STRFTIME(3)

NAME
strftime - generate formatted time information

SYNOPSIS
#include <sys/types.h>
#include <time.h>

size_t strftime(char *s, size_t maxsize, const char *format,
	const struct tm *timeptr);

DESCRIPTION
The following description is transcribed verbatim from the December 7, 1988 draft standard for ANSI C. This draft is essentially identical in technical content to the final version of the standard.

The strftime function places characters into the array pointed to by s as controlled by the string pointed to by format. The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format string consists of zero or more conversion specifiers and ordinary multibyte characters. A conversion specifier consists of a % character followed by a character that determines the behavior of the conversion specifier. All ordinary multibyte characters (including the terminating null character) are copied unchanged into the array. If copying takes place between objects that overlap the behavior is undefined. No more than maxsize characters are placed into the array. Each conversion specifier is replaced by appropriate characters as described in the following list. The appropriate characters are determined by the LC_TIME category of the current locale and by the values contained in the structure pointed to by timeptr.

%a  is replaced by the locale's abbreviated weekday name.

%A  is replaced by the locale's full weekday name.

%b  is replaced by the locale's abbreviated month name.

%B  is replaced by the locale's full month name.

%c  is replaced by the locale's appropriate date and time representation.

%d  is replaced by the day of the month as a decimal number (01-31).

%H  is replaced by the hour (24-hour clock) as a decimal number (00-23).

%I  is replaced by the hour (12-hour clock) as a decimal number (01-12).

%j  is replaced by the day of the year as a decimal number (001-366).

%m  is replaced by the month as a decimal number (01-12).

%M  is replaced by the minute as a decimal number (00-59).

%p  is replaced by the locale's equivalent of the AM/PM designations associated with a 12-hour clock.

%S  is replaced by the second as a decimal number (00-61).

%U  is replaced by the week number of the year (the first Sunday as the first day of week 1) as a decimal number (00-53).

%w  is replaced by the weekday as a decimal number [0 (Sunday)-6].

%W  is replaced by the week number of the year (the first Monday as the first day of week 1) as a decimal number (00-53).

%x  is replaced by the locale's appropriate date representation.

%X  is replaced by the locale's appropriate time representation.

%y  is replaced by the year without century as a decimal number (00-99).

%Y  is replaced by the year with century as a decimal number.

%Z  is replaced by the time zone name or abbreviation, or by no characters if no time zone is determinable.

%%  is replaced by %.

If a conversion specifier is not one of the above, the behavior is undefined.

RETURNS
If the total number of resulting characters including the terminating null character is not more than maxsize, the strftime function returns the number of characters placed into the array pointed to by s not including the terminating null character. Otherwise, zero is returned and the contents of the array are indeterminate.

NON-ANSI EXTENSIONS
If SYSV_EXT is defined when the routine is compiled, then the following additional conversions will be available. These are borrowed from the System V cftime(3) and ascftime(3) routines.

%D  is equivalent to specifying %m/%d/%y.

%e  is replaced by the day of the month, padded with a blank if it is only one digit.

%h  is equivalent to %b, above.

%n  is replaced with a newline character (ASCII LF).

%r  is equivalent to specifying %I:%M:%S %p.

%R  is equivalent to specifying %H:%M.

%T  is equivalent to specifying %H:%M:%S.

%t  is replaced with a TAB character.

If SUNOS_EXT is defined when the routine is compiled, then the following additional conversions will be available. These are borrowed from the SunOS version of strftime.

%k  is replaced by the hour (24-hour clock) as a decimal number (0-23). Single digit numbers are padded with a blank.

%l  is replaced by the hour (12-hour clock) as a decimal number (1-12). Single digit numbers are padded with a blank.

POSIX 1003.2 EXTENSIONS
If POSIX2_DATE is defined, then all of the conversions available with SYSV_EXT and SUNOS_EXT are available, as well as the following additional conversions:

%C  The century, as a number between 00 and 99.

%u  is replaced by the weekday as a decimal number [1 (Monday)-7].

%V  is replaced by the week number of the year (the first Monday as the first day of week 1) as a decimal number (01-53). The method for determining the week number is as specified by ISO 8601 (to wit: if the week containing January 1 has four or more days in the new year, then it is week 1, otherwise it is the highest numbered week of the previous year (52 or 53) and the next week is week 1).

The text of the POSIX standard for the date utility describes %U and %W this way:

%U  is replaced by the week number of the year (the first Sunday as the first day of week 1) as a decimal number (00-53). All days in a new year preceding the first Sunday are considered to be in week 0.

%W  is replaced by the week number of the year (the first Monday as the first day of week 1) as a decimal number (00-53). All days in a new year preceding the first Monday are considered to be in week 0.

In addition, the alternate representations %Ec, %EC, %Ex, %Ey, %EY, %Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, and %Oy are recognized, but their normal representations are used.

VMS EXTENSIONS
If VMS_EXT is defined, then the following additional conversion is available:

%v  The date in VMS format (e.g. 20-JUN-1991).

MAIL HEADER EXTENSIONS
If MAILHEADER_EXT is defined, then the following additional conversion is available:

%z  The timezone offset in a +HHMM format (e.g. the format necessary to produce RFC-822/RFC-1036 date headers).

ISO DATE FORMAT EXTENSIONS
If ISO_DATE_EXT is defined, then all of the conversions available with POSIX2_DATE, SYSV_EXT, and SUNOS_EXT are available, as well as the following additional conversions:

%G  is replaced by the year with century of the ISO week number (see %V, above) as a decimal number.

%g  is replaced by the year without century of the ISO week number, as a decimal number (00-99).

For example, January 1, 1993, is in week 53 of 1992. Thus, the year of its ISO week number is 1992, even though its year is 1993. Similarly, December 31, 1973, is in week 1 of 1974. Thus, the year of its ISO week number is 1974, even though its year is 1973.

SEE ALSO
time(2), ctime(3), localtime(3), tzset(3)

BUGS
This version does not handle multibyte characters or pay attention to the setting of the LC_TIME environment variable.

The ``appropriate'' values used for %c, %x, are %X are those specified by the 1003.2 standard for the POSIX locale.

CAVEATS
The pre-processor symbol POSIX_SEMANTICS is automatically defined, which forces the code to call tzset(3) whenever the TZ environment variable has changed. If this routine will be used in an application that will not be changing TZ, then there may be some performance improvements by not defining POSIX_SEMANTICS.

AUTHOR
Arnold Robbins

INTERNET: arnold@gnu.ai.mit.edu

ACKNOWLEDGEMENTS
Thanks to Geoff Clare <gwc@root.co.uk> for helping debug earlier versions of this routine, and for advice about POSIX semantics. Additional thanks to Arthur David Olsen <ado@elsie.nci.nih.gov> for some code improvements. Thanks also to Tor Lillqvist <tml@tik.vtt.fi> for code fixes to the ISO 8601 code. Thanks to Hume Smith for pointing out a problem with the ISO 8601 code and to Arthur David Olsen for further discussions.