Provided by: manpages-posix-dev_2.16-1_all 
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 .