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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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ctime(3) Library Functions 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
Standard C library (libc, -lc)
#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
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 tzname, timezone, and
daylight as if it called tzset(3). 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 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 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. 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 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 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 │
│ localtime() │ │ locale │
└────────────────┴───────────────┴──────────────────────────────┘
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().
asctime()
ctime()
gmtime()
localtime()
mktime()
C23, POSIX.1-2024.
gmtime_r()
localtime_r()
POSIX.1-2024.
asctime_r()
ctime_r()
None.
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)).
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()
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 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.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
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Linux man-pages 6.9.1 2024-06-15 ctime(3)
Pages that refer to this page: find(1), pcp-iostat(1), pcp-ss(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), pmnotifyerr(3), __pmparsectime(3), strftime(3), timegm(3), time_t(3type), tm(3type), tzset(3), rtc(4), localtime(5), tzfile(5), environ(7), time(7), audispd-zos-remote(8), lsof(8), zdump(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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