| CTIME(3) | Library Functions Manual | CTIME(3) |
asctime,
asctime_r, ctime,
ctime_r, difftime,
gmtime, gmtime_r,
localtime, localtime_r,
mktime, timegm,
timelocal — transform binary
date and time values
Standard C Library (libc, -lc)
#include
<time.h>
extern char *tzname[2];
char *
asctime(const
struct tm *timeptr);
char *
asctime_r(const
struct tm *restrict timeptr,
char *restrict buf);
char *
ctime(const
time_t *clock);
char *
ctime_r(const
time_t *clock, char
*buf);
double
difftime(time_t
time1, time_t
time0);
struct tm *
gmtime(const
time_t *clock);
struct tm *
gmtime_r(const
time_t *clock, struct tm
*result);
struct tm *
localtime(const
time_t *clock);
struct tm *
localtime_r(const
time_t *clock, struct tm
*result);
time_t
mktime(struct
tm *timeptr);
time_t
timegm(struct
tm *timeptr);
time_t
timelocal(struct
tm *timeptr);
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)).
When encountering an error, these functions return
NULL and set errno to an
appropriate value.
The function
localtime()
converts the time value pointed at by clock. It
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 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 tm_isdst'th element of
tzname to a pointer to an ASCII string containing the
time zone abbreviation to be used with localtime()'s
return value.
The function
gmtime()
also converts the time value, but makes no time zone adjustment. It 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(). It returns a pointer to a
26-character string of the form:
Thu Nov 24 18:22:48 1986\n\0
All of the fields have constant width.
The
ctime_r()
function provides the same functionality as ctime(),
except that the caller must provide the output buffer
buf (which must be at least 26 characters long) to
store the result. The
localtime_r()
and
gmtime_r()
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-out time in the structure
tm (pointed at by *timeptr) to
the form shown in the example above.
The
asctime_r()
function provides the same functionality as
asctime(), except that the caller provides the
output buffer buf (which must be at least 26
characters long) to store the result.
The functions
mktime() and
timegm()
convert the broken-out time (in the structure pointed to by
*timeptr) 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
timelocal()
function is like
timegm(),
but uses the current timezone setting. This makes
timelocal() equivalent to
mktime().
The original values of the
tm_wday and tm_yday components
of the structure are ignored. The original values of the other components
are not restricted to their normal ranges and will be normalized, if need
be. For example, October 40 is changed into November 9, a
tm_hour of -1 means 1 hour before midnight,
tm_mday of 0 means the day preceding the current
month, and tm_mon of -2 means 2 months before January
of tm_year. (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 (respectively) in effect for the specified time. A negative value
for tm_isdst causes the
mktime() function to attempt to divine whether
summer time is in effect for the specified time. The
tm_isdst and tm_gmtoff members
are forced to zero by timegm().)
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. 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
contained 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 (i.e., Daylight Saving) time is in effect.
The field tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating locations east of the Prime Meridian.
date(1), gettimeofday(2), getenv(3), time(3), tzset(3), tzfile(5)
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 asctime_r(),
ctime_r(), gmtime_r(), and
localtime_r() 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 cannot be completely emulated using the standard
functions described above.
This manual page is derived from the time package contributed to Berkeley by Arthur Olson and which appeared in 4.3BSD.
Except for difftime(),
mktime(), and the _r()
variants of the other functions, 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 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.
| January 2, 1999 | Mac OS X 12 |