TM(3) BSD Library Functions Manual TM(3)
NAME
tm -- time structure
SYNOPSIS
#include <time.h>
DESCRIPTION
The <time.h> header defines the tm structure that contains calendar dates and time broken down into components. The following standards-com-
pliant fields are present:
Type Field Represents Range
int tm_sec Seconds [0, 61]
int tm_min Minutes [0, 59]
int tm_hour Hours 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_isdt Positive if daylight savings >= 0
The tm structure is used by various common library routines such as mktime(3), localtime(3), and strptime(3). All fields described above are
defined in the IEEE Std 1003.1-2008 (``POSIX.1'') standard.
NetBSD Extensions
In addition, the following NetBSD-specific fields are available:
Type Field Represents
int tm_gmtoff Offset from UTC in seconds
__aconst char tm_zone Timezone abbreviation
The tm_zone and tm_gmtoff fields exist, and are filled in by applicable library routines, only if arrangements to do so were made when the
library containing these functions was created. There is no guarantee that these fields will continue to exist in this form in future
releases of NetBSD.
The tm_gmtoff field denotes the offset (in seconds) of the time represented from UTC, with positive values indicating east of the Prime
Meridian. The tm_zone field will become invalid and point to freed storage if the corresponding struct tm was returned by localtime_rz(3)
and the const timezone_t tz argument has been freed by tzfree(3).
SEE ALSO
asctime(3), offtime(3), timeval(3), wcsftime(3)
STANDARDS
The tm structure conforms to IEEE Std 1003.1-2008 (``POSIX.1'') with respect to the described standard structure members.
BSD
April 14, 2011 BSD
Check Out this Related Man Page
CTIME(3) BSD 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 val-
ues
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <time.h>
extern char *tzname[2];
char *
ctime(const time_t *clock);
double
difftime(time_t time1, time_t time0);
char *
asctime(const struct tm *tm);
struct tm *
localtime(const time_t *clock);
struct tm *
gmtime(const time_t *clock);
time_t
mktime(struct tm *tm);
time_t
timegm(struct tm *tm);
char *
ctime_r(const time_t *clock, char *buf);
struct tm *
localtime_r(const time_t *clock, struct tm *result);
struct tm *
gmtime_r(const time_t *clock, struct tm *result);
char *
asctime_r(const struct tm *tm, char *buf);
DESCRIPTION
The functions ctime(), gmtime() and localtime() all take as an argument a time value representing the time in seconds since the Epoch
(00:00:00 UTC, January 1, 1970; see time(3)).
The function localtime() converts the time value pointed at by clock, and returns a pointer to a ``struct tm'' (described below) which con-
tains the broken-out time information for the value after adjusting for the current time zone (and any other factors such as Daylight Saving
Time). Time zone adjustments are performed as specified by the TZ environment variable (see tzset(3)). The function localtime() uses
tzset(3) to initialize time conversion information if tzset(3) has not already been called by the process.
After filling in the tm structure, localtime() 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.
The function gmtime() similarly converts the time value, but without any time zone adjustment, and returns a pointer to a tm structure
(described below).
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-char-
acter string of the form:
Thu Nov 24 18:22:48 1986
All the fields have constant width.
The ctime_r() function provides the same functionality as ctime() except the caller must provide the output buffer buf to store the result,
which must be at least 26 characters long. 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 asctime() function converts the broken down time in the structure tm pointed at by *tm to the form shown in the example above.
The asctime_r() function provides the same functionality as asctime() except the caller provide the output buffer buf to store the result,
which must be at least 26 characters long.
The functions mktime() and timegm() convert the broken-down time in the structure 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)). 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 divine
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 repre-
sented, it returns -1;
The difftime() function returns the difference between two calendar times, (time1 - time0), expressed in seconds.
External declarations as well as the tm structure definition are in the <time.h> include file. The tm structure includes at least the fol-
lowing 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 summer time in effect? */
char *tm_zone; /* abbreviation of timezone name */
long tm_gmtoff; /* offset from UTC in seconds */
The field tm_isdst is non-zero if summer time is in effect.
The field tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating east of the Prime Meridian.
SEE ALSO
date(1), 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 pro-
vided 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.
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 reen-
trant. (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) and tzsetwall(3)).
Use of the external variable tzname is discouraged; the tm_zone entry in the tm structure is preferred.
BSD
January 2, 1999 BSD