path: root/man3p/strftime.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "STRFTIME" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" strftime 
.SH NAME
strftime \- convert date and time to a string
.SH SYNOPSIS
.LP
\fB#include <time.h>
.br
.sp
size_t strftime(char *restrict\fP \fIs\fP\fB, size_t\fP \fImaxsize\fP\fB,
.br
\ \ \ \ \ \  const char *restrict\fP \fIformat\fP\fB, const struct
tm *restrict\fP
\fItimeptr\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIstrftime\fP() function shall place bytes into the array pointed
to by \fIs\fP as controlled by the string pointed to by
\fIformat\fP. The format is a character string, beginning and ending
in its initial shift state, if any. The \fIformat\fP string
consists of zero or more conversion specifications and ordinary characters.
A conversion specification consists of a \fB'%'\fP
character, possibly followed by an \fBE\fP or \fBO\fP modifier, and
a terminating conversion specifier character that
determines the conversion specification's behavior. All ordinary characters
(including the terminating null byte) are copied
unchanged into the array. If copying takes place between objects that
overlap, the behavior is undefined. No more than
\fImaxsize\fP bytes are placed into the array. Each conversion specifier
is replaced by appropriate characters as described in the
following list. The appropriate characters are determined using the
\fILC_TIME\fP category of the current locale and by the values
of zero or more members of the broken-down time structure pointed
to by \fItimeptr\fP, as specified in brackets in the
description. If any of the specified values are outside the normal
range, the characters stored are unspecified.
.LP
Local timezone information is used as though \fIstrftime\fP() called
\fItzset\fP(). 
.LP
The following conversion specifications are supported:
.TP 7
\fB%a\fP
Replaced by the locale's abbreviated weekday name. [ \fItm_wday\fP]
.TP 7
\fB%A\fP
Replaced by the locale's full weekday name. [ \fItm_wday\fP]
.TP 7
\fB%b\fP
Replaced by the locale's abbreviated month name. [ \fItm_mon\fP]
.TP 7
\fB%B\fP
Replaced by the locale's full month name. [ \fItm_mon\fP]
.TP 7
\fB%c\fP
Replaced by the locale's appropriate date and time representation.
(See the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<time.h>\fP.)
.TP 7
\fB%C\fP
Replaced by the year divided by 100 and truncated to an integer, as
a decimal number [00,99]. [ \fItm_year\fP]
.TP 7
\fB%d\fP
Replaced by the day of the month as a decimal number [01,31]. [ \fItm_mday\fP]
.TP 7
\fB%D\fP
Equivalent to \fB%m\fP / \fB%d\fP / \fB%y\fP . [ \fItm_mon\fP, \fItm_mday\fP,
\fItm_year\fP]
.TP 7
\fB%e\fP
Replaced by the day of the month as a decimal number [1,31]; a single
digit is preceded by a space. [ \fItm_mday\fP]
.TP 7
\fB%F\fP
Equivalent to \fB%Y\fP - \fB%m\fP - \fB%d\fP (the ISO\ 8601:2000 standard
date format). [ \fItm_year\fP,
\fItm_mon\fP, \fItm_mday\fP]
.TP 7
\fB%g\fP
Replaced by the last 2 digits of the week-based year (see below) as
a decimal number [00,99]. [ \fItm_year\fP, \fItm_wday\fP,
\fItm_yday\fP]
.TP 7
\fB%G\fP
Replaced by the week-based year (see below) as a decimal number (for
example, 1977). [ \fItm_year\fP, \fItm_wday\fP,
\fItm_yday\fP]
.TP 7
\fB%h\fP
Equivalent to \fB%b\fP . [ \fItm_mon\fP]
.TP 7
\fB%H\fP
Replaced by the hour (24-hour clock) as a decimal number [00,23].
[ \fItm_hour\fP]
.TP 7
\fB%I\fP
Replaced by the hour (12-hour clock) as a decimal number [01,12].
