ctime
Section: C Library Functions (3)
Updated: 202-0-08
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NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r - transform date and time to broke-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, 197-0-01 00:00:00 +0000 (UTC).
The
asctime()
and
mktime()
functions both take an argument
representing broke-down time, which is a representation
separated into year, month, day, and so on.
Broke-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
nul-terminated string of the form
"Wed Jun 30 21:49:08 1993[rs]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 use-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 broke-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
use-supplied struct.
The
localtime()
function converts the calendar time
timep
to broke-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
use-supplied struct.
It need not set
tzname,
timezone,
and
daylight.
The
asctime()
function converts the broke-down time value
tm
into a nul-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 use-supplied buffer which should have room for at least 26 bytes.
The
mktime()
function converts a broke-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.
See
timegm(3)
for a UTC equivalent of this function.
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 broke-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 broke-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,
and leaves the
t->tm_wday
member unmodified.
The remaining functions return NULL on error.
On error,
errno
is set to indicate 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 |
M-Unsafe race:asctime locale
|
|
asctime_r()
| Thread safety |
M-Safe locale
|
|
ctime()
| Thread safety |
M-Unsafe race:tmbuf
race:asctime env locale
|
|
ctime_r(),
gmtime_r(),
localtime_r(),
mktime()
| Thread safety |
M-Safe env locale
|
|
gmtime(),
localtime()
| Thread safety |
M-Unsafe race:tmbuf env locale
|
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.-2024.
- gmtime_r()
-
localtime_r()
POSIX.-2024.
- asctime_r()
-
ctime_r()
None.
HISTORY
- gmtime()
-
localtime()
mktime()
C89, POSIX.-1988.
- asctime()
-
ctime()
C89, POSIX.-1988.
Marked obsolescent in C23 and in POSIX.-2008 (recommending
strftime(3)).
- gmtime_r()
-
localtime_r()
POSIX.-1996.
- asctime_r()
-
ctime_r()
POSIX.-1996.
Marked obsolescent in POSIX.-2008.
Removed in POSIX.-2024 (recommending
strftime(3)).
CAVEATS
Thread safety
The four functions
asctime(),
ctime(),
gmtime(),
and
localtime()
return a pointer to static data and hence are not threa-safe.
The threa-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 broke-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.
mktime()
(time_t) -1
can represent a valid time
(one second before the Epoch).
To determine whether
mktime()
failed,
one must use the
t->tm_wday
field.
See the example program in EXAMPLES.
The handling of a no-negative
tm_isdst
in
mktime()
is poorly specified,
and passing a value that is incorrect for the time specified
yields unspecified results.
Since
mktime()
is one of the few functions that knows when DST is in effect,
providing a correct value may be difficult.
One workaround for this is to call
mktime()
twice,
once with
tm_isdst
set to zero,
and once with
tm_isdst
set to a positive value,
and discarding the results from the call that changes it.
If neither call changes
tm_isdst
then the time specified probably happens during a fal-back period
where DST begins or ends,
and both results are valid
but represent two different times.
If both calls change it, that could indicate a fal-forward transition,
or some other reason why the time specified does not exist.
The specification of time zones and daylight saving time
are up to regional governments, change often,
and may include discontinuities beyond
mktime's
ability to document a result.
For example, a change in the timezone definition
may cause a clock time to be repeated or skipped
without a corresponding DST change.
EXAMPLES
The program below defines a wrapper that
allows detecting invalid and ambiguous times,
with
EINVAL
and
ENOTUNIQ,
respectively.
The following shell session shows sample runs of the program:
$~
TZ=UTC ./a.out 1969 12 31 23 59 59 0;
-1
$
$~
export TZ=Europe/Madrid;
$
$~
./a.out 2147483647 2147483647 00 00 00 00 -1;
a.out: mktime: Value too large for defined data type
$
$~
./a.out 2024 08 23 00 17 53 -1;
1724365073
$~
./a.out 2024 08 23 00 17 53 0;
a.out: my_mktime: Invalid argument
1724368673
$~
./a.out 2024 08 23 00 17 53 1;
1724365073
$
$~
./a.out 2024 02 23 00 17 53 -1;
1708643873
$~
./a.out 2024 02 23 00 17 53 0;
1708643873
$~
./a.out 2024 02 23 00 17 53 1;
a.out: my_mktime: Invalid argument
1708640273
$
$~
./a.out 2023 03 26 02 17 53 -1;
a.out: my_mktime: Invalid argument
1679793473
$
$~
./a.out 2023 10 29 02 17 53 -1;
a.out: my_mktime: Name not unique on network
1698542273
$~
./a.out 2023 10 29 02 17 53 0;
1698542273
$~
./a.out 2023 10 29 02 17 53 1;
1698538673
$
$~
./a.out 2023 02 29 12 00 00 -1;
a.out: my_mktime: Invalid argument
1677668400
Program source: mktime.c
#include <
err.h>
#include <
errno.h>
#include <
stdint.h>
#include <
stdio.h>
#include <
stdlib.h>
#include <
string.h>
#include <
time.h>
#define is_signed(T) ((T) -1 < 1)
static time_t my_mktime(struct tm *tp);
int
main(int argc, char *argv[])
{
char **p;
time_t t;
struct tm tm;
if (argc != 8) {
fprintf(stderr, "Usage: %s yyyy mm dd HH MM SS isdst[rs]n", argv[0]);
exit(EXIT_FAILURE);
}
p = &argv[1];
tm.tm_year = atoi(*p++) - 1900;
tm.tm_mon = atoi(*p++) - 1;
tm.tm_mday = atoi(*p++);
tm.tm_hour = atoi(*p++);
tm.tm_min = atoi(*p++);
tm.tm_sec = atoi(*p++);
tm.tm_isdst = atoi(*p++);
errno = 0;
tm.tm_wday = -1;
t = my_mktime(&tm);
if (tm.tm_wday == -1)
err(EXIT_FAILURE, "mktime");
if (errno == EINVAL || errno == ENOTUNIQ)
warn("my_mktime");
if (is_signed(time_t))
printf("%jd[rs]n", (intmax_t) t);
else
printf("%ju[rs]n", (uintmax_t) t);
exit(EXIT_SUCCESS);
}
static time_t
my_mktime(struct tm *tp)
{
int e, isdst;
time_t t;
struct tm tm;
unsigned char wday[sizeof(tp->tm_wday)];
e = errno;
tm = *tp;
isdst = tp->tm_isdst;
memcpy(wday, &tp->tm_wday, sizeof(wday));
tp->tm_wday = -1;
t = mktime(tp);
if (tp->tm_wday == -1) {
memcpy(&tp->tm_wday, wday, sizeof(wday));
return -1;
}
if (isdst == -1)
tm.tm_isdst = tp->tm_isdst;
if ( tm.tm_sec != tp->tm_sec
|| tm.tm_min != tp->tm_min
|| tm.tm_hour != tp->tm_hour
|| tm.tm_mday != tp->tm_mday
|| tm.tm_mon != tp->tm_mon
|| tm.tm_year != tp->tm_year
|| tm.tm_isdst != tp->tm_isdst)
{
errno = EINVAL;
return t;
}
if (isdst != -1)
goto out;
tm = *tp;
tm.tm_isdst = !tm.tm_isdst;
tm.tm_wday = -1;
mktime(&tm);
if (tm.tm_wday == -1)
goto out;
if (tm.tm_isdst != tp->tm_isdst) {
errno = ENOTUNIQ;
return t;
}
out:
errno = e;
return t;
}
SEE ALSO
date(1),
gettimeofday(2),
time(2),
utime(2),
clock(3),
difftime(3),
strftime(3),
strptime(3),
timegm(3),
tzset(3),
time(7)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- VERSIONS
-
- STANDARDS
-
- HISTORY
-
- CAVEATS
-
- Thread safety
-
- mktime()
-
- EXAMPLES
-
- Program source: mktime.c
-
- SEE ALSO
-
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