CTIME(3) Library Functions Manual CTIME(3)
asctime, asctime_r, ctime, ctime_r, ctime_rz, difftime, gmtime, gmtime_r,
localtime, localtime_r, localtime_rz, mktime, mktime_z, tzalloc,
tzgetname, tzfree, -- convert date and time to ASCII
Standard C Library (libc, -lc)
extern char *tzname;
asctime(const struct tm *tm);
asctime_r(const struct tm restrict tm, char * restrict buf);
ctime(const time_t *clock);
ctime_r(const time_t *clock, char *buf);
ctime_rz(const timezone_t tz, const time_t *clock, char *buf);
difftime(time_t time1, time_t time0);
struct tm *
gmtime(const time_t *clock);
struct tm *
gmtime_r(const time_t * restrict clock, struct tm * restrict result);
struct tm *
localtime(const time_t *clock);
struct tm *
localtime_r(const time_t * restrict clock, struct tm * restrict result);
struct tm *
localtime_rz(const timezone_t tz, const time_t * restrict clock, struct
tm * restrict result);
mktime(struct tm *tm);
mktime_z(const timezone_t tz, struct tm *tm);
tzalloc(const char *zone);
tzfree(const timezone_t tz);
const char *
tzgetname(const timezone_t tz, int isdst);
The asctime family of functions provide various standard library routines
to operate with time and conversions related to time.
The asctime() function converts a time value contained in the tm
structure to a string with the following general format:
Thu Nov 24 18:22:48 1986\n\0
The tm structure is described in tm(3).
The asctime_r() has the same behavior as asctime(), but the result
is stored to buf, which should have a size of at least 26 bytes.
The ctime() function converts a time_t, pointed to by clock,
representing the time in seconds since 00:00:00 UTC, 1970-01-01,
and returns a pointer to a string with the format described above.
Years requiring fewer than four characters are padded with leading
zeroes. For years longer than four characters, the string is of
Thu Nov 24 18:22:48 81986\n\0
with five spaces before the year. These unusual formats are
designed to make it less likely that older software that expects
exactly 26 bytes of output will mistakenly output misleading values
for out-of-range years.
The ctime_r() is similar to ctime(), except it places the result of
the conversion on the buf argument, which should be 26 or more
bytes long, instead of using a global static buffer.
ctime_rz(tz, clock, buf)
The ctime_rz() function is similar to ctime_r(), but it also takes
a const timezone_t argument, as returned by a previous call to
The difftime() function returns the difference between two calendar
times, (time1 - time0), expressed in seconds.
The gmtime() function converts to Coordinated Universal Time (UTC)
and returns a pointer to the tm structure described in tm(3).
The gmtime_r() provides the same functionality as gmtime(),
differing in that the caller must supply a buffer area result to
which the result is stored.
Also localtime() is comparable to gmtime(). However, localtime()
corrects for the time zone and any time zone adjustments (such as
Daylight Saving Time in the U.S.A.). After filling in the tm
structure, the function sets the tm_isdst'th element of tzname to a
pointer to an ASCII string that is the time zone abbreviation to be
used with localtime()'s return value.
As gmtime_r(), the localtime_r() takes an additional buffer result
as a parameter and stores the result to it. Note however that
localtime_r() does not imply initialization of the local time
conversion information; the application may need to do so by
localtime_rz(tz, clock, result)
The localtime_rz() function is similar to localtime_r(), but it
also takes a const timezone_t argument, returned by a previous call
The mktime() function converts the broken-down time, expressed as
local time in the tm(3) structure, into a calendar time value with
the same encoding as that of the values returned by the time(3)
function. The following remarks should be taken into account.
o 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
in the U.S.A.) respectively, is or is not in effect for the
o A negative value for tm_isdst causes the mktime() function to
attempt to divine whether summer time is in effect for the
specified time; in this case it does not use a consistent rule
and may give a different answer when later presented with the
o 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
The function returns the specified calendar time; if the calendar
time cannot be represented, it returns (time_t)-1. This can happen
either because the resulting conversion would not fit in a time_t
variable, or because the time specified happens to be in the
daylight savings gap and tm_isdst was set to -1. Other mktime()
implementations do not return an error in the second case and
return the appropriate time offset after the daylight savings gap.