[ \fItm_hour\fP]
.TP 7
\fB%j\fP
Replaced by the day of the year as a decimal number [001,366]. [ \fItm_yday\fP]
.TP 7
\fB%m\fP
Replaced by the month as a decimal number [01,12]. [ \fItm_mon\fP]
.TP 7
\fB%M\fP
Replaced by the minute as a decimal number [00,59]. [ \fItm_min\fP]
.TP 7
\fB%n\fP
Replaced by a <newline>.
.TP 7
\fB%p\fP
Replaced by the locale's equivalent of either a.m. or p.m. [ \fItm_hour\fP]
.TP 7
\fB%r\fP
Replaced by the time in a.m. and p.m. notation;   \ in the POSIX locale
this shall be equivalent to \fB%I\fP :
\fB%M\fP : \fB%S\fP \fB%p\fP .  [ \fItm_hour\fP,
\fItm_min\fP, \fItm_sec\fP]
.TP 7
\fB%R\fP
Replaced by the time in 24-hour notation ( \fB%H\fP : \fB%M\fP ).
[ \fItm_hour\fP, \fItm_min\fP]
.TP 7
\fB%S\fP
Replaced by the second as a decimal number [00,60]. [ \fItm_sec\fP]
.TP 7
\fB%t\fP
Replaced by a <tab>.
.TP 7
\fB%T\fP
Replaced by the time ( \fB%H\fP : \fB%M\fP : \fB%S\fP ). [ \fItm_hour\fP,
\fItm_min\fP, \fItm_sec\fP]
.TP 7
\fB%u\fP
Replaced by the weekday as a decimal number [1,7], with 1 representing
Monday. [ \fItm_wday\fP]
.TP 7
\fB%U\fP
Replaced by the week number of the year as a decimal number [00,53].
The first Sunday of January is the first day of week 1;
days in the new year before this are in week 0. [ \fItm_year\fP, \fItm_wday\fP,
\fItm_yday\fP]
.TP 7
\fB%V\fP
Replaced by the week number of the year (Monday as the first day of
the week) as a decimal number [01,53]. If the week
containing 1 January has four or more days in the new year, then it
is considered week 1. Otherwise, it is the last week of the
previous year, and the next week is week 1. Both January 4th and the
first Thursday of January are always in week 1. [
\fItm_year\fP, \fItm_wday\fP, \fItm_yday\fP]
.TP 7
\fB%w\fP
Replaced by the weekday as a decimal number [0,6], with 0 representing
Sunday. [ \fItm_wday\fP]
.TP 7
\fB%W\fP
Replaced by the week number of the year as a decimal number [00,53].
The first Monday of January is the first day of week 1;
days in the new year before this are in week 0. [ \fItm_year\fP, \fItm_wday\fP,
\fItm_yday\fP]
.TP 7
\fB%x\fP
Replaced by the locale's appropriate date representation. (See the
Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<time.h>\fP.)
.TP 7
\fB%X\fP
Replaced by the locale's appropriate time representation. (See the
Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<time.h>\fP.)
.TP 7
\fB%y\fP
Replaced by the last two digits of the year as a decimal number [00,99].
[ \fItm_year\fP]
.TP 7
\fB%Y\fP
Replaced by the year as a decimal number (for example, 1997). [ \fItm_year\fP]
.TP 7
\fB%z\fP
Replaced by the offset from UTC in the ISO\ 8601:2000 standard format
( \fB+hhmm\fP or \fB-hhmm\fP ), or by no
characters if no timezone is determinable. For example, \fB"-0430"\fP
means 4 hours 30 minutes behind UTC (west of Greenwich).
