.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
+.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
-.\"
+.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
-.\"
+.\"
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.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" References consulted:
.\" Linux libc source code
.\" Modified 2001-12-13, joey, aeb
.\" Modified 2004-11-16, mtk
.\"
-.TH CTIME 3 2004-11-16 "" "Linux Programmer's Manual"
+.TH CTIME 3 2019-03-06 "" "Linux Programmer's Manual"
.SH NAME
asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r,
localtime_r \- transform date and time to broken-down time or ASCII
.SH SYNOPSIS
.nf
.B #include <time.h>
-.sp
+.PP
.BI "char *asctime(const struct tm *" tm );
-.br
.BI "char *asctime_r(const struct tm *" tm ", char *" buf );
-.sp
+.PP
.BI "char *ctime(const time_t *" timep );
-.br
.BI "char *ctime_r(const time_t *" timep ", char *" buf );
-.sp
+.PP
.BI "struct tm *gmtime(const time_t *" timep );
-.br
.BI "struct tm *gmtime_r(const time_t *" timep ", struct tm *" result );
-.sp
+.PP
.BI "struct tm *localtime(const time_t *" timep );
-.br
.BI "struct tm *localtime_r(const time_t *" timep ", struct tm *" result );
-.sp
+.PP
.BI "time_t mktime(struct tm *" tm );
.fi
+.PP
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.ad l
+.PP
+.BR asctime_r (),
+.BR ctime_r (),
+.BR gmtime_r (),
+.BR localtime_r ():
+.RS
+_POSIX_C_SOURCE
+ || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
+.RE
+.ad
.SH DESCRIPTION
-The \fBctime()\fP, \fBgmtime()\fP and \fBlocaltime()\fP functions all take
-an argument of data type \fItime_t\fP which represents calendar time.
+The
+.BR ctime (),
+.BR gmtime ()
+and
+.BR localtime ()
+functions all take
+an argument of data type \fItime_t\fP, which represents calendar time.
When interpreted as an absolute time value, it represents the number of
-seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal
-Time (UTC).
+seconds elapsed since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
.PP
-The \fBasctime()\fP and \fBmktime()\fP functions both take an argument
-representing broken-down time which is a representation
-separated into year, month, day, etc.
+The
+.BR asctime ()
+and
+.BR mktime ()
+functions both take an argument
+representing broken-down time, which is a representation
+separated into year, month, day, and so on.
.PP
Broken-down time is stored
-in the structure \fItm\fP which is defined in \fI<time.h>\fP as follows:
-.sp
-.RS
-.nf
-.ne 11
-.ta 8n 16n 32n
+in the structure \fItm\fP, which is defined in \fI<time.h>\fP as follows:
+.PP
+.in +4n
+.EX
struct tm {
- int tm_sec; /* seconds */
- int tm_min; /* minutes */
- int tm_hour; /* hours */
- int tm_mday; /* day of the month */
- int tm_mon; /* month */
- int tm_year; /* year */
- int tm_wday; /* day of the week */
- int tm_yday; /* day in the year */
- int tm_isdst; /* daylight saving time */
+ 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 */
};
-.ta
-.fi
-.RE
+.EE
+.in
.PP
The members of the \fItm\fP structure are:
-.TP
+.TP 10
.I tm_sec
-The number of seconds after the minute, normally in the range 0 to 59,
-but can be up to 61 to allow for leap seconds.
+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.
.TP
.I tm_min
-The number of minutes after the hour, in the range 0 to 59.
+The number of minutes after the hour, in the range 0 to 59.
.TP
.I tm_hour
The number of hours past midnight, in the range 0 to 23.
.TP
.I 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
+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.
.PP
.BI ctime( t )
is equivalent to
.BI asctime(localtime( t )) \fR.
-It converts the calendar time \fIt\fP into a string of the form
-.sp
-.RS
-"Wed Jun 30 21:49:08 1993\\n"
-.RE
-.sp
-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
+It converts the calendar time \fIt\fP into a
+null-terminated string of the form
+.PP
+.in +4n
+.EX
+"Wed Jun 30 21:49:08 1993\en"
+.EE
+.in
+.PP
+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 variable \fItzname\fP (see
+functions.
+The function also sets the external
+variables \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP (see
.BR tzset (3))
-with information about the current time zone.
-The re-entrant version \fBctime_r()\fP does the same, but stores the
-string in a user-supplied buffer of length at least 26. It need not
-set \fItzname\fP.