There is code to mimick this behavior, but it is not enabled by
The mktime_z() function is similar to mktime() but it also takes a
const timezone_t argument, returned by a previous call to
The tzalloc() function takes as an argument a timezone name and
returns a timezone_t object suitable to be used in the ctime_rz(),
localtime_rz(), and mktime_z() functions.
Note that instead of setting the environment variable TZ, and
globally changing the behavior of the calling program, one can use
multiple timezones at the same time by using separate timezone_t
objects allocated by tzalloc() and calling the ``z'' variants of
The tzfree() function deallocates tz, which was previously
allocated by tzalloc().
Finally, tzgetname() returns the name for the given tz. If isdst
is 0, the call is equivalent to tzname. If isdst is set to 1
the call is equivalent to tzname.
o On success the asctime() and ctime() functions return a pointer to a
static character buffer, and the asctime_r(), ctime_r(), and
ctime_rz() function return a pointer to the user-supplied buffer. On
failure they all return NULL and no errors are defined for them.
o On success the gmtime(), and localtime() functions return a pointer
to a statically allocated struct tm whereas the gmtime_r(),
localtime_r(), and localtime_rz(), functions return a pointer to the
user-supplied struct tm. On failure they all return NULL and the
global variable errno is set to indicate the error.
o The mktime() and mktime_z() function returns the specified time since
the Epoch as a time_t type value. If the time cannot be represented,
then mktime() and mktime_z() return (time_t)-1 setting the global
variable errno to indicate the error.
o The tzalloc() function returns a pointer to a timezone_t object or
NULL on failure, setting errno to indicate the error.
o tzgetzone() function returns string containing the name of the
timezone given in tz.
/etc/localtime local time zone file
/usr/share/zoneinfo time zone information directory
/usr/share/zoneinfo/posixrules used with POSIX-style TZ's
/usr/share/zoneinfo/GMT for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
The described functions may fail with
[EINVAL] The result cannot be represented because a parameter
is incorrect, or the conversion failed because no such
time exists (for example a time in the DST gap).
[EOVERFLOW] The result cannot be represented because the time
requested is out of bounds and the time calculation
resulted in overflow.
All functions that return values, except their ``z'' variants, can also
return the same errors as open(2) and malloc(3).
getenv(3), strftime(3), time(3), tm(3), tzset(3), tzfile(5)
The ctime(), difftime(), asctime(), localtime(), gmtime() and mktime()
functions conform to ANSI X3.159-1989 (``ANSI C89''). Rest of the
functions conform to IEEE Std 1003.1-2008 (``POSIX.1'').
The functions that do not take an explicit timezone_t argument return
values point to static data; the data is overwritten by each call. For
the above functions the tm_zone field of a returned struct tm points to a
static array of characters, which will also be overwritten at the next
call (and by calls to tzset(3)). The functions that do take an explicit
timezone_t argument and set the fields of a supplied struct tm should not
call tzfree() since the tm_zone field of the struct tm points to data
allocated by tzalloc().
The asctime() and ctime() functions behave strangely for years before
1000 or after 9999. The 1989 and 1999 editions of the C Standard say
that years from -99 through 999 are converted without extra spaces, but
this conflicts with longstanding tradition and with this implementation.
Traditional implementations of these two functions are restricted to
years in the range 1900 through 2099. To avoid this portability mess,
new programs should use strftime() instead.
Avoid using out-of-range values with mktime() when setting up lunch with
promptness sticklers in Riyadh.
NetBSD 6.1.5 November 2, 2011 NetBSD 6.1.5