\ If \fItm_isdst\fP is zero, the standard time offset is used. If
\fItm_isdst\fP is greater than zero, the daylight savings
time offset is used. If \fItm_isdst\fP is negative, no characters
are returned.  [ \fItm_isdst\fP]
.TP 7
\fB%Z\fP
Replaced by the timezone name or abbreviation, or by no bytes if no
timezone information exists. [ \fItm_isdst\fP]
.TP 7
\fB%%\fP
Replaced by \fB%\fP .
.sp
.LP
If a conversion specification does not correspond to any of the above,
the behavior is undefined.
.LP
If
a \fBstruct tm\fP broken-down time structure is created by \fIlocaltime\fP()
or \fIlocaltime_r\fP(), or modified by \fImktime\fP(), and the value
of \fITZ\fP is subsequently modified, the results of the \fB%Z\fP
and \fB%z\fP \fIstrftime\fP() conversion specifiers are undefined,
when \fIstrftime\fP() is called with such a broken-down
time structure.
.LP
If a \fBstruct tm\fP broken-down time structure is created or modified
by \fIgmtime\fP() or \fIgmtime_r\fP(), it is unspecified
whether the result of the \fB%Z\fP and \fB%z\fP conversion specifiers
shall refer to UTC or the current local timezone, when
\fIstrftime\fP() is called with such a broken-down time structure.
.SS Modified Conversion Specifiers
.LP
Some conversion specifiers can be modified by the \fBE\fP or \fBO\fP
modifier characters to indicate that an alternative
format or specification should be used rather than the one normally
used by the unmodified conversion specifier. If the alternative
format or specification does not exist for the current locale (see
ERA in the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Section 7.3.5, LC_TIME), the behavior shall
be as if the unmodified conversion specification were used.
.TP 7
\fB%Ec\fP
Replaced by the locale's alternative appropriate date and time representation.
.TP 7
\fB%EC\fP
Replaced by the name of the base year (period) in the locale's alternative
representation.
.TP 7
\fB%Ex\fP
Replaced by the locale's alternative date representation.
.TP 7
\fB%EX\fP
Replaced by the locale's alternative time representation.
.TP 7
\fB%Ey\fP
Replaced by the offset from \fB%EC\fP (year only) in the locale's
alternative representation.
.TP 7
\fB%EY\fP
Replaced by the full alternative year representation.
.TP 7
\fB%Od\fP
Replaced by the day of the month, using the locale's alternative numeric
symbols, filled as needed with leading zeros if there
is any alternative symbol for zero; otherwise, with leading spaces.
.TP 7
\fB%Oe\fP
Replaced by the day of the month, using the locale's alternative numeric
symbols, filled as needed with leading spaces.
.TP 7
\fB%OH\fP
Replaced by the hour (24-hour clock) using the locale's alternative
numeric symbols.
.TP 7
\fB%OI\fP
Replaced by the hour (12-hour clock) using the locale's alternative
numeric symbols.
.TP 7
\fB%Om\fP
Replaced by the month using the locale's alternative numeric symbols.
.TP 7
\fB%OM\fP
Replaced by the minutes using the locale's alternative numeric symbols.
.TP 7
\fB%OS\fP
Replaced by the seconds using the locale's alternative numeric symbols.
.TP 7
\fB%Ou\fP
Replaced by the weekday as a number in the locale's alternative representation
(Monday=1).
.TP 7
\fB%OU\fP
Replaced by the week number of the year (Sunday as the first day of
the week, rules corresponding to \fB%U\fP ) using the
locale's alternative numeric symbols.
.TP 7
\fB%OV\fP
Replaced by the week number of the year (Monday as the first day of
the week, rules corresponding to \fB%V\fP ) using the
locale's alternative numeric symbols.
.TP 7
\fB%Ow\fP
Replaced by the number of the weekday (Sunday=0) using the locale's
alternative numeric symbols.
.TP 7
\fB%OW\fP
Replaced by the week number of the year (Monday as the first day of
the week) using the locale's alternative numeric
symbols.
.TP 7
\fB%Oy\fP
Replaced by the year (offset from \fB%C\fP ) using the locale's alternative
numeric symbols.
