The
strftime() function shall place bytes into the array
pointed to by s as controlled by the string pointed
to by format. The format is a character string,
beginning and ending in its initial shift state, if any. The
format string consists of zero or more conversion
specifications and ordinary characters. A conversion
specification consists of a ’%’
character, possibly followed by an E or O
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 maxsize 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
LC_TIME category of the current locale and by the
values of zero or more members of the broken-down time
structure pointed to by timeptr, as specified in
brackets in the description. If any of the specified values
are outside the normal range, the characters stored are
unspecified.
Local timezone
information is used as though strftime() called
tzset().
The following
conversion specifications are supported:
%a
Replaced by the locale’s abbreviated weekday name.
[ tm_wday]
%A
Replaced by the locale’s full weekday name. [
tm_wday]
%b
Replaced by the locale’s abbreviated month name. [
tm_mon]
%B
Replaced by the locale’s full month name. [
tm_mon]
%c
Replaced by the locale’s appropriate date and time
representation. (See the Base Definitions volume of
IEEE Std 1003.1-2001, <time.h>.)
%C
Replaced by the year divided by 100 and truncated to an
integer, as a decimal number [00,99]. [ tm_year]
%d
Replaced by the day of the month as a decimal number
[01,31]. [ tm_mday]
Replaced by the day of the month as a decimal number
[1,31]; a single digit is preceded by a space. [
tm_mday]
%F
Equivalent to %Y - %m - %d (the
ISO 8601:2000 standard date format). [ tm_year,
tm_mon, tm_mday]
%g
Replaced by the last 2 digits of the week-based year
(see below) as a decimal number [00,99]. [ tm_year,
tm_wday, tm_yday]
%G
Replaced by the week-based year (see below) as a decimal
number (for example, 1977). [ tm_year,
tm_wday, tm_yday]
%h
Equivalent to %b . [ tm_mon]
%H
Replaced by the hour (24-hour clock) as a decimal number
[00,23]. [ tm_hour]
%I
Replaced by the hour (12-hour clock) as a decimal number
[01,12]. [ tm_hour]
%j
Replaced by the day of the year as a decimal number
[001,366]. [ tm_yday]
%m
Replaced by the month as a decimal number [01,12]. [
tm_mon]
%M
Replaced by the minute as a decimal number [00,59]. [
tm_min]
%n
Replaced by a <newline>.
%p
Replaced by the locale’s equivalent of either a.m.
or p.m. [ tm_hour]
%r
Replaced by the time in a.m. and p.m. notation; in
the POSIX locale this shall be equivalent to %I :
%M : %S %p . [ tm_hour, tm_min,
tm_sec]
%R
Replaced by the time in 24-hour notation ( %H :
%M ). [ tm_hour, tm_min]
%S
Replaced by the second as a decimal number [00,60]. [
tm_sec]
%t
Replaced by a <tab>.
%T
Replaced by the time ( %H : %M : %S
). [ tm_hour, tm_min, tm_sec]
%u
Replaced by the weekday as a decimal number [1,7], with
1 representing Monday. [ tm_wday]
%U
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. [
tm_year, tm_wday, tm_yday]
%V
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. [ tm_year, tm_wday,
tm_yday]
%w
Replaced by the weekday as a decimal number [0,6], with
0 representing Sunday. [ tm_wday]
%W
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. [
tm_year, tm_wday, tm_yday]
%x
Replaced by the locale’s appropriate date
representation. (See the Base Definitions volume of
IEEE Std 1003.1-2001, <time.h>.)
%X
Replaced by the locale’s appropriate time
representation. (See the Base Definitions volume of
IEEE Std 1003.1-2001, <time.h>.)
%y
Replaced by the last two digits of the year as a decimal
number [00,99]. [ tm_year]
%Y
Replaced by the year as a decimal number (for example,
1997). [ tm_year]
%z
Replaced by the offset from UTC in the
ISO 8601:2000 standard format ( +hhmm or
-hhmm ), or by no characters if no timezone is
determinable. For example, "-0430" means 4
hours 30 minutes behind UTC (west of Greenwich). If
tm_isdst is zero, the standard time offset is used.
