NAME

ctime, ctime_r, localtime, localtime_r, gmtime, gmtime_r, asctime, asctime_r, tzset, tzsetwall − 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);

void tzsetwall(void);

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

struct tm ∗localtime_r(const time_t ∗clock, struct tm ∗res);

struct tm ∗gmtime_r(const time_t ∗clock, struct tm ∗res);

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

POSIX

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(), localtime(), and gmtime() functions accept arguments of type time_t, pointed to by clock(), representing the time in seconds since 00:00:00 UTC, January 1, 1970.  The ctime() function returns a pointer to a 26-character string as shown below.  Time zone and daylight savings corrections are made before string generation.  The fields are constant width:

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

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 POSIX 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 above 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 POSIX 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:

inttm_sec; /∗ seconds after the minute — [0, 61] ∗/
/∗ for leap seconds ∗/
inttm_min; /∗ minutes after the hour — [0, 59] ∗/
inttm_hour;/∗ hour since midnight — [0, 23] ∗/
inttm_mday;/∗ day of the month — [1, 31] ∗/
inttm_mon; /∗ months since January — [0, 11] ∗/
inttm_year;/∗ years since 1900 ∗/
inttm_wday;/∗ days since Sunday — [0, 6] ∗/
inttm_yday;/∗ days since January 1 — [0, 365] ∗/
inttm_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 will handle 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 may also be called by the user.  See environ(5) 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(4)). 

The tzsetwall() function sets things up so that localtime() returns the best available approximation of local wall clock time. 

ERRORS

The ctime_r() and asctime_r() functions will fail if the following is true:

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

USAGE

These functions are included for compatibility with older implementations, and do not support localized date and time formats. 

EXAMPLES

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 will be set to 5∗60∗60, tzname[1] is EDT, altzone will be 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 will be 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 will 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. 

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

SEE ALSO

time(2), Intro(3), getenv(3C), mktime(3C), printf(3S), putenv(3C), setlocale(3C), strftime(3C), TIMEZONE(4), attributes(5), environ(5)

NOTES

When compiling multithread programs, see Intro(3), Notes On Multithread Applications.

The return values for ctime(), localtime(), and gmtime() point to static data whose content is overwritten by each call. 

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. 

The asctime(), ctime(), gmtime(), and localtime() functions are unsafe in multithread applications.  The asctime_r() and gmtime_r() functions are MT-Safe.  The ctime_r(), localtime_r(), tzset(), and tzsetwall() 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(). 

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 may not be supported in future releases.  New applications and libraries should use the POSIX standard interface. 

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

SunOS 5.6  —  Last change: 19 May 1997