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 broken-down time structure
tm is defined in
<time.h>. See also
ctime(3).
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 shown in the list below. In this
list, the field(s) employed from the
tm structure are also shown.
- %a
- The abbreviated name of the day of the week according to the current
locale. (Calculated from tm_wday.)
- %A
- The full name of the day of the week according to the current locale.
(Calculated from tm_wday.)
- %b
- The abbreviated month name according to the current locale. (Calculated
from tm_mon.)
- %B
- The full month name according to the current locale. (Calculated from
tm_mon.)
- %c
- The preferred date and time representation for the current locale.
- %C
- The century number (year/100) as a 2-digit integer. (SU) (Calculated from
tm_year.)
- %d
- The day of the month as a decimal number (range 01 to 31). (Calculated
from tm_mday.)
- %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) (Calculated from tm_mday.)
- %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) (Calculated from tm_year, tm_yday, and
tm_wday.)
- %g
- Like %G, but without century, that is, with a 2-digit year
(00–99). (TZ) (Calculated from tm_year, tm_yday, and
tm_wday.)
- %h
- Equivalent to %b. (SU)
- %H
- The hour as a decimal number using a 24-hour clock (range 00 to 23).
(Calculated from tm_hour.)
- %I
- The hour as a decimal number using a 12-hour clock (range 01 to 12).
(Calculated from tm_hour.)
- %j
- The day of the year as a decimal number (range 001 to 366). (Calculated
from tm_yday.)
- %k
- The hour (24-hour clock) as a decimal number (range 0 to 23); single
digits are preceded by a blank. (See also %H.) (Calculated from
tm_hour.) (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.) (Calculated from
tm_hour.) (TZ)
- %m
- The month as a decimal number (range 01 to 12). (Calculated from
tm_mon.)
- %M
- The minute as a decimal number (range 00 to 59). (Calculated from
tm_min.)
- %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". (Calculated from
tm_hour.)
- %P
- Like %p but in lowercase: "am" or "pm" or a
corresponding string for the current locale. (Calculated from
tm_hour.) (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, 1970-01-01 00:00:00 +0000 (UTC).
(TZ) (Calculated from mktime(tm).)
- %S
- The second as a decimal number (range 00 to 60). (The range is up to 60 to
allow for occasional leap seconds.) (Calculated from tm_sec.)
- %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. (Calculated from tm_wday.) (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. (Calculated from tm_yday and
tm_wday.)
- %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.
(Calculated from tm_year, tm_yday, and tm_wday.)
(SU)
- %w
- The day of the week as a decimal, range 0 to 6, Sunday being 0. See also
%u. (Calculated from tm_wday.)
- %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. (Calculated
from tm_yday and tm_wday.)
- %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).
(Calculated from tm_year)
- %Y
- The year as a decimal number including the century. (Calculated from
tm_year)
- %z
- The +hhmm or -hhmm numeric timezone (that is, the hour and
minute offset from UTC). (SU)
- %Z
- 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.
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.
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.
The environment variables
TZ and
LC_TIME are used.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| strftime () |
Thread safety |
MT-Safe env locale |
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.
- 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 uppercase.
- #
- 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"
#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);
}
date(1),
time(2),
ctime(3),
setlocale(3),
sprintf(3),
strptime(3)
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