Originální popis anglicky:
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r - transform date and time to broken-down time or ASCII
Návod, kniha: Linux Programmer's Manual
#include <time.h>
char *asctime(const struct tm *tm);
char *asctime_r(const struct tm *tm, char *buf);
char *ctime(const time_t *timep);
char *ctime_r(const time_t *timep, char *buf);
struct tm *gmtime(const time_t *timep);
struct tm *gmtime_r(const time_t *timep, struct tm *result);
struct tm *localtime(const time_t *timep);
struct tm *localtime_r(const time_t *timep, struct tm *result);
time_t mktime(struct tm *tm);
The
ctime(),
gmtime() and
localtime() functions all take an
argument of data type
time_t which represents calendar time. When
interpreted as an absolute time value, it represents the number of seconds
elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
The
asctime() and
mktime() functions both take an argument
representing broken-down time which is a representation separated into year,
month, day, etc.
Broken-down time is stored in the structure
tm which is defined in
<time.h> as follows:
struct tm {
int tm_sec; /* seconds */
int tm_min; /* minutes */
int tm_hour; /* hours */
int tm_mday; /* day of the month */
int tm_mon; /* month */
int tm_year; /* year */
int tm_wday; /* day of the week */
int tm_yday; /* day in the year */
int tm_isdst; /* daylight saving time */
};
The members of the
tm structure are:
- tm_sec
- The number of seconds after the minute, normally in the
range 0 to 59, but can be up to 61 to allow for leap seconds.
- tm_min
- The number of minutes after the hour, in the range 0 to
59.
- tm_hour
- The number of hours past midnight, in the range 0 to
23.
- tm_mday
- The day of the month, in the range 1 to 31.
- tm_mon
- The number of months since January, in the range 0 to
11.
- tm_year
- The number of years since 1900.
- tm_wday
- The number of days since Sunday, in the range 0 to 6.
- tm_yday
- The number of days since January 1, in the range 0 to
365.
- tm_isdst
- A flag that indicates whether daylight saving time is in
effect at the time described. The value is positive if daylight saving
time is in effect, zero if it is not, and negative if the information is
not available.
The call
ctime(t) is equivalent to
asctime(localtime(t)). It converts the calendar
time
t into a string of the form
"Wed Jun 30 21:49:08 1993\n"
The abbreviations for the days of the week are `Sun', `Mon', `Tue', `Wed',
`Thu', `Fri', and `Sat'. The abbreviations for the months are `Jan', `Feb',
`Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'. The
return value points to a statically allocated string which might be
overwritten by subsequent calls to any of the date and time functions. The
function also sets the external variable
tzname (see
tzset(3))
with information about the current time zone. The re-entrant version
ctime_r() does the same, but stores the string in a user-supplied
buffer of length at least 26. It need not set
tzname.
The
gmtime() function converts the calendar time
timep to
broken-down time representation, expressed in Coordinated Universal Time
(UTC). It may return NULL when the year does not fit into an integer. The
return value points to a statically allocated struct which might be
overwritten by subsequent calls to any of the date and time functions. The
gmtime_r() function does the same, but stores the data in a
user-supplied struct.
The
localtime() function converts the calendar time
timep to
broken-time representation, expressed relative to the user's specified time
zone. The function acts as if it called
tzset(3) and sets the external
variables
tzname with information about the current time zone,
timezone with the difference between Coordinated Universal Time (UTC)
and local standard time in seconds, and
daylight to a non-zero value if
daylight savings time rules apply during some part of the year. The return
value points to a statically allocated struct which might be overwritten by
subsequent calls to any of the date and time functions. The
localtime_r() function does the same, but stores the data in a
user-supplied struct. It need not set
tzname.
The
asctime() function converts the broken-down time value
tm into
a string with the same format as
ctime(). The return value points to a
statically allocated string which might be overwritten by subsequent calls to
any of the date and time functions. The
asctime_r() function does the
same, but stores the string in a user-supplied buffer of length at least 26.
The
mktime() function converts a broken-down time structure, expressed as
local time, to calendar time representation. The function ignores the
specified contents of the structure members
tm_wday and
tm_yday
and recomputes them from the other information in the broken-down time
structure. If structure members are outside their legal interval, they will be
normalized (so that, e.g., 40 October is changed into 9 November). Calling
mktime() also sets the external variable
tzname with information
about the current time zone. If the specified broken-down time cannot be
represented as calendar time (seconds since the epoch),
mktime()
returns a value of (time_t)(-1) and does not alter the
tm_wday and
tm_yday members of the broken-down time structure.
Each of these functions returns the value described, or NULL (-1 in case of
mktime()) in case an error was detected.
The four functions
asctime(),
ctime(),
gmtime() and
localtime() return a pointer to static data and hence are not
thread-safe. Thread-safe versions
asctime_r(),
ctime_r(),
gmtime_r() and
localtime_r() are specified by SUSv2, and
available since libc 5.2.5.
The glibc version of struct tm has additional fields
long tm_gmtoff; /* Seconds east of UTC */
const char *tm_zone; /* Timezone abbreviation */
defined when _BSD_SOURCE was set before including
<time.h>. This is
a BSD extension, present in 4.3BSD-Reno.
SVID 3, POSIX, BSD 4.3, ISO 9899
date(1),
gettimeofday(2),
time(2),
utime(2),
clock(3),
difftime(3),
newctime(3),
strftime(3),
strptime(3),
tzset(3)
