NAME
strftime - convert date and time to a string
SYNOPSIS
#include <time.h>
size_t strftime(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr);
DESCRIPTION
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]
%D Equivalent to %m / %d / %y . [ tm_mon, tm_mday, tm_year]
%e 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 %z strftime()
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.
RETURN VALUE
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.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
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.
#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);
...
APPLICATION USAGE
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:
%a The first three characters of %A .
%A One of Sunday, Monday, ..., Saturday.
%b The first three characters of %B .
%B One of January, February, ..., December.
%c Equivalent to %a %b %e %T %Y .
%p One of AM or PM.
%r Equivalent to %I : %M : %S %p .
%x Equivalent to %m / %d / %y .
%X Equivalent to %T .
%Z Implementation-defined.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
asctime() , clock() , ctime() , difftime() , getdate() , gmtime() ,
localtime() , mktime() , strptime() , time() , tzset() , utime() , Base
Definitions volume of IEEE Std 1003.1-2001, Section 7.3.5, LC_TIME,
<time.h>
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 .