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STRFTIME(3) Linux Programmer's Manual STRFTIME(3)
NAME
strftime - format date and time
SYNOPSIS
#include <time.h>
size_t strftime(char *s, size_t max, const char *format,
const struct tm *tm);
DESCRIPTION
The strftime() function formats the broken-down time tm according to the format
specification format and places the result in the character array s of size max.
Ordinary characters placed in the format string are copied to s without conversion.
Conversion specifications are introduced by a '%' character, and terminated by a
conversion specifier character, and are replaced in s as follows:
%a The abbreviated weekday name according to the current locale.
%A The full weekday name according to the current locale.
%b The abbreviated month name according to the current locale.
%B The full month name according to the current locale.
%c The preferred date and time representation for the current locale.
%C The century number (year/100) as a 2-digit integer. (SU)
%d The day of the month as a decimal number (range 01 to 31).
%D Equivalent to %m/%d/%y. (Yecch -- for Americans only. Americans should note
that in other countries %d/%m/%y is rather common. This means that in inter-
national context this format is ambiguous and should not be used.) (SU)
%e Like %d, the day of the month as a decimal number, but a leading zero is
replaced by a space. (SU)
%E Modifier: use alternative format, see below. (SU)
%F Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)
%G The ISO 8601 year with century as a decimal number. The 4-digit year corre-
sponding to the ISO week number (see %V). This has the same format and
value as %y, except that if the ISO week number belongs to the previous or
next year, that year is used instead. (TZ)
%g Like %G, but without century, i.e., with a 2-digit year (00-99). (TZ)
%h Equivalent to %b. (SU)
%H The hour as a decimal number using a 24-hour clock (range 00 to 23).
%I The hour as a decimal number using a 12-hour clock (range 01 to 12).
%j The day of the year as a decimal number (range 001 to 366).
%k The hour (24-hour clock) as a decimal number (range 0 to 23); single digits
are preceded by a blank. (See also %H.) (TZ)
%l The hour (12-hour clock) as a decimal number (range 1 to 12); single digits
are preceded by a blank. (See also %I.) (TZ)
%m The month as a decimal number (range 01 to 12).
%M The minute as a decimal number (range 00 to 59).
%n A newline character. (SU)
%O Modifier: use alternative format, see below. (SU)
%p Either 'AM' or 'PM' according to the given time value, or the corresponding
strings for the current locale. Noon is treated as 'pm' and midnight as
'am'.
%P Like %p but in lowercase: 'am' or 'pm' or a corresponding string for the
current locale. (GNU)
%r The time in a.m. or p.m. notation. In the POSIX locale this is equivalent
to '%I:%M:%S %p'. (SU)
%R The time in 24-hour notation (%H:%M). (SU) For a version including the sec-
onds, see %T below.
%s The number of seconds since the Epoch, i.e., since 1970-01-01 00:00:00 UTC.
(TZ)
%S The second as a decimal number (range 00 to 60). (The range is up to 60 to
allow for occasional leap seconds.)
%t A tab character. (SU)
%T The time in 24-hour notation (%H:%M:%S). (SU)
%u The day of the week as a decimal, range 1 to 7, Monday being 1. See also
%w. (SU)
%U The week number of the current year as a decimal number, range 00 to 53,
starting with the first Sunday as the first day of week 01. See also %V and
%W.
%V The ISO 8601:1988 week number of the current year as a decimal number, range
01 to 53, where week 1 is the first week that has at least 4 days in the
current year, and with Monday as the first day of the week. See also %U and
%W. (SU)
%w The day of the week as a decimal, range 0 to 6, Sunday being 0. See also
%u.
%W The week number of the current year as a decimal number, range 00 to 53,
starting with the first Monday as the first day of week 01.
%x The preferred date representation for the current locale without the time.
%X The preferred time representation for the current locale without the date.
%y The year as a decimal number without a century (range 00 to 99).
%Y The year as a decimal number including the century.
%z The time-zone as hour offset from GMT. Required to emit RFC 822-conformant
dates (using "%a, %d %b %Y %H:%M:%S %z"). (GNU)
%Z The time zone or name or abbreviation.
