illumos: manual page: ctime.3c

CTIME(3C) Standard C Library Functions CTIME(3C)

NAME

ctime, ctime_r, localtime, localtime_r, gmtime, gmtime_r, asctime,
asctime_r, tzset - convert date and time to string

SYNOPSIS

#include <time.h>

char *ctime(const time_t *clock);

struct tm *localtime(const time_t *clock);

struct tm *gmtime(const time_t *clock);

char *asctime(const struct tm *tm);

extern time_t timezone, altzone;
extern int daylight;
extern char *tzname[2];

void tzset(void);

char *ctime_r(const time_t *clock, char *buf, int buflen);

struct tm *localtime_r(const time_t *restrict clock,
struct tm *restrict res);

struct tm *gmtime_r(const time_t *restrict clock,
struct tm *restrict res);

char *asctime_r(const struct tm *restrict tm, char *restrict buf,
int buflen);

Standard conforming

cc [ flag... ] file... -D_POSIX_PTHREAD_SEMANTICS [ library... ]

char *ctime_r(const time_t *clock, char *buf);

char *asctime_r(const struct tm *tm, char *buf);

DESCRIPTION

The ctime() function converts the time pointed to by clock, representing
the time in seconds since the Epoch (00:00:00 UTC, January 1, 1970), to
local time in the form of a 26-character string, as shown below. Time
zone and daylight savings corrections are made before string generation.
The fields are in constant width:

Fri Sep 13 00:00:00 1986\n\0

The ctime() function is equivalent to:

asctime(localtime(clock))

The ctime(), asctime(), gmtime(), and localtime() functions return values
in one of two thread-specific data objects: a broken-down time structure
and an array of char. Execution of any of the functions can overwrite the
information returned in either of these objects by any of the other
functions executed by the same thread.

The ctime_r() function has the same functionality as ctime() except that
the caller must supply a buffer buf with length buflen to store the
result; buf must be at least 26 bytes. The standard-conforming ctime_r()
function does not take a buflen parameter.

The localtime() and gmtime() functions return pointers to tm structures
(see below). The localtime() function corrects for the main time zone and
possible alternate ("daylight savings") time zone; the gmtime() function
converts directly to Coordinated Universal Time (UTC), which is what the
UNIX system uses internally.

The localtime_r() and gmtime_r() functions have the same functionality as
localtime() and gmtime() respectively, except that the caller must supply
a buffer res to store the result.

The asctime() function converts a tm structure to a 26-character string,
as shown in the previous example, and returns a pointer to the string.

The asctime_r() function has the same functionality as asctime() except
that the caller must supply a buffer buf with length buflen for the
result to be stored. The buf argument must be at least 26 bytes. The
standard-conforming asctime_r() function does not take a buflen
parameter. The asctime_r() function returns a pointer to buf upon
success. In case of failure, NULL is returned and errno is set.

Declarations of all the functions and externals, and the tm structure,
are in the <time.h> header. The members of the tm structure are:

int tm_sec; /* seconds after the minute -- [0, 60] */
/* for leap seconds */
int tm_min; /* minutes after the hour -- [0, 59] */
int tm_hour; /* hour since midnight -- [0, 23] */
int tm_mday; /* day of the month -- [1, 31] */
int tm_mon; /* months since January -- [0, 11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday -- [0, 6] */
int tm_yday; /* days since January 1 -- [0, 365] */
int tm_isdst; /* flag for alternate daylight savings time */

The value of tm_isdst is positive if daylight savings time is in effect,
zero if daylight savings time is not in effect, and negative if the
information is not available. Previously, the value of tm_isdst was
defined as non-zero if daylight savings was in effect.

The external time_t variable altzone contains the difference, in seconds,
between Coordinated Universal Time and the alternate time zone. The
external variable timezone contains the difference, in seconds, between
UTC and local standard time. The external variable daylight indicates
whether time should reflect daylight savings time. Both timezone and
altzone default to 0 (UTC). The external variable daylight is non-zero if
an alternate time zone exists. The time zone names are contained in the
external variable tzname, which by default is set to:

char *tzname[2] = { "GMT", "" };

These functions know about the peculiarities of this conversion for
various time periods for the U.S. (specifically, the years 1974, 1975,
and 1987). They start handling the new daylight savings time starting
with the first Sunday in April, 1987.

The tzset() function uses the contents of the environment variable TZ to
override the value of the different external variables. It is called by
asctime() and can also be called by the user. If TZ is not specified or
has an invalid setting, tzset() uses GMT0. See environ(7) for a
description of the TZ environment variable.

Starting and ending times are relative to the current local time zone. If
the alternate time zone start and end dates and the time are not
provided, the days for the United States that year will be used and the
time will be 2 AM. If the start and end dates are provided but the time
is not provided, the time will be 2 AM. The effects of tzset() change the
values of the external variables timezone, altzone, daylight, and tzname.

