238 lines
5.6 KiB
Groff
238 lines
5.6 KiB
Groff
.TH NEWCTIME 3
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.SH NAME
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asctime, ctime, difftime, gmtime, localtime, mktime \- convert date and time to ASCII
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.SH SYNOPSIS
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.nf
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.B extern char *tzname[2];
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.PP
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.B void tzset()
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.PP
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.B #include <sys/types.h>
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.PP
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.B char *ctime(clock)
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.B const time_t *clock;
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.PP
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.B double difftime(time1, time0)
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.B time_t time1;
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.B time_t time0;
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.PP
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.B #include <time.h>
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.PP
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.B char *asctime(tm)
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.B const struct tm *tm;
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.PP
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.B struct tm *localtime(clock)
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.B const time_t *clock;
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.PP
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.B struct tm *gmtime(clock)
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.B const time_t *clock;
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.PP
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.B time_t mktime(tm)
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.B struct tm *tm;
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.PP
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.B cc ... -ltz
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.fi
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.SH DESCRIPTION
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.I Ctime\^
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converts a long integer, pointed to by
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.IR clock ,
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representing the time in seconds since
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00:00:00 UTC, 1970-01-01,
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and returns a pointer to a
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string of the form
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.br
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.ce
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.eo
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Thu Nov 24 18:22:48 1986\n\0
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.br
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.ec
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Years requiring fewer than four characters are padded with leading zeroes.
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For years longer than four characters, the string is of the form
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.br
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.ce
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.eo
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Thu Nov 24 18:22:48 81986\n\0
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.ec
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.br
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with five spaces before the year.
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These unusual formats are designed to make it less likely that older
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software that expects exactly 26 bytes of output will mistakenly output
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misleading values for out-of-range years.
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.PP
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.I Localtime\^
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and
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.I gmtime\^
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return pointers to ``tm'' structures, described below.
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.I Localtime\^
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corrects for the time zone and any time zone adjustments
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(such as Daylight Saving Time in the United States).
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After filling in the ``tm'' structure,
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.I localtime
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sets the
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.BR tm_isdst 'th
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element of
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.B tzname
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to a pointer to an
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ASCII string that's the time zone abbreviation to be used with
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.IR localtime 's
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return value.
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.PP
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.I Gmtime\^
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converts to Coordinated Universal Time.
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.PP
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.I Asctime\^
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converts a time value contained in a
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``tm'' structure to a string,
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as shown in the above example,
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and returns a pointer to the string.
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.PP
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.I Mktime\^
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converts the broken-down time,
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expressed as local time,
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in the structure pointed to by
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.I tm
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into a calendar time value with the same encoding as that of the values
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returned by the
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.I time
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function.
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The original values of the
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.B tm_wday
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and
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.B tm_yday
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components of the structure are ignored,
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and the original values of the other components are not restricted
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to their normal ranges.
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(A positive or zero value for
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.B tm_isdst
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causes
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.I mktime
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to presume initially that summer time (for example, Daylight Saving Time
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in the U.S.A.)
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respectively,
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is or is not in effect for the specified time.
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A negative value for
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.B tm_isdst
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causes the
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.I mktime
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function to attempt to divine whether summer time is in effect
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for the specified time.)
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On successful completion, the values of the
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.B tm_wday
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and
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.B tm_yday
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components of the structure are set appropriately,
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and the other components are set to represent the specified calendar time,
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but with their values forced to their normal ranges; the final value of
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.B tm_mday
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is not set until
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.B tm_mon
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and
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.B tm_year
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are determined.
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.I Mktime\^
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returns the specified calendar time;
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If the calendar time cannot be represented,
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it returns
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.BR -1 .
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.PP
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.I Difftime\^
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returns the difference between two calendar times,
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.RI ( time1
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-
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.IR time0 ),
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expressed in seconds.
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.PP
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Declarations of all the functions and externals, and the ``tm'' structure,
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are in the
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.B <time.h>\^
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header file.
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The structure (of type)
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.B struct tm
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includes the following fields:
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.RS
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.PP
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.nf
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.ta .5i +\w'long tm_gmtoff;\0\0'u
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int tm_sec; /\(** seconds (0 - 60) \(**/
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int tm_min; /\(** minutes (0 - 59) \(**/
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int tm_hour; /\(** hours (0 - 23) \(**/
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int tm_mday; /\(** day of month (1 - 31) \(**/
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int tm_mon; /\(** month of year (0 - 11) \(**/
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int tm_year; /\(** year \- 1900 \(**/
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int tm_wday; /\(** day of week (Sunday = 0) \(**/
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int tm_yday; /\(** day of year (0 - 365) \(**/
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int tm_isdst; /\(** is summer time in effect? \(**/
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char \(**tm_zone; /\(** abbreviation of timezone name \(**/
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long tm_gmtoff; /\(** offset from UTC in seconds \(**/
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.fi
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.RE
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.PP
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The
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.I tm_zone
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and
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.I tm_gmtoff
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fields exist, and are filled in, only if arrangements to do
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so were made when the library containing these functions was
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created.
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There is no guarantee that these fields will continue to exist
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in this form in future releases of this code.
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.PP
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.I Tm_isdst\^
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is non-zero if summer time is in effect.
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.PP
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.I Tm_gmtoff
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is the offset (in seconds) of the time represented
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from UTC, with positive values indicating east
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of the Prime Meridian.
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.SH FILES
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.ta \w'/usr/share/zoneinfo/posixrules\0\0'u
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/usr/share/zoneinfo time zone information directory
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.br
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/usr/share/zoneinfo/localtime local time zone file
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.br
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/usr/share/zoneinfo/posixrules used with POSIX-style TZ's
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.br
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/usr/share/zoneinfo/GMT for UTC leap seconds
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.sp
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If
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.B /usr/share/zoneinfo/GMT
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is absent,
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UTC leap seconds are loaded from
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.BR /usr/share/zoneinfo/posixrules .
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.SH SEE ALSO
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getenv(3),
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newstrftime(3),
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newtzset(3),
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time(2),
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tzfile(5)
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.SH NOTES
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The return values point to static data;
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the data is overwritten by each call.
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The
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.B tm_zone
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field of a returned
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.B "struct tm"
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points to a static array of characters, which
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will also be overwritten at the next call
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(and by calls to
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.IR tzset ).
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.PP
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.I Asctime\^
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and
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.I ctime\^
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behave strangely for years before 1000 or after 9999.
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The 1989 and 1999 editions of the C Standard say
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that years from \-99 through 999 are converted without
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extra spaces, but this conflicts with longstanding
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tradition and with this implementation.
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Traditional implementations of these two functions are
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restricted to years in the range 1900 through 2099.
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To avoid this portability mess, new programs should use
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.I strftime\^
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instead.
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.PP
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Avoid using out-of-range values with
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.I mktime
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when setting up lunch with promptness sticklers in Riyadh.
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.\" @(#)newctime.3 7.17
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