CTIME(3) BSD Library Functions Manual CTIME(3)
NAME
asctime, asctimer, ctime, ctimer, difftime, gmtime, gmtimer,
localtime, localtimer, mktime, timegm -- transform binary date and time
values
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
##include <>
extern char *tzname[2];
char *
ctime(const timet *clock);
double
difftime(timet time1, timet time0);
char *
asctime(const struct tm *tm);
struct tm *
localtime(const timet *clock);
struct tm *
gmtime(const timet *clock);
timet
mktime(struct tm *tm);
timet
timegm(struct tm *tm);
char *
ctimer(const timet *clock, char *buf);
struct tm *
localtimer(const timet *clock, struct tm *result);
struct tm *
gmtimer(const timet *clock, struct tm *result);
char *
asctimer(const struct tm *tm, char *buf);
DESCRIPTION
The functions ctime(), gmtime() and localtime() all take as an argument a
time value representing the time in seconds since the Epoch (00:00:00
UTC, January 1, 1970; see time(3)).
The function localtime() converts the time value pointed at by clock, and
returns a pointer to a ``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 vari-
able (see tzset(3)). The function localtime() uses tzset(3) to initial-
ize time conversion information if tzset(3) has not already been called
by the process.
After filling in the tm structure, localtime() sets the tmisdst'th ele-
ment of tzname to a pointer to an ASCI string that's the time zone
abbreviation to be used with localtime()'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 ctimer() function provides the same functionality as ctime() except
the caller must provide the output buffer buf to store the result, which
must be at least 26 characters long. The localtimer() and gmtimer()
functions provide the same functionality as localtime() and gmtime()
respectively, except the caller must provide the output buffer result.
The asctime() function converts the broken down time in the structure tm
pointed at by *tm to the form shown in the example above.
The asctimer() function provides the same functionality as asctime()
except the caller provide the output buffer 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 struc-
ture according to the current timezone setting (see tzset(3)). The
timegm() function interprets the input structure as representing Univer-
sal Coordinated Time (UTC).
The original values of the tmwday and tmyday components of the struc-
ture 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 tmhour of -1 means 1
hour before midnight, tmmday of 0 means the day preceding the current
month, and tmmon of -2 means 2 months before January of tmyear. (A
positive or zero value for tmisdst 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
tmisdst causes the mktime() function to attempt to divine whether summer
time is in effect for the specified time. The tmisdst and tmgmtoff
members are forced to zero by timegm().)
On successful completion, the values of the tmwday and tmyday compo-
nents 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 tmmday is not set
until tmmon and tmyear 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, (time1 - time0), expressed in seconds.
External declarations as well as the tm structure definition are in the
include file. The tm structure includes at least the following
fields:
int tmsec; /* seconds (0 - 60) */
int tmmin; /* minutes (0 - 59) */
int tmhour; /* hours (0 - 23) */
int tmmday; /* day of month (1 - 31) */
int tmmon; /* month of year (0 - 11) */
int tmyear; /* year - 1900 */
int tmwday; /* day of week (Sunday = 0) */
int tmyday; /* day of year (0 - 365) */
int tmisdst; /* is summer time in effect? */
char *tmzone; /* abbreviation of timezone name */
long tmgmtoff; /* offset from UTC in seconds */
The field tmisdst is non-zero if summer time is in effect.
The field tmgmtoff is the offset (in seconds) of the time represented
from UTC, with positive values indicating east of the Prime Meridian.
SEE ALSO
date(1), gettimeofday(2), getenv(3), time(3), tzset(3), tzfile(5)
STANDARDS
The asctime(), ctime(), difftime(), gmtime(), localtime(), and mktime()
functions conform to ISO/IEC 9899:1990 (``ISO C90''), and conform to
ISO/IEC 9945-1:1996 (``POSIX.1'') provided the selected local timezone
does not contain a leap-second table (see zic(8)).
The asctimer(), ctimer(), gmtimer(), and localtimer() functions are
expected to conform to ISO/IEC 9945-1:1996 (``POSIX.1'') (again provided
the selected local timezone does not contain a leap-second table).
The timegm() function is not specified by any standard; its function can-
not be completely emulated using the standard functions described above.
HISTORY
This manual page is derived from the time package contributed to Berkeley
by Arthur Olson and which appeared in 4.3BSD.
BUGS
Except for difftime(), mktime(), and the r() variants of the other func-
tions, 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.
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 tmzone field of a returned 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 tzname is discouraged; the tmzone entry in
the tm structure is preferred.
BSD January 2, 1999 BSD
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