
.I time
is not given, is
.BR 02:00:00 .
.RE
.LP
If no
.I rule
is present in
.BR TZ ,
the rules specified
by the
.IR tzfile (5)-format
file
.B 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
.I offset
values in
.BR TZ .
.PP
For compatibility with System V Release 3.1, a semicolon
.RB ( ; )
may be used to separate the
.I rule
from the rest of the specification.
.PP
If the
.B TZ
environment variable does not specify a
.IR tzfile (5)-format
and cannot be interpreted as a direct specification,
UTC is used.
.PP
.I Tzsetwall
sets things up so that
.I localtime
returns the best available approximation of local wall clock time.
.PP
.I Ctime\^
converts a long integer, pointed to by
.IR 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
.br
.ce
.eo
Thu Nov 24 18:22:48 1986\n\0
.ec
.br
All the fields have constant width.
.PP
.IR Localtime\^
and
.I gmtime\^
return pointers to ``tm'' structures, described below.
.I Localtime\^
corrects for the time zone and any time zone adjustments
(such as Daylight Saving Time in the U.S.A.).
Before doing so,
.I localtime\^
calls
.I tzset\^
(if
.I tzset\^
has not been called in the current process).
After filling in the ``tm'' structure,
.I localtime
sets the
.BR tm_isdst 'th
element of
.B tzname
to a pointer to an 
ASCII string that's the time zone abbreviation to be used with
.IR localtime 's
return value.
.PP
.I Gmtime\^
converts to Coordinated Universal Time.
.PP
.I Asctime\^
converts a time value contained in a
``tm'' structure to a 26-character string,
as shown in the above example,
and returns a pointer
to the string.
.PP
.I Mktime\^
converts the broken-down time,
expressed as local time,
in the structure pointed to by
.I tm
into a calendar time value with the same encoding as that of the values
returned by the
.I time
function.
The original values of the
.B tm_wday
and
.B 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
.B tm_isdst
causes
.I 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
.B tm_isdst
causes the
.I mktime
function to attempt to divine whether summer time is in effect
for the specified time.)
On successful completion, the values of the
.B tm_wday
and
.B 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
.B tm_mday
is not set until
.B tm_mon
and
.B tm_year
are determined.
.I Mktime\^
returns the specified calendar time;
If the calendar time cannot be represented,
it returns
.BR -1 .
.PP
.I Difftime\^
returns the difference between two calendar times,
.I time1
-
.IR time0,
expressed in seconds.
.PP
Declarations of all the functions and externals, and the ``tm'' structure,
are in the
.B <time.h>\^
header file.
The structure (of type)
.B struct tm
includes the following fields:
.RS
.PP
.nf
.ta .5i +\w'long tm_gmtoff;\0\0'u
	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 \(**/
.fi
.RE
.PP
The
.I tm_zone
and
.I 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
.I Tm_isdst\^
is non-zero if summer time is in effect.
.PP
.I Tm_gmtoff
is the offset (in seconds) of the time represented
from UTC, with positive values indicating east
of the Prime Meridian.
.SH FILES
.ta \w'/etc/zoneinfo/posixrules\0\0'u
/etc/zoneinfo	time zone information directory
.br
/etc/zoneinfo/localtime	local time zone file
.br
/etc/zoneinfo/posixrules	used in converting POSIX-style TZ's
.br
/etc/zoneinfo/GMT	for UTC leap seconds
.sp
If
.I /etc/zoneinfo/GMT
is absent,
UTC leap seconds are loaded from
.IR /etc/zoneinfo/posixrules .
.SH SEE ALSO
time(2), getenv(3), tzfile(5)
.SH NOTES
The return values point to static data
whose content is overwritten by each call.
The
.B tm_zone
field of a returned
.B "struct tm"
points to a static array of characters, which
will also be overwritten at the next call
(and by calls to
.I tzset
or
.IR tzsetwall ).
.PP
Avoid using out-of-range values with
.I mktime
when setting up lunch with promptness sticklers in Riyadh.