| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
CTIME(3) Linux Programmer's Manual CTIME(3)
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r - transform date and time to broken-down time or ASCII
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
asctime_r(), ctime_r(), gmtime_r(), localtime_r():
_POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE ||
_POSIX_SOURCE
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 the Epoch,
1970-01-01 00:00:00 +0000 (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 60 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 null-terminated 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 variables tzname, timezone, and daylight (see
tzset(3)) with information about the current timezone. The reentrant version
ctime_r() does the same, but stores the string in a user-supplied buffer which
should have room for at least 26 bytes. It need not set tzname, timezone, and
daylight.
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-down time
representation, expressed relative to the user's specified timezone. The
function acts as if it called tzset(3) and sets the external variables tzname
with information about the current timezone, timezone with the difference
between Coordinated Universal Time (UTC) and local standard time in seconds,
and daylight to a nonzero 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, timezone, and daylight.
The asctime() function converts the broken-down time value tm into a null-
terminated 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 which should have room
for at least 26 bytes.
The mktime() function converts a broken-down time structure, expressed as
local time, to calendar time representation. The function ignores the values
supplied by the caller in the tm_wday and tm_yday fields. The value specified
in the tm_isdst field informs mktime() whether or not daylight saving time
(DST) is in effect for the time supplied in the tm structure: a positive value
means DST is in effect; zero means that DST is not in effect; and a negative
value means that mktime() should (use timezone information and system
databases to) attempt to determine whether DST is in effect at the specified
time.
The mktime() function modifies the fields of the tm structure as follows:
tm_wday and tm_yday are set to values determined from the contents of the
other fields; if structure members are outside their valid interval, they will
be normalized (so that, for example, 40 October is changed into 9 November);
tm_isdst is set (regardless of its initial value) to a positive value or to 0,
respectively, to indicate whether DST is or is not in effect at the specified
time. Calling mktime() also sets the external variable tzname with
information about the current timezone.
If the specified broken-down time cannot be represented as calendar time
(seconds since the Epoch), mktime() returns (time_t) -1 and does not alter the
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.
POSIX.1-2001. C89 and C99 specify asctime(), ctime(), gmtime(), localtime(),
and mktime(). POSIX.1-2008 marks asctime(), asctime_r(), ctime(), and
ctime_r() as obsolete, recommending the use of strftime(3) instead.
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.
POSIX.1-2001 says: "The asctime(), ctime(), gmtime(), and localtime()
functions shall return values in one of two static objects: a broken-down time
structure and an array of type char. Execution of any of the functions may
overwrite the information returned in either of these objects by any of the
other functions." This can occur in the glibc implementation.
In many implementations, including glibc, a 0 in tm_mday is interpreted as
meaning the last day of the preceding month.
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.
According to POSIX.1-2004, localtime() is required to behave as though
tzset(3) was called, while localtime_r() does not have this requirement. For
portable code tzset(3) should be called before localtime_r().
date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3),
strftime(3), strptime(3), timegm(3), tzset(3), time(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2010-02-25 CTIME(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface