NAME
asctime, ctime,
difftime, gmtime,
localtime, mktime —
transform binary date and time value to
ASCII
SYNOPSIS
#include
<sys/types.h>
#include <time.h>
extern char *tzname[2];
char *
ctime(const
time_t *clock);
double
difftime(time_t
time1, time_t
time0);
char *
asctime(const
struct tm *tm);
struct tm *
localtime(const
time_t *clock);
struct tm *
gmtime(const
time_t *clock);
time_t
mktime(struct
tm *tm);
DESCRIPTION
The functionsctime(),
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 environmental variable (see
tzset(3)). The function localtime() uses
tzset to initialize time
conversion information if
tzset has not already been
called by the process.
After filling in the tm structure,
localtime()
sets the tm_isdst'th element of
tzname to a pointer to an ASCII 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
asctime()
function converts the broken down time in the structure
tm pointed at by *tm to the form
shown in the example above.
The function
mktime()
converts the broken-down time, expressed as local 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 original values of the
tm_wday and 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 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 tm_isdst causes the
mktime() function to attempt to divine whether
summer time is in effect for the specified time.)
On successful completion, the values of the
tm_wday and 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 tm_mday is not set
until tm_mon and tm_year are
determined.
Mktime()
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 ⟨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 tm_isdst is non-zero if summer time is in effect.
The field tm_gmtoff 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)
HISTORY
This manual page is derived from the time package contributed to Berkeley by Arthur Olsen and which appeared in 4.3BSD.
BUGS
Except for difftime() and
mktime(), 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 tm_zone 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 tm_zone entry in the tm structure is preferred.
Avoid using out-of-range values with
mktime() when setting up lunch with promptness
sticklers in Riyadh.