NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ENVIRONMENT | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON
STRFTIME(3) Linux Programmer's Manual STRFTIME(3)
strftime - format date and time
#include <time.h>
size_t strftime(char *s, size_t max, const char *format,
const struct tm *tm);
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 international 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 week-based year (see NOTES) with century as a decimal
number. The 4-digit year corresponding 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, that is, 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
seconds, see %T below.
%s The number of seconds since the Epoch, that is, 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 week number (see NOTES) 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 new year. 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 timezone 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
specifier 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 behavior 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).
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 undefined. (This behavior applies since at least 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.
The environment variables TZ and LC_TIME are used.
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 specifier allowed a range of 00 to 61, to allow for the
theoretical possibility of a minute that included a double leap second (there
never has been such a minute).
%G, %g, and %V yield values calculated from the week-based year defined by the
ISO 8601 standard. In this system, weeks start on a Monday, and are numbered
from 01, for the first week, up to 52 or 53, for the last week. Week 1 is the
first week where four or more days fall within the new year (or, synonymously,
week 01 is: the first week of the year that contains a Thursday; or, the week
that has 4 January in it). When three of fewer days of the first calendar
week of the new year fall within that year, then the ISO 8601 week-based
system counts those days as part of week 53 of the preceding year. For
example, 1 January 2010 is a Friday, meaning that just three days of that
calendar week fall in 2010. Thus, the ISO 8601 week-based system considers
these days to be part of week 53 (%V) of the year 2009 (%G) ; week 01 of
ISO 8601 year 2010 starts on Thursday, 4 January 2010.
Glibc provides some extensions for conversion specifications. (These
extensions are not specified in POSIX.1-2001, but a few other systems provide
similar features.) Between the '%' character and the conversion specifier
character, an optional flag and field width may be specified. (These precede
the E or O modifiers, 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
conversion 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.
Some buggy versions of gcc(1) 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(1) 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);
}
Nowadays, gcc(1) provides the -Wno-format-y2k option to prevent the warning,
so that the above workaround is no longer required.
The program below can be used to experiment with strftime().
Some examples of the result string produced by the glibc implementation of
strftime() are as follows:
$ ./a.out '%m'
Result string is "11"
$ ./a.out '%5m'
Result string is "00011"
$ ./a.out '%_5m'
Result string is " 11"
#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 */
date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)
This page is part of release 3.21 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2009-02-24 STRFTIME(3)