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CTIME(3) FreeBSD Library Functions Manual CTIME(3)
NAME asctime, asctime_r, ctime, ctime_r, difftime, gmtime, gmtime_r, localtime, localtime_r, mktime, timegm - transform binary date and time values
LIBRARY Standard C Library (libc, -lc)
SYNOPSIS #include <time.h>
extern char *tzname[2];
char * asctime(const struct tm *tm);
char * asctime_r(const struct tm *tm, char *buf);
char * ctime(const time_t *clock);
char * ctime_r(const time_t *clock, char *buf);
double difftime(time_t time1, time_t time0);
struct tm * gmtime(const time_t *clock);
struct tm * gmtime_r(const time_t *clock, struct tm *result);
struct tm * localtime(const time_t *clock);
struct tm * localtime_r(const time_t *clock, struct tm *result);
time_t mktime(struct tm *tm);
time_t timegm(struct tm *tm);
DESCRIPTION The ctime(), gmtime(), and localtime() functions all take as argument a pointer to a time value representing the time in seconds since the Epoch (00:00:00 UTC on January 1, 1970; see time(3)).
The localtime() function converts the time value pointed to by clock, and returns a pointer to a struct tm (described below) which contains the broken-out time information for the value after adjusting for the current time zone (see tzset(3)). When the specified time translates to a year that will not fit in an int, localtime() returns NULL. The localtime() function uses tzset(3) to initialize time conversion information if time zone adjustment, and returns a pointer to a struct tm.
The ctime() function adjusts the time value for the current time zone in the same manner as localtime(), and returns a pointer to a 26-character string of the form:
Thu Nov 24 18:22:48 1986\n\0
All the fields have constant width.
The asctime() function converts the broken down time in the struct tm pointed to by tm to the form shown in the example above.
The ctime_r() and asctime_r() functions provide the same functionality as ctime() and asctime() except the caller must provide the output buffer buf, which must be at least 26 characters long, to store the result in. The localtime_r() and gmtime_r() functions provide the same functionality as localtime() and gmtime() respectively, except the caller must provide the output buffer result.
The functions mktime() and timegm() convert the broken-down time in the struct tm pointed to by tm into a time value with the same encoding as that of the values returned by the time(3) function (that is, seconds from the Epoch, UTC). The mktime() function interprets the input structure according to the current timezone setting (see tzset(3)) while the timegm() function interprets the input structure as representing Universal Coordinated Time (UTC).
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 needed. For example, October 40 is changed into November 9, a tm_hour of -1 means 1 hour before midnight, tm_mday of 0 means the day preceding the current month, and tm_mon of -2 means 2 months before January of tm_year. (A positive or zero value for tm_isdst causes mktime() to presume initially that summer time (for example, Daylight Saving Time) is or is not in effect for the specified time, respectively. A negative value for tm_isdst causes the mktime() function to attempt to guess whether summer time is in effect for the specified time. The tm_isdst and tm_gmtoff members are forced to zero by timegm().)
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 mktime() function returns the specified calendar time; if the calendar time cannot be represented, it returns -1;
The difftime() function returns the difference in seconds between two time values, time1 - time0.
External declarations as well as the definition of struct tm are in the <time.h> header. The tm structure includes at least the following fields:
int tm_sec; /* seconds (0 - 60) */ int tm_min; /* minutes (0 - 59) */ int tm_hour; /* hours (0 - 23) */ long tm_gmtoff; /* offset from UTC in seconds */
The tm_isdst field is non-zero if summer time is in effect.
The tm_gmtoff field is the offset in seconds of the time represented from UTC, with positive values indicating a time zone ahead of UTC (east of the Prime Meridian).
SEE ALSO date(1), clock_gettime(2), gettimeofday(2), getenv(3), time(3), tzset(3), tzfile(5)
STANDARDS The asctime(), ctime(), difftime(), gmtime(), localtime(), and mktime() functions conform to ISO/IEC 9899:1990 ("ISO C90"), and conform to ISO/IEC 9945-1:1996 ("POSIX.1") provided the selected local timezone does not contain a leap-second table (see zic(8)).
The asctime_r(), ctime_r(), gmtime_r(), and localtime_r() functions are expected to conform to ISO/IEC 9945-1:1996 ("POSIX.1") (again provided the selected local timezone does not contain a leap-second table).
The timegm() function is not specified by any standard; its function cannot be completely emulated using the standard functions described above.
HISTORY This manual page is derived from the time package contributed to Berkeley by Arthur Olson and which appeared in 4.3BSD.
The functions asctime(), gmtime(), and localtime() first appeared in Version 5 AT&T UNIX, difftime() and mktime() in 4.3BSD-Reno, and timegm() and timelocal() in SunOS 4.0.
The asctime_r(), ctime_r(), gmtime_r() and localtime_r() functions have been available since FreeBSD 8.0.
BUGS Except for difftime(), mktime(), and the _r() variants of the other functions, these functions leave their result in an internal static object and return a pointer to that object. Subsequent calls to these function will modify the same object.
The C Standard provides no mechanism for a program to modify its current local timezone setting, and the POSIX-standard method is not reentrant. (However, thread-safe implementations are provided in the POSIX threaded environment.)
The tm_zone field of a returned tm structure points to a static array of characters, which will also be overwritten by any subsequent calls (as well as by subsequent calls to tzset(3)).
Use of the external variable tzname is discouraged; the tm_zone entry in the tm structure is preferred.
FreeBSD 14.0-RELEASE-p11 April 20, 2023 FreeBSD 14.0-RELEASE-p11