ctime(3) Library Functions Manual ctime(3)
NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r - transform date and time to broken-down time or ASCII
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <time.h>
char *asctime(const struct tm *tm);
char *asctime_r(const struct tm *restrict tm,
char buf[restrict 26]);
char *ctime(const time_t *timep);
char *ctime_r(const time_t *restrict timep,
char buf[restrict 26]);
struct tm *gmtime(const time_t *timep);
struct tm *gmtime_r(const time_t *restrict timep,
struct tm *restrict result);
struct tm *localtime(const time_t *timep);
struct tm *localtime_r(const time_t *restrict timep,
struct tm *restrict 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 <= 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, described in tm(3type).
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 tz-
name, timezone, and daylight as if it called tzset(3). The reentrant
version ctime_r() does the same, but stores the string in a user-sup-
plied 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 gm-
time_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 time-
zone. The function also sets the external variables tzname, timezone,
and daylight as if it called tzset(3). 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 asc-
time_r() function does the same, but stores the string in a user-sup-
plied 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 fol-
lows: tm_wday and tm_yday are set to values determined from the contents
of the other fields; if structure members are outside their valid inter-
val, 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. The function also
sets the external variables tzname, timezone, and daylight as if it
called tzset(3).
If the specified broken-down time cannot be represented as calendar time
(seconds since the Epoch), mktime() returns (time_t) -1 and does not al-
ter 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 func-
tions return NULL on error. On error, errno is set to indicate the er-
ror.
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() │ │ │
└────────────────┴───────────────┴─────────────────────────────────────┘
VERSIONS
POSIX doesn't specify the parameters of ctime_r() to be restrict; that
is specific to glibc.
In many implementations, including glibc, a 0 in tm_mday is interpreted
as meaning the last day of the preceding month.
According to POSIX.1, 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().
STANDARDS
asctime()
ctime()
gmtime()
localtime()
mktime()
C23, POSIX.1-2024.
gmtime_r()
localtime_r()
POSIX.1-2024.
asctime_r()
ctime_r()
None.
HISTORY
gmtime()
localtime()
mktime()
C89, POSIX.1-1988.
asctime()
ctime()
C89, POSIX.1-1988. Marked obsolescent in C23 and in POSIX.1-2008
(recommending strftime(3)).
gmtime_r()
localtime_r()
POSIX.1-1996.
asctime_r()
ctime_r()
POSIX.1-1996. Marked obsolescent in POSIX.1-2008. Removed in
POSIX.1-2024 (recommending strftime(3)).
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 says: "The asctime(), ctime(), gmtime(), and localtime() func-
tions 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 func-
tions that return a pointer to one of these object types may overwrite
the information in any object of the same type pointed to by the value
returned from any previous call to any of them." This can occur in the
glibc implementation.
SEE ALSO
date(1), gettimeofday(2), time(2), utime(2), clock(3), difftime(3),
strftime(3), strptime(3), timegm(3), tzset(3), time(7)
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