libc/time/ctime.3

.\" $NetBSD: ctime.3,v 1.72 2025/01/23 22:44:22 christos Exp $
.\"
.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.
.\"
.Dd January 23, 2025
.Dt CTIME 3
.Os
.Sh NAME
.Nm asctime ,
.Nm asctime_r ,
.Nm ctime ,
.Nm ctime_r ,
.Nm ctime_rz ,
.Nm difftime ,
.Nm gmtime ,
.Nm gmtime_r ,
.Nm localtime ,
.Nm localtime_r ,
.Nm localtime_rz ,
.Nm mktime ,
.Nm mktime_z
.Nd convert date and time
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Vt extern char *tzname[2];
.Ft [[deprecated]] char *
.Fn asctime "const struct tm *tm"
.Ft [[only in POSIX.1-2017 and earlier]] char *
.Fn asctime_r "const struct tm *restrict tm" "char * restrict buf"
.Ft [[deprecated]] char *
.Fn ctime "const time_t *clock"
.Ft [[only in POSIX.1-2017 and earlier]] char *
.Fn ctime_r "const time_t *clock"  "char *buf"
.Ft char *
.Fn ctime_rz "timezone_t restrict tz" "const time_t *clock"  "char *buf"
.Ft double
.Fn difftime "time_t time1" "time_t time0"
.Ft struct tm *
.Fn gmtime "const time_t *clock"
.Ft struct tm *
.Fn gmtime_r "const time_t * restrict clock" "struct tm * restrict result"
.Ft struct tm *
.Fn localtime "const time_t *clock"
.Ft struct tm *
.Fn localtime_r "const time_t * restrict clock" "struct tm * restrict result"
.Ft struct tm *
.Fn localtime_rz "timezone_t restrict tz" "const time_t * restrict clock" "struct tm * restrict result"
.Ft time_t
.Fn mktime "struct tm *tm"
.Ft time_t
.Fn mktime_z "timezone_t restrict tz" "struct tm *restrict tm"
.Sh DESCRIPTION
The
.Nm
family of functions provide various standard library routines
to operate with time and conversions related to time.
.Sh FUNCTIONS
.Bl -tag -width abcd
.It Fn asctime "tm"
The
.Fn asctime
function converts a time value contained in the
.Fa tm
structure to a string with the following general format:
.Bd -literal -offset indent
Thu Nov 24 18:22:48 1986\en\e0
.Ed
Years requiring fewer than four characters are padded with leading zeroes.
For years longer than four characters, the string is of the form:
.Bd -literal -offset indent
Thu Nov 24 18:22:48     81986\n\0
.Ed
with five spaces before the year.
This unusual format is designed to make it less likely that older
software that expects exactly 26 bytes of output will mistakenly output
misleading values for out-of-range years.
This function is deprecated starting in C23.
Callers can use
.Fn strftime
instead.
.Pp
The
.Fa tm
structure is described in
.Xr tm 3 .
.Pp
This function is deprecated starting in C23.
Callers can use
.Fn strftime
instead.
.It Fn asctime_r "tm" "buf"
The
.Fn asctime_r
has the same behavior as
.Fn asctime ,
but the result is stored in
.Fa buf ,
which should have a size of at least 26 bytes.
.It Fn ctime "clock"
The
.Fn ctime
function converts a
.Vt time_t ,
pointed to by
.Fa clock ,
and returns a pointer to a string with the format described above.
Years requiring fewer than four characters are padded with leading zeroes.
For years longer than four characters, the string is of the form
.Bd -literal -offset indent
Thu Nov 24 18:22:48     81986\en\e0
.Ed
.Pp
with five spaces before the year.
These unusual formats are designed to make it less likely that older
software that expects exactly 26 bytes of output will mistakenly output
misleading values for out-of-range years.
.Pp
The
.Fa clock
time stamp represents the time in seconds since 1970-01-01 00:00:00
Coordinated Universal Time (UTC).
The POSIX standard says that time stamps must be nonnegative
and must ignore leap seconds.
Many implementations extend POSIX by allowing negative time stamps,
and can therefore represent time stamps that predate the
introduction of UTC and are some other flavor of Universal Time (UT).
Some implementations support leap seconds, in contradiction to POSIX.
.Pp
The
.Fn ctime
function is deprecated starting in C23.
Callers can use
.Fn localtime_r
and
.Fn strftime
instead.
.It Fn ctime_r "clock" "buf"
The
.Fn ctime_r
is similar to
.Fn ctime ,
except it places the result of the conversion in the
.Fa buf
argument, which should be 26 or more bytes long,
instead of using a global static buffer.
.It Fn ctime_rz "tz" "clock" "buf"
The
.Fn ctime_rz
function is similar to
.Fn ctime_r ,
but it also takes a
.Ft "timezone_t"
argument, as returned by a previous call to
.Fn tzalloc ,
or a
.Dv NULL
pointer denoting
Coordinated Universal Time
.Pq UTC .
.It Fn difftime "time1" "time2"
The
.Fn difftime
function returns the difference between two calendar times,
.Fa ( time1 No - Fa time0 ) ,
expressed in seconds.
.It Fn gmtime "clock"
The
.Fn gmtime
function converts to Coordinated Universal Time
.Pq UTC
and returns a pointer to the
.Va tm
structure described in
.Xr tm 3 .
.It Fn gmtime_r "clock" "result"
The
.Fn gmtime_r
function provides the same functionality as
.Fn gmtime ,
differing in that the caller must supply a buffer area
.Fa result
in which the result is stored.
.It Fn localtime "clock"
Also
.Fn localtime
is comparable to
.Fn gmtime .
However,
.Fn localtime
corrects for the timezone and any timezone adjustments
(such as Daylight Saving Time in the U.S.A.).
After filling in the
.Va tm
structure, the function sets the
.Fa tm_isdst Ns 'th
element of
.Fa tzname
to a pointer to an
ASCII string that is the timezone abbreviation to be used with
.Fn localtime Ns 's
return value.
.It Fn localtime_r "clock" "result"
As
.Fn gmtime_r ,
the
.Fn localtime_r
takes an additional buffer
.Fa result
as a parameter and stores the result in it.
Note however that
.Fn localtime_r
does not imply initialization of the local time conversion information;
the application may need to do so by calling
.Xr tzset 3 .
.It Fn localtime_rz "tz" "clock" "result"
The
.Fn localtime_rz
function is similar to
.Fn localtime_r ,
but it also takes a
.Ft "timezone_t"
argument, returned by a previous call to
.Fn tzalloc ,
or a
.Dv NULL
pointer denoting Coordinated Universal Time
.Pq UTC .
.It Fn mktime "tm"
The
.Fn mktime
function converts the broken-down time,
expressed as local time in the
.Xr tm 3
structure, into a calendar time value with
the same encoding as that of the values returned by the
.Xr time 3
function.
The following remarks should be taken into account.
.Bl -bullet
.It
The original values of the
.Fa tm_wday
and
.Fa tm_yday
components of the structure are ignored,
and the original values of the other components are not restricted
to their normal ranges and will be normalized,
if need be.
.Pp
For example,
consider a
.Fa struct tm
initialized with
.Fa tm_year
= 122,
.Fa tm_mon
= 10,
.Fa tm_mday
= 30,
.Fa tm_hour
= 22,
.Fa tm_min
= 57,
and
.Fa tm_sec
= 0.
Incrementing
.Fa tm_min
by 13 and calling
.Fn mktime
would lead to
.Fa tm_hour
= 23 and
.Fa tm_min
= 10.
.Pp
This normalizing can lead to cascading changes:
Again using a
.Fa struct tm
initialized as in the above example but with
.Fa tm_hour
= 23,
the same change would lead to
.Fa tm_mon
= 11,
.Fa tm_mday
= 1,
.Fa tm_hour
= 0, and
.Fa tm_min
= 10.
.Pp
Negative values may also be normalized with similar
cascading effect such that e.g.,
a
.Fa tm_hour
of \-1 means 1 hour before midnight on the previous
day and so on.
.It
A positive or zero value for
.Fa tm_isdst
causes
.Fn mktime
to presume initially that daylight saving time
respectively,
is or is not in effect for the specified time.
.It
A negative value for
.Fa tm_isdst
causes the
.Fn mktime
function to attempt to divine whether daylight saving time is in effect
for the specified time; in this case it does not use a consistent
rule and may give a different answer when later
presented with the same argument.
.It
On successful completion, the values of the
.Fa tm_wday
and
.Fa tm_yday
components of the structure are set appropriately,
and the other components are set to represent the specified calendar time,
but with their values forced to their normal ranges; the final value of
.Fa tm_mday
is not set until
.Fa tm_mon
and
.Fa tm_year
are determined.
.El
.Pp
The function returns the specified calendar time;
if the calendar time cannot be represented, it returns
.Va "(time_t)\-1" .
This can happen either because the resulting conversion would not fit
in a
.Vt time_t
variable, or because the time specified happens to be in the daylight
savings gap and
.Fa tm_isdst
was set to
.Dv \-1 .
Other
.Fn mktime
implementations do not return an error in the second case and return
the appropriate time offset after the daylight savings gap.
There is code to mimick this behavior, but it is not enabled by default.
.It Fn mktime_z "tz" "tm"
The
.Fn mktime_z
function is similar to
.Fn mktime
but it also takes a
.Ft "const timezone_t"
argument, returned by a previous call to
.Fn tzalloc ,
or a null pointer denoting
Coordinated Universal Time
.Pq UTC .
.El
.Pp
Declarations of all the functions and externals, and the
.Ft tm
structure, are in the
.In time.h
header file.
The structure (of type)
.Ft struct tm
includes the following fields:
.Bd -literal
       int tm_sec;      /* seconds (0 - 60) */
       int tm_min;      /* minutes (0 - 59) */
       int tm_hour;     /* hours (0 - 23) */
       int tm_mday;     /* day of month (1 - 31) */
       int tm_mon;      /* month of year (0 - 11) */
       int tm_year;     /* year - 1900 */
       int tm_wday;     /* day of week (Sunday = 0) */
       int tm_yday;     /* day of year (0 - 365) */
       int tm_isdst;    /* is daylight saving time in effect? */
       char *tm_zone;   /* abbreviation of timezone name (optional) */
       long tm_gmtoff;  /* offset from UT in seconds (optional) */
.Ed
.Bl -bullet
.It
.Va tm_isdst
is non-zero if daylight saving time is in effect.
.It
.Va tm_gmtoff
is the offset (in seconds) of the time represented from UT,
with positive values indicating east of the Prime Meridian.
The field's name is derived from Greenwich Mean Time, a precursor of UT.
.El
In
.Ft struct tm
the
.Fa tm_zone
and
.Fa tm_gmtoff
fields exist, and are filled in, only if arrangements to do
so were made when the library containing these functions  was
created.
Similarly, the
.Va tzname
variable is optional; also, there is no guarantee that
.Dv tzname
will
continue to exist in this form in future releases of this code.
.Pp
The
.Fn ctime_r ,
.Fn localtime_r ,
.Fn gmtime_r ,
and
.Fn asctime_r
functions
are like their unsuffixed counterparts, except that they accept an
additional argument specifying where to store the result if successful.
.Pp
The
.Fn ctime_rz ,
.Fn localtime_rz ,
and
.Fn mktime_z
functions
are like their unsuffixed counterparts, except that they accept an
extra initial
.Ar zone
argument specifying the timezone to be used for conversion.
If
.Fa zone
is
.Dv NULL ,
UT is used; otherwise,
.Fa zone
should have been allocated by
.Fn tzalloc
and should not be freed until after all uses (e.g., by calls to
.Fn strftime )
of the filled-in
.Fn tm_zone
fields.
.Sh RETURN VALUES
.Bl -bullet
.It
On success the
.Fn asctime
and
.Fn ctime
functions return a pointer to a static character buffer, and the
.Fn asctime_r ,
.Fn ctime_r ,
and
.Fn ctime_rz
function return a pointer to the user-supplied buffer.
On failure they all return
.Dv NULL
and no errors are defined for them.
.It
On success the
.Fn gmtime ,
and
.Fn localtime
functions return a pointer to a statically allocated
.Va "struct tm"
whereas the
.Fn gmtime_r ,
.Fn localtime_r ,
and
.Fn localtime_rz ,
functions return a pointer to the user-supplied
.Va "struct tm" .