+with information about the current timezone.
+The reentrant version
+.BR 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 \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP.
.PP
-The \fBgmtime()\fP function converts the calendar time \fItimep\fP to
+The
+.BR gmtime ()
+function converts the calendar time \fItimep\fP to
broken-down time representation, expressed in Coordinated Universal Time
-(UTC). It may return NULL when the year does not fit into an integer.
+(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 \fBgmtime_r()\fP function does the same, but stores the data in a
+The
+.BR gmtime_r ()
+function does the same, but stores the data in a
user-supplied struct.
.PP
-The \fBlocaltime()\fP function converts the calendar time \fItimep\fP to
-broken-time representation, expressed relative to the user's specified
-time zone. The function acts as if it called
+The
+.BR localtime ()
+function converts the calendar time \fItimep\fP to
+broken-down time representation,
+expressed relative to the user's specified timezone.
+The function acts as if it called
.BR tzset (3)
-and sets the external variables \fItzname\fP with
-information about the current time zone, \fItimezone\fP with the difference
+and sets the external variables \fItzname\fP with
+information about the current timezone, \fItimezone\fP with the difference
between Coordinated Universal Time (UTC) and local standard time in
-seconds, and \fIdaylight\fP to a non-zero value if daylight savings
+seconds, and \fIdaylight\fP 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 \fBlocaltime_r()\fP function does the same, but stores the data in a
-user-supplied struct. It need not set \fItzname\fP.
+The
+.BR localtime_r ()
+function does the same, but stores the data in a
+user-supplied struct.
+It need not set \fItzname\fP, \fItimezone\fP, and \fIdaylight\fP.
.PP
-The \fBasctime()\fP function converts the broken-down time value
-\fItm\fP into a string with the same format as \fBctime()\fP.
-The return value points to a statically allocated string which might be
+The
+.BR asctime ()
+function converts the broken-down time value
+\fItm\fP into a null-terminated string with the same format as
+.BR 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 \fBasctime_r()\fP function does the same, but stores the string in
-a user-supplied buffer of length at least 26.
-.PP
-The \fBmktime()\fP function converts a broken-down time structure, expressed
-as local time, to calendar time representation. The function ignores
-the specified contents of the structure members \fItm_wday\fP and \fItm_yday\fP
-and recomputes them from the other information in the broken-down time
-structure.
-If structure members are outside their legal interval, they will be
-normalized (so that, e.g., 40 October is changed into 9 November).
-Calling \fBmktime()\fP also sets the external variable \fItzname\fP with
-information about the current time zone. If the specified broken-down
-time cannot be represented as calendar time (seconds since the epoch),
-\fBmktime()\fP returns a value of (time_t)(\-1) and does not alter the
-\fItm_wday\fP and \fItm_yday\fP members of the broken-down time structure.
-.SH "RETURN VALUE"
-Each of these functions returns the value described, or NULL
-(\-1 in case of \fBmktime()\fP) in case an error was detected.
+The
+.BR asctime_r ()
+function does the same, but stores the string in
+a user-supplied buffer which should have room for at least 26 bytes.
+.PP
+The
+.BR 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
+.I tm_wday
+and
+.I tm_yday
+fields.
+The value specified in the
+.I tm_isdst
+field informs
+.BR mktime ()
+whether or not daylight saving time (DST)
+is in effect for the time supplied in the
+.I tm
+structure:
+a positive value means DST is in effect;
+zero means that DST is not in effect;
+and a negative value means that
+.BR mktime ()
+should (use timezone information and system databases to)
+attempt to determine whether DST is in effect at the specified time.
+.PP
+The
+.BR mktime ()
+function modifies the fields of the
+.IR tm
+structure as follows:
+.I tm_wday
+and
+.I 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);
+.I 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
+.BR mktime ()
+also sets the external variable \fItzname\fP with
+information about the current timezone.
+.PP
+If the specified broken-down
+time cannot be represented as calendar time (seconds since the Epoch),
+.BR mktime ()
+returns
+.I (time_t)\ \-1
+and does not alter the
+members of the broken-down time structure.
+.SH RETURN VALUE
+On success,
+.BR gmtime ()
+and
+.BR localtime ()
+return a pointer to a
+.IR "struct\ tm" .
+.PP
+On success,
+.BR gmtime_r ()
+and
+.BR localtime_r ()
+return the address of the structure pointed to by
+.IR result .
+.PP
+On success,
+.BR asctime ()
+and
+.BR ctime ()
+return a pointer to a string.