.sp
.LP
\fB%g\fP , \fB%G\fP , and \fB%V\fP give values according to the ISO\ 8601:2000
standard week-based year. In this
system, weeks begin on a Monday and week 1 of the year is the week
that includes January 4th, which is also the week that includes
the first Thursday of the year, and is also the first week that contains
at least four days in the year. If the first Monday of
January is the 2nd, 3rd, or 4th, the preceding days are part of the
last week of the preceding year; thus, for Saturday 2nd January
1999, \fB%G\fP is replaced by 1998 and \fB%V\fP is replaced by 53.
If December 29th, 30th, or 31st is a Monday, it and any
following days are part of week 1 of the following year. Thus, for
Tuesday 30th December 1997, \fB%G\fP is replaced by 1998 and
\fB%V\fP is replaced by 01.
.LP
If a conversion specifier is not one of the above, the behavior is
undefined.
.SH RETURN VALUE
.LP
If the total number of resulting bytes including the terminating null
byte is not more than \fImaxsize\fP, \fIstrftime\fP()
shall return the number of bytes placed into the array pointed to
by \fIs\fP, not including the terminating null byte. Otherwise,
0 shall be returned and the contents of the array are unspecified.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Getting a Localized Date String
.LP
The following example first sets the locale to the user's default.
The locale information will be used in the \fInl_langinfo\fP() and
\fIstrftime\fP() functions. The \fInl_langinfo\fP() function returns
the localized date string which specifies how the date is
laid out. The \fIstrftime\fP() function takes this information and,
using the \fBtm\fP structure for values, places the date and
time information into \fIdatestring\fP.
.sp
.RS
.nf

\fB#include <time.h>
#include <locale.h>
#include <langinfo.h>
\&...
struct tm *tm;
char datestring[256];
\&...
setlocale (LC_ALL, "");
\&...
strftime (datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm);
\&...
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
The range of values for \fB%S\fP is [00,60] rather than [00,59] to
allow for the occasional leap second.
.LP
Some of the conversion specifications are duplicates of others. They
are included for compatibility with \fInl_cxtime\fP() and
\fInl_ascxtime\fP(), which were published in Issue 2.
.LP
Applications should use \fB%Y\fP (4-digit years) in preference to
\fB%y\fP (2-digit years).
.LP
In the C locale, the \fBE\fP and \fBO\fP modifiers are ignored and
the replacement strings for the following specifiers
are:
.TP 7
\fB%a\fP
The first three characters of \fB%A\fP .
.TP 7
\fB%A\fP
One of Sunday, Monday, ..., Saturday.
.TP 7
\fB%b\fP
The first three characters of \fB%B\fP .
.TP 7
\fB%B\fP
One of January, February, ..., December.
.TP 7
\fB%c\fP
Equivalent to \fB%a\fP \fB%b\fP \fB%e\fP \fB%T\fP \fB%Y\fP .
.TP 7
\fB%p\fP
One of AM or PM.
.TP 7
\fB%r\fP
Equivalent to \fB%I\fP : \fB%M\fP : \fB%S\fP \fB%p\fP .
.TP 7
\fB%x\fP
Equivalent to \fB%m\fP / \fB%d\fP / \fB%y\fP .
.TP 7
\fB%X\fP
Equivalent to \fB%T\fP .
.TP 7
\fB%Z\fP
Implementation-defined.
.sp
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIasctime\fP() , \fIclock\fP() , \fIctime\fP()
, \fIdifftime\fP() , \fIgetdate\fP() , \fIgmtime\fP() , \fIlocaltime\fP()
, \fImktime\fP() ,
\fIstrptime\fP() , \fItime\fP() , \fItzset\fP() ,
\fIutime\fP() , Base Definitions volume of IEEE\ Std\ 1003.1-2001,
Section 7.3.5, LC_TIME, \fI<time.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .