CTIME(3) BSD 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 the form
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
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 tzalloc().
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 initializa-
tion 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 to tzalloc().
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 val-
ues returned by the time(3) function. The following remarks should be taken into
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 specified time.
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 same argument.
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 determined.
The function returns the specified calendar time; if the calendar time cannot be rep-
resented, it returns (time_t)-1. This can happen either because the resulting conver-
sion 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() implementa-
tions 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 default.
The mktime_z() function is similar to mktime() but it also takes a const timezone_t
argument, returned by a previous call to tzalloc().
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() func-
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'' vari-
ants of the functions.
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
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, set-
ting 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
[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
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 con-
verted 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
Avoid using out-of-range values with mktime() when setting up lunch with promptness stick-
lers in Riyadh.
BSD November 2, 2011 BSD