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asctime asctime_r ctime ctime_r difftime gmtime gmtime_r localtime localtime_r mktime timegm - transform binary date and time values
Lb libc
The function
localtime ();
converts the time value pointed at by
Fa clock ,
and returns a pointer to a
``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
TZ
environment variable (see
tzset(3)).
The function
localtime ();
uses
tzset(3)
to initialize time conversion information if
tzset(3)
has not already been called by the process.
After filling in the tm structure,
localtime ();
sets the
Fa tm_isdst Ns 'th
element of
Fa tzname
to a pointer to an
ASCII
string that is the time zone abbreviation to be
used with
localtime (Ns 's);
return value.
The function
gmtime ();
similarly converts the time value, but without any time zone adjustment,
and returns a pointer to a tm structure (described below).
The
ctime ();
function
adjusts the time value for the current time zone in the same manner as
localtime (,);
and returns a pointer to a 26-character string of the form:
Thu Nov 24 18:22:48 1986\n\0
All the fields have constant width.
The
ctime_r ();
function
provides the same functionality as
ctime ();
except the caller must provide the output buffer
Fa buf
to store the result, which must be at least 26 characters long.
The
localtime_r ();
and
gmtime_r ();
functions
provide the same functionality as
localtime ();
and
gmtime ();
respectively, except the caller must provide the output buffer
Fa result .
The
asctime ();
function
converts the broken down time in the structure
Fa tm
pointed at by
Fa *tm
to the form
shown in the example above.
The
asctime_r ();
function
provides the same functionality as
asctime ();
except the caller provide the output buffer
Fa buf
to store the result, which must be at least 26 characters long.
The functions
mktime ();
and
timegm ();
convert the broken-down time in the structure
pointed to by tm into a time value with the same encoding as that of the
values returned by the
time(3)
function (that is, seconds from the Epoch,
UTC )
The
mktime ();
function
interprets the input structure according to the current timezone setting
(see
tzset(3)).
The
timegm ();
function
interprets the input structure as representing Universal Coordinated Time
(UTC
)
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, and will be
normalized if needed.
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
mktime ();
to presume initially that summer time (for example, Daylight Saving Time)
is or is not in effect for the specified time, respectively.
A negative value for
Fa tm_isdst
causes the
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
timegm (. ));
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
mktime ();
function
returns the specified calendar time; if the calendar time cannot be
represented, it returns -1;
The
difftime ();
function
returns the difference between two calendar times,
( Fa time1
-
Fa time0 ) ,
expressed in seconds.
External declarations as well as the tm structure definition are in the
#include <time.h>
include file.
The tm structure includes at least 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 summer time in effect? */ char *tm_zone; /* abbreviation of timezone name */ long tm_gmtoff; /* offset from UTC in seconds */
The field Fa tm_isdst is non-zero if summer time is in effect.
The field Fa tm_gmtoff is the offset (in seconds) of the time represented from UTC with positive values indicating east of the Prime Meridian.
The
asctime_r (,);
ctime_r (,);
gmtime_r (,);
and
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).
The
timegm ();
function is not specified by any standard; its function cannot be
completely emulated using the standard functions described above.
The C Standard provides no mechanism for a program to modify its current local timezone setting, and the POSIX -standard method is not reentrant. (However, thread-safe implementations are provided in the POSIX threaded environment.)
The 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 tzset(3) and tzsetwall(3)).
Use of the external variable Fa tzname is discouraged; the Fa tm_zone entry in the tm structure is preferred.
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