%+ The date and time in date(1) format. (TZ) (Not supported in glibc2.)
%% A literal '%' character.
Some conversion specifications can be modified by preceding the conversion speci-
fier character by the E or O modifier to indicate that an alternative format should
be used. If the alternative format or specification does not exist for the current
locale, the behaviour will be as if the unmodified conversion specification were
used. (SU) The Single Unix Specification mentions %Ec, %EC, %Ex, %EX, %Ey, %EY,
%Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect
of the O modifier is to use alternative numeric symbols (say, roman numerals), and
that of the E modifier is to use a locale-dependent alternative representation.
The broken-down time structure tm is defined in <time.h>. See also ctime(3).
RETURN VALUE
The strftime() function returns the number of characters placed in the array s, not
including the terminating null byte, provided the string, including the terminating
null byte, fits. Otherwise, it returns 0, and the contents of the array is unde-
fined. (Thus at least since libc 4.4.4; very old versions of libc, such as libc
4.4.1, would return max if the array was too small.)
Note that the return value 0 does not necessarily indicate an error; for example,
in many locales %p yields an empty string.
ENVIRONMENT
The environment variables TZ and LC_TIME are used.
CONFORMING TO
SVr4, C89, C99. There are strict inclusions between the set of conversions given
in ANSI C (unmarked), those given in the Single Unix Specification (marked SU),
those given in Olson's timezone package (marked TZ), and those given in glibc
(marked GNU), except that %+ is not supported in glibc2. On the other hand glibc2
has several more extensions. POSIX.1 only refers to ANSI C; POSIX.2 describes
under date(1) several extensions that could apply to strftime() as well. The %F
conversion is in C99 and POSIX.1-2001.
In SUSv2, the %S specified allowed a range of 00 to 61, to allow for the theoreti-
cal possibility of a minute that included a double leap second (there never has
been such a minute).
GLIBC NOTES
Glibc provides some extensions for conversion specifications. (These extensions
are not specified in POSIX.1-2001, but a few other systems provide similar fea-
tures.) Between the % character and the conversion specifier character, an
optional flag and field width may be specified. (These precede the E or O modi-
fiers, if present.)
The following flag characters are permitted:
_ (underscore) Pad a numeric result string with spaces.
- (dash) Do not pad a numeric result string.
0 Pad a numeric result string with zeros even if the conversion specifier
character uses space-padding by default.
^ Convert alphabetic characters in result string to upper case.
# Swap the case of the result string. (This flag only works with certain con-
version specifier characters, and of these, it is only really useful with
%Z).
An optional decimal width specifier may follow the (possibly absent) flag. If the
natural size of the field is smaller than this width, then the result string is
padded (on the left) to the specified width.
BUGS
Some buggy versions of gcc complain about the use of %c: warning: '%c' yields only
last 2 digits of year in some locales. Of course programmers are encouraged to use
%c, it gives the preferred date and time representation. One meets all kinds of
strange obfuscations to circumvent this gcc problem. A relatively clean one is to
add an intermediate function
size_t my_strftime(char *s, size_t max, const char *fmt, const struct tm
*tm) {
return strftime(s, max, fmt, tm);
}
EXAMPLE
The program below can be used to experiment with strftime().
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
char outstr[200];
time_t t;
struct tm *tmp;
t = time(NULL);
tmp = localtime(&t);
if (tmp == NULL) {
perror("localtime");
exit(EXIT_FAILURE);
}
if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0) {
fprintf(stderr, "strftime returned 0");
exit(EXIT_FAILURE);
}
printf("Result string is \"%s\"\n", outstr);
exit(EXIT_SUCCESS);
} /* main */
Some examples of the result string produced by the glibc implementation of strf-
time() are as follows:
$ ./a.out "%m"
Result string is "11"
$ ./a.out "%5m"
Result string is "00011"
$ ./a.out "%_5m"
Result string is " 11"
SEE ALSO
date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)
GNU 2005-11-23 STRFTIME(3)
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On Apache/2.2.3 (Red Hat)
Under GNU General Public License
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