Note that in most installations, TZ is set to the correct value by
default when the user logs on, using the local /etc/default/init file
(see TIMEZONE(5)).

RETURN VALUES

Upon successful completion, the gmtime() and localtime() functions return
a pointer to a struct tm. If an error is detected, gmtime() and
localtime() return a null pointer.

Upon successful completion, the gmtime_r() and localtime_r() functions
return the address of the structure pointed to by the res argument. If an
error is detected, gmtime_r() and localtime_r() return a null pointer and
set errno to indicate the error.

ERRORS

The ctime_r() and asctime_r() functions will fail if:

ERANGE
The length of the buffer supplied by the caller is not large
enough to store the result.

The gmtime(), gmtime_r(), localtime(), and localtime_r() functions will
fail if:

EOVERFLOW
The result cannot be represented.

USAGE

These functions do not support localized date and time formats. The
strftime(3C) function can be used when localization is required.

The localtime(), localtime_r(), gmtime(), gmtime_r(), ctime(), and
ctime_r() functions assume Gregorian dates. Times before the adoption of
the Gregorian calendar will not match historical records.

EXAMPLES

Example 1: Examples of the tzset() function.

The tzset() function scans the contents of the environment variable and
assigns the different fields to the respective variable. For example, the
most complete setting for New Jersey in 1986 could be:

EST5EDT4,116/2:00:00,298/2:00:00

or simply

EST5EDT

An example of a southern hemisphere setting such as the Cook Islands
could be

KDT9:30KST10:00,63/5:00,302/20:00

In the longer version of the New Jersey example of TZ, tzname[0] is EST,
timezone is set to 5*60*60, tzname[1] is EDT, altzone is set to 4*60*60,
the starting date of the alternate time zone is the 117th day at 2 AM,
the ending date of the alternate time zone is the 299th day at 2 AM
(using zero-based Julian days), and daylight is set positive. Starting
and ending times are relative to the current local time zone. If the
alternate time zone start and end dates and the time are not provided,
the days for the United States that year will be used and the time will
be 2 AM. If the start and end dates are provided but the time is not
provided, the time will be 2 AM. The effects of tzset() are thus to
change the values of the external variables timezone, altzone, daylight,
and tzname. The ctime(), localtime(), mktime(), and strftime() functions
also update these external variables as if they had called tzset() at the
time specified by the time_t or struct tm value that they are converting.

BUGS

The zoneinfo timezone data files do not transition past Tue Jan 19
03:14:07 2038 UTC. Therefore for 64-bit applications using zoneinfo
timezones, calculations beyond this date might not use the correct offset
from standard time, and could return incorrect values. This affects the
64-bit version of localtime(), localtime_r(), ctime(), and ctime_r().

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:

+--------------------+-------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------------+
|CSI | Enabled |
+--------------------+-------------------------+
|Interface Stability | Standard |
+--------------------+-------------------------+
|MT-Level | MT-Safe with exceptions |
+--------------------+-------------------------+

The asctime(), ctime(), gmtime(), and localtime() functions are safe to
use in multithread applications because they employ thread-specific data.
However, their use is discouraged because standards do not require them
to be thread-safe. The asctime_r() and gmtime_r() functions are MT-Safe.
The ctime_r(), localtime_r(), and tzset() functions are MT-Safe in
multithread applications, as long as no user-defined function directly
modifies one of the following variables: timezone, altzone, daylight, and
tzname. These four variables are not MT-Safe to access. They are modified
by the tzset() function in an MT-Safe manner. The mktime(),
localtime_r(), and ctime_r() functions call tzset().

NOTES

When compiling multithreaded programs, see Intro(3).

The return values for asctime(), ctime(), gmtime(), and localtime() point
to thread-specific data whose content is overwritten by each call by the
same thread.

Setting the time during the interval of change from timezone to altzone
or vice versa can produce unpredictable results. The system administrator
must change the Julian start and end days annually.

If tzset() has previously evaluated the timezone identified by the value
of the TZ environment variable, tzset() can reuse the previous settings
of the external variables altzone, daylight, timezone, and tzname[]
associated with that timezone.

Solaris 2.4 and earlier releases provided definitions of the ctime_r(),
localtime_r(), gmtime_r(), and asctime_r() functions as specified in
POSIX.1c Draft 6. The final POSIX.1c standard changed the interface for
ctime_r() and asctime_r(). Support for the Draft 6 interface is provided
for compatibility only and might not be supported in future releases. New
applications and libraries should use the standard-conforming interface.

For POSIX.1c-conforming applications, the _POSIX_PTHREAD_SEMANTICS and
_REENTRANT flags are automatically turned on by defining the
_POSIX_C_SOURCE flag with a value >= 199506L.

In Solaris 10, gmtime(), gmtime_r(), localtime(), and localtime_r() were
updated to return a null pointer if an error is detected. This change was
based on the SUSv3 specification. See standards(7).

February 17, 2023 CTIME(3C)