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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE ||
_SVID_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, and so on.
Broken-down time is stored in the structure tm, which is defined in
<time.h> as follows:
struct tm {
int tm_sec; /* Seconds (0-60) */
int tm_min; /* Minutes (0-59) */
int tm_hour; /* Hours (0-23) */
int tm_mday; /* Day of the month (1-31) */
int tm_mon; /* Month (0-11) */
int tm_year; /* Year - 1900 */
int tm_wday; /* Day of the week (0-6, Sunday = 0) */
int tm_yday; /* Day in the year (0-365, 1 Jan = 0) */
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.
On success, gmtime() and localtime() return a pointer to a struct tm.
On success, gmtime_r() and localtime_r() return the address of the
structure pointed to by result.
On success, asctime() and ctime() return a pointer to a string.
On success, asctime_r() and ctime_r() return a pointer to the string
pointed to by buf.
On success, mktime() returns the calendar time (seconds since the
Epoch), expressed as a value of type time_t.
On error, mktime() returns the value (time_t) -1. The remaining
functions return NULL on error. On error, errno is set to indicate
the cause of the error.
EOVERFLOW
The result cannot be represented.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────┬───────────────┬─────────────────────────────────┐
│Interface │ Attribute │ Value │
├───────────────┼───────────────┼─────────────────────────────────┤
│asctime() │ Thread safety │ MT-Unsafe race:asctime locale │
├───────────────┼───────────────┼─────────────────────────────────┤
│asctime_r() │ Thread safety │ MT-Safe locale │
├───────────────┼───────────────┼─────────────────────────────────┤
│ctime() │ Thread safety │ MT-Unsafe race:tmbuf │
│ │ │ race:asctime env locale │
├───────────────┼───────────────┼─────────────────────────────────┤
│ctime_r(), │ Thread safety │ MT-Safe env locale │
│gmtime_r(), │ │ │
│localtime_r(), │ │ │
│mktime() │ │ │
├───────────────┼───────────────┼─────────────────────────────────┤
│gmtime(), │ Thread safety │ MT-Unsafe race:tmbuf env locale │
│localtime() │ │ │
└───────────────┴───────────────┴─────────────────────────────────┘
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. The
thread-safe versions, asctime_r(), ctime_r(), gmtime_r() and
localtime_r(), are specified by SUSv2.
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 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
2016-12-12 CTIME(3)
Pages that refer to this page: find(1), pcp-iostat(1), pcp-tapestat(1), pmie(1), utmpdump(1), clock_getres(2), gettimeofday(2), time(2), difftime(3), getdate(3), pmctime(3), pmlocaltime(3), pmmktime(3), pmparsectime(3), strftime(3), strptime(3), timegm(3), tzset(3), rtc(4), localtime(5), tzfile(5), environ(7), time(7), audispd-zos-remote(8), lsof(8)