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NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r, localtime_r - transform date and
time to broken-down time or ASCII
SYNOPSIS
#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
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
EOVERFLOW
The result cannot be represented.
ATTRIBUTES
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() │ │ │
└───────────────┴───────────────┴─────────────────────────────────┘
CONFORMING TO
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.
NOTES
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
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.1-2001, 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().
SEE ALSO
date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3), strftime(3), strptime(3), timegm(3),
tzset(3), time(7)
COLOPHON
This page is part of release 5.10 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/.
2020-12-21 CTIME(3)