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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.
The format specification is a null-terminated string and may contain special character sequences called conversion specifications, each of which is introduced by a '%' character and terminated by some other character known as a conversion specifier character. All other character sequences are ordinary character sequences.
The characters of ordinary character sequences (including
the null byte) are copied verbatim from format to s. However, the characters of
conversion specifications are replaced as follows:
The abbreviated weekday name according to the current locale.
The full weekday name according to the current locale.
The abbreviated month name according to the current locale.
The full month name according to the current locale.
The preferred date and time representation for the current locale.
The century number (year/100) as a 2-digit integer. (SU)
The day of the month as a decimal number (range 01 to 31).
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)
Like %d,
the day of the month as a decimal number, but a leading
zero is replaced by a space. (SU)
Modifier: use alternative format, see below. (SU)
Equivalent to %Y-%m-%d (the ISO 8601
date format). (C99)
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)
Like %G, but without
century, that is, with a 2-digit year (00-99). (TZ)
Equivalent to %b. (SU)
The hour as a decimal number using a 24-hour clock (range 00 to 23).
The hour as a decimal number using a 12-hour clock (range 01 to 12).
The day of the year as a decimal number (range 001 to 366).
The hour (24-hour clock) as a decimal number (range
0 to 23); single digits are preceded by a blank. (See
also %H.) (TZ)
The hour (12-hour clock) as a decimal number (range
1 to 12); single digits are preceded by a blank. (See
also %I.) (TZ)
The month as a decimal number (range 01 to 12).
The minute as a decimal number (range 00 to 59).
A newline character. (SU)
Modifier: use alternative format, see below. (SU)
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".
Like %p
but in lowercase: "am" or "pm" or a corresponding
string for the current locale. (GNU)
The time in a.m. or p.m. notation. In the POSIX locale this is equivalent to %I:%M:%S %p. (SU)
The time in 24-hour notation (%H:%M). (SU) For a
version including the seconds, see %T below.
The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). (TZ)
The second as a decimal number (range 00 to 60). (The range is up to 60 to allow for occasional leap seconds.)
A tab character. (SU)
The time in 24-hour notation (%H:%M:%S). (SU)
The day of the week as a decimal, range 1 to 7,
Monday being 1. See also %w. (SU)
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.
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)
The day of the week as a decimal, range 0 to 6,
Sunday being 0. See also %u.
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.
The preferred date representation for the current locale without the time.
The preferred time representation for the current locale without the date.
The year as a decimal number without a century (range 00 to 99).
The year as a decimal number including the century.
The +hhmm
or −hhmm numeric
timezone (that is, the hour and minute offset from
UTC). (SU)
The timezone 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).
Provided that the result string, including the terminating
null byte, does not exceed max bytes, strftime() returns the number of bytes
(excluding the terminating null byte) placed in the array
s. If the length of
the result string (including the terminating null byte) would
exceed max bytes,
then strftime() returns 0, and
the contents of the array are 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. An
empty format string
will likewise yield an empty string.
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 Monday, 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.
0Pad 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
works only 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.
If the output string would exceed max bytes, errno is not set. This makes it impossible
to distinguish this error case from cases where the
format string
legitimately produces a zero-length output string.
POSIX.1-2001 does not
specify any errno settings for
strftime().
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.
RFC 2822-compliant date format (with an English locale for %a and %b)
"%a, %d %b %Y %T %z"
RFC 822-compliant date format (with an English locale for %a and %b)
"%a, %d %b %y %T %z"
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"
Here's the program source:
#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);
}
This page is part of release 3.52 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/.
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Copyright 1993 David Metcalfe (davidprism.demon.co.uk) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END References consulted: Linux libc source code Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) 386BSD man pages GNU texinfo documentation on glibc date/time functions. Modified Sat Jul 24 18:03:44 1993 by Rik Faith (faithcs.unc.edu) Applied fix by Wolfgang Franke, aeb, 961011 Corrected return value, aeb, 970307 Added Single UNIX Spec conversions and %z, aeb/esr, 990329. 2005-11-22 mtk, added Glibc Notes covering optional 'flag' and 'width' components of conversion specifications. |