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CTIME(3) NetBSD Library Functions Manual CTIME(3)
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by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
asctime, asctime_r, ctime, ctime_r, ctime_rz, difftime, gmtime, gmtime_r,
localtime, localtime_r, localtime_rz, mktime, mktime_z -- convert date
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(timezone_t restrict 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(timezone_t restrict tz, const time_t * restrict clock,
struct tm * restrict result);
mktime(struct tm *tm);
mktime_z(timezone_t restrict tz, struct tm *restrict tm);
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 in buf, which should have a size of at least 26 bytes.
The ctime() function converts a time_t, pointed to by clock, 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 clock time stamp represents the time in seconds since
1970-01-01 00:00:00 Coordinated Universal Time (UTC). The POSIX
standard says that time stamps must be nonnegative and must ignore
leap seconds. Many implementations extend POSIX by allowing nega-
tive time stamps, and can therefore represent time stamps that pre-
date the introduction of UTC and are some other flavor of Universal
Time (UT). Some implementations support leap seconds, in contra-
diction to POSIX.
The ctime_r() is similar to ctime(), except it places the result of
the conversion in 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 timezone_t argument, as returned by a previous call to tzalloc(),
or a NULL pointer denoting Coordinated Universal Time (UTC).
The difftime() function returns the difference between two calendar
times, (time1 - time0), expressed in seconds.
The ctime_r(), localtime_r(), gmtime_r(), and asctime_r() functions
are like their unsuffixed counterparts, except that they accept an
additional argument specifying where to store the result if suc-
The ctime_rz(), localtime_rz(), and mktime_z() functions are like
their unsuffixed counterparts, except that they accept an extra
initial zone argument specifying the timezone to be used for con-
version. If zone is NULL, UT is used; otherwise, zone should have
been allocated by tzalloc() and should not be freed until after all
uses (e.g., by calls to strftime()) of the filled-in tm_zone()
The gmtime() function converts to Coordinated Universal Time (UTC)
and returns a pointer to the tm structure described in tm(3).
The gmtime_r() function provides the same functionality as
gmtime(), differing in that the caller must supply a buffer area
result in which the result is stored.
Also localtime() is comparable to gmtime(). However, localtime()
corrects for the timezone and any timezone 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 timezone 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 in it. Note however that
localtime_r() does not imply initialization of the local time con-
version information; the application may need to do so by calling
localtime_rz(tz, clock, result)
The localtime_rz() function is similar to localtime_r(), but it
also takes a timezone_t argument, returned by a previous call to
tzalloc(), or a NULL pointer denoting Coordinated Universal Time
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.
· 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 and will
be normalized, if need be.
For example, consider a struct tm initialized with tm_year =
122, tm_mon = 10, tm_mday = 30, tm_hour = 22, tm_min = 57, and
tm_sec = 0. Incrementing tm_min by 13 and calling mktime()
would lead to tm_hour = 23 and tm_min = 10.
This normalizing can lead to cascading changes: Again using a
struct tm initialized as in the above example but with tm_hour
= 23, the same change would lead to tm_mon = 11, tm_mday = 1,
tm_hour = 0, and tm_min = 10.
Negative values may also be normalized with similar cascading
effect such that e.g., a tm_hour of -1 means 1 hour before mid-
night on the previous day and so on.
· A positive or zero value for tm_isdst causes mktime() to pre-
sume initially that daylight saving time respectively, is or is
not in effect for the specified time.
· A negative value for tm_isdst causes the mktime() function to
attempt to divine whether daylight saving 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.
· 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 day-
light 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
tzalloc(), or a null pointer denoting Coordinated Universal Time
Declarations of all the functions and externals, and the tm structure,
are in the <time.h> header file. The structure (of type) struct tm
includes 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 daylight saving time in effect? */
char *tm_zone; /* abbreviation of timezone name (optional) */
long tm_gmtoff; /* offset from UT in seconds (optional) */
· tm_isdst is non-zero if daylight saving time is in effect.
· tm_gmtoff is the offset (in seconds) of the time represented from UT,
with positive values indicating east of the Prime Meridian. The
field's name is derived from Greenwich Mean Time, a precursor of UT.
In struct tm the tm_zone and tm_gmtoff fields exist, and are filled in,
only if arrangements to do so were made when the library containing these
functions was created. Similarly, the tzname variable is optional;
also, there is no guarantee that tzname will continue to exist in this
form in future releases of this code.
· 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.
· 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.
· 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.
· The tzalloc() function returns a pointer to a timezone_t object or
NULL on failure, setting errno to indicate the error. It may also
return NULL when the name argument is NULL, and this is not an error,
but a way of referring to Coordinated Universal Time (UTC).
· tzgetzone() function returns string containing the name of the time-
zone given in tz.
/etc/localtime local timezone file
/usr/share/zoneinfo timezone information directory
/usr/share/zoneinfo/localtime local timezone file
/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 func-
tions conform to IEEE Std 1003.1-2008 (``POSIX.1'').
A ctime() function appeared in Version 1 AT&T UNIX.
The functions that do not take an explicit timezone_t argument return
values pointing to static data; the data is overwritten by each call.
For the above functions the tzname variable (once set) and the tm_zone
field of a returned struct tm point to an array of characters that can be
freed or overwritten by later calls to the functions localtime(),
tzfree(), and tzset(), if these functions affect the timezone information
that specifies the abbreviation in question. The remaining functions and
data are thread-safe. 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 allo-
cated by tzalloc().
The asctime(), asctime_r(), ctime(), ctime_r(), and ctime_rz(), 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 tradi-
tion and with this implementation. The 2011 edition says that the behav-
ior is undefined if the year is before 1000 or after 9999. 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.
NetBSD 10.99 October 22, 2022 NetBSD 10.99