+.PP
+On success,
+.BR asctime_r ()
+and
+.BR ctime_r ()
+return a pointer to the string pointed to by
+.IR buf .
+.PP
+On success,
+.BR mktime ()
+returns the calendar time (seconds since the Epoch),
+expressed as a value of type
+.IR time_t .
+.PP
+On error,
+.BR mktime ()
+returns the value
+.IR "(time_t)\ -1" .
+The remaining functions return NULL on error.
+On error,
+.I errno
+is set to indicate the cause of the error.
+.SH ERRORS
+.TP
+.B EOVERFLOW
+The result cannot be represented.
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.ad l
+.TS
+allbox;
+lbw14 lb lbw31
+l l l.
+Interface Attribute Value
+T{
+.BR asctime ()
+T} Thread safety MT-Unsafe race:asctime locale
+T{
+.BR asctime_r ()
+T} Thread safety MT-Safe locale
+T{
+.BR ctime ()
+T} Thread safety T{
+MT-Unsafe race:tmbuf
+.br
+race:asctime env locale
+T}
+T{
+.BR ctime_r (),
+.BR gmtime_r (),
+.BR localtime_r (),
+.BR mktime ()
+T} Thread safety MT-Safe env locale
+T{
+.BR gmtime (),
+.BR localtime ()
+T} Thread safety MT-Unsafe race:tmbuf env locale
+.TE
+.ad
+.SH CONFORMING TO
+POSIX.1-2001.
+C89 and C99 specify
+.BR asctime (),
+.BR ctime (),
+.BR gmtime (),
+.BR localtime (),
+and
+.BR mktime ().
+POSIX.1-2008 marks
+.BR asctime (),
+.BR asctime_r (),
+.BR ctime (),
+and
+.BR ctime_r ()
+as obsolete,
+recommending the use of
+.BR strftime (3)
+instead.
.SH NOTES
The four functions
-.BR asctime() ,
-.BR ctime() ,
-.B gmtime()
+.BR asctime (),
+.BR ctime (),
+.BR gmtime ()
and
-.B localtime()
+.BR localtime ()
return a pointer to static data and hence are not thread-safe.
-Thread-safe versions
-.BR asctime_r() ,
-.BR ctime_r() ,
-.B gmtime_r()
+The thread-safe versions,
+.BR asctime_r (),
+.BR ctime_r (),
+.BR gmtime_r ()
+and
+.BR localtime_r (),
+are specified by SUSv2.
+.PP
+POSIX.1-2001 says:
+"The
+.BR asctime (),
+.BR ctime (),
+.BR gmtime (),
and
-.BR localtime_r()
-are specified by SUSv2, and available since libc 5.2.5.
-.LP
-In many implementations, including
-.IR glibc ,
-a 0 in
+.BR localtime ()
+functions shall return values in one of two static objects:
+a broken-down time structure and an array of type
+.IR 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.
+.PP
+In many implementations, including glibc, a 0 in
.I tm_mday
is interpreted as meaning the last day of the preceding month.
-.LP
-The glibc version of struct tm has additional fields
-.sp
-.RS
-.nf
-long tm_gmtoff; /* Seconds east of UTC */
+.PP
+The glibc version of \fIstruct tm\fP has additional fields
+.PP
+.in +4n
+.EX
const char *tm_zone; /* Timezone abbreviation */
-.fi
-.RE
-.sp
-defined when _BSD_SOURCE was set before including
+.EE
+.in
+.PP
+defined when
+.B _BSD_SOURCE
+was set before including
.IR <time.h> .
This is a BSD extension, present in 4.3BSD-Reno.
-.SH "CONFORMING TO"
-SVID 3, POSIX, 4.3BSD, ISO 9899
-.SH "SEE ALSO"
+.PP
+According to POSIX.1-2004,
+.BR localtime ()
+is required to behave as though
+.BR tzset (3)
+was called, while
+.BR localtime_r ()
+does not have this requirement.
+.\" See http://thread.gmane.org/gmane.comp.time.tz/2034/
+For portable code,
+.BR tzset (3)
+should be called before
+.BR localtime_r ().
+.SH SEE ALSO
.BR date (1),
.BR gettimeofday (2),
.BR time (2),
.BR difftime (3),
.BR strftime (3),
.BR strptime (3),
-.BR tzset (3)
+.BR timegm (3),
+.BR tzset (3),
+.BR time (7)
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