If tm_isdst is greater than zero, the daylight
savings time offset is used. If tm_isdst is negative,
no characters are returned. [ tm_isdst]
%Z
Replaced by the timezone name or abbreviation, or by no
bytes if no timezone information exists. [
tm_isdst]
%%
Replaced by % .
If a conversion
specification does not correspond to any of the above, the
behavior is undefined.
If a struct
tm broken-down time structure is created by
localtime() or localtime_r(), or modified by
mktime(), and the value of TZ is subsequently
modified, the results of the %Z and %zstrftime() conversion specifiers are undefined, when
strftime() is called with such a broken-down time
structure.
If a struct
tm broken-down time structure is created or modified by
gmtime() or gmtime_r(), it is unspecified
whether the result of the %Z and %z conversion
specifiers shall refer to UTC or the current local timezone,
when strftime() is called with such a broken-down
time structure.
Modified
Conversion Specifiers
Some conversion specifiers can be modified by the E
or O 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.
%Ec
Replaced by the locale’s
alternative appropriate date and time representation.
%EC
Replaced by the name of the base year (period) in the
locale’s alternative representation.
%Ex
Replaced by the locale’s alternative date
representation.
%EX
Replaced by the locale’s alternative time
representation.
%Ey
Replaced by the offset from %EC (year only) in
the locale’s alternative representation.
%EY
Replaced by the full alternative year
representation.
%Od
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.
%Oe
Replaced by the day of the month, using the
locale’s alternative numeric symbols, filled as needed
with leading spaces.
%OH
Replaced by the hour (24-hour clock) using the
locale’s alternative numeric symbols.
%OI
Replaced by the hour (12-hour clock) using the
locale’s alternative numeric symbols.
%Om
Replaced by the month using the locale’s
alternative numeric symbols.
%OM
Replaced by the minutes using the locale’s
alternative numeric symbols.
%OS
Replaced by the seconds using the locale’s
alternative numeric symbols.
%Ou
Replaced by the weekday as a number in the
locale’s alternative representation (Monday=1).
%OU
Replaced by the week number of the year (Sunday as the
first day of the week, rules corresponding to %U )
using the locale’s alternative numeric symbols.
%OV
Replaced by the week number of the year (Monday as the
first day of the week, rules corresponding to %V )
using the locale’s alternative numeric symbols.
%Ow
Replaced by the number of the weekday (Sunday=0) using
the locale’s alternative numeric symbols.
%OW
Replaced by the week number of the year (Monday as the
first day of the week) using the locale’s alternative
numeric symbols.
%Oy
Replaced by the year (offset from %C ) using the
locale’s alternative numeric symbols.
%g ,
%G , and %V 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, %G is
replaced by 1998 and %V 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, %G is replaced
by 1998 and %V is replaced by 01.
If a conversion
specifier is not one of the above, the behavior is
undefined.
If the total
number of resulting bytes including the terminating null
byte is not more than maxsize, strftime()
shall return the number of bytes placed into the array
pointed to by s, not including the terminating null
byte. Otherwise, 0 shall be returned and the contents of the
array are unspecified.
Getting a
Localized Date String
The following example first sets the locale to the
user’s default. The locale information will be used in
the nl_langinfo() and strftime() functions.
The nl_langinfo() function returns the localized date
string which specifies how the date is laid out. The
strftime() function takes this information and, using
the tm structure for values, places the date and time
information into datestring.
The range of
values for %S is [00,60] rather than [00,59] to allow
for the occasional leap second.
Some of the
conversion specifications are duplicates of others. They are
included for compatibility with nl_cxtime() and
nl_ascxtime(), which were published in Issue 2.
Applications
should use %Y (4-digit years) in preference to
%y (2-digit years).
In the C
locale, the E and O modifiers are ignored and
the replacement strings for the following specifiers
are:
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./unix/online.html .