On failure they all return
.Dv NULL
and the global variable
.Va errno
is set to indicate the error.
.It
The
.Fn mktime
and
.Fn mktime_z
function returns the specified time since the Epoch as a
.Vt time_t
type value.
If the time cannot be represented, then
.Fn mktime
and
.Fn mktime_z
return
.Va "(time_t)-1"
setting the global variable
.Va errno
to indicate the error.
.It
The
.Fn tzalloc
function returns a pointer to a
.Ft timezone_t
object or
.Dv NULL
on failure, setting
.Va errno
to indicate the error.
It may also return
.Dv NULL
when the
.Fa name
argument is
.Dv NULL ,
and this is not an error, but a way of referring to
Coordinated Universal Time
.Pq UTC .
.It
.Fn tzgetzone
function returns string containing the name of the timezone given in
.Fa tz .
.El
.Sh FILES
.Bl -tag -width /usr/share/zoneinfo/posixrules -compact
.It Pa /etc/localtime
local timezone file
.It Pa /usr/share/zoneinfo
timezone information directory
.It Pa /usr/share/zoneinfo/localtime
local timezone file
.It Pa /usr/share/zoneinfo/posixrules
default DST rules (obsolete,
and can cause bugs if present)
.It Pa /usr/share/zoneinfo/GMT
for UTC leap seconds
.El
.Pp
If
.Pa /usr/share/zoneinfo/GMT
is absent, UTC leap seconds are loaded from
.Pa /usr/share/zoneinfo/posixrules .
.Sh ERRORS
The described functions may fail with
.Bl -tag -width Er
.It Bq Er EINVAL
The result cannot be represented because a parameter is incorrect, or
the conversion failed because no such time exists (for example a time
in the DST gap).
.It Bq Er EOVERFLOW
The result cannot be represented because the time requested is out of bounds
and the time calculation resulted in overflow.
.El
.Pp
All functions that return values, except their
.Dq z
variants, can also return the same errors as
.Xr open 2
and
.Xr malloc 3 .
.Sh SEE ALSO
.Xr getenv 3 ,
.Xr strftime 3 ,
.Xr time 3 ,
.Xr tm 3 ,
.Xr tzset 3 ,
.Xr tzfile 5
.Sh STANDARDS
The
.Fn ctime ,
.Fn difftime ,
.Fn asctime ,
.Fn localtime ,
.Fn gmtime
and
.Fn mktime
functions conform to
.St -ansiC .
Rest of the functions conform to
.St -p1003.1-2008 .
.Sh HISTORY
A
.Fn ctime
function appeared in
.At v1 .
.Sh CAVEATS
The functions that do not take an explicit
.Ft timezone_t
argument return values pointing to static data; the data is overwritten by
each call.
For the above functions the
.Dv tzname
variable (once set) and the
.Fa tm_zone
field of a returned
.Va "struct tm"
point to an array of characters that
can be freed or overwritten by later calls to the functions
.Fn localtime ,
.Fn tzfree ,
and
.Fn tzset ,
if these functions affect the timezone information that specifies the
abbreviation in question.
The remaining functions and data are thread-safe.
The functions that do take an explicit
.Ft timezone_t
argument and set the fields of a supplied
.Va "struct tm"
should not call
.Fn tzfree
since the
.Fa tm_zone
field of the
.Va "struct tm"
points to data allocated by
.Fn tzalloc .
.Pp
The
.Fn asctime ,
.Fn asctime_r ,
.Fn ctime ,
.Fn ctime_r ,
and
.Fn ctime_rz ,
functions behave strangely for years before 1000 or after 9999.
The 1989 and 1999 editions of the C Standard say
that years from \-99 through 999 are converted without
extra spaces, but this conflicts with longstanding
tradition and with this implementation.
The 2011 edition says that the behavior
is undefined if the year is before 1000 or after 9999.
Traditional implementations of these two functions are
restricted to years in the range 1900 through 2099.
To avoid this portability mess, new programs should use
.Fn strftime
instead.
.\" 2009-05-17 by Arthur David Olson.