strftime(3) - Linux manual page (original) (raw)
strftime(3) Library Functions Manual strftime(3)
NAME top
strftime - format date and timeLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <time.h>**
**size_t strftime(char** _s_**[restrict .**_max_**], size_t** _max_**,**
**const char *restrict** _format_**,**
**const struct tm *restrict** _tm_**);**
**size_t strftime_l(char** _s_**[restrict .**_max_**], size_t** _max_**,**
**const char *restrict** _format_**,**
**const struct tm *restrict** _tm_**,**
**locale_t** _locale_**);**DESCRIPTION top
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)](../man3/ctime.3.html).
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 _tmwday_.) (The
specific names used in the current locale can be obtained
by calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **ABDAY_**{**1**–**7**} as an argument.)
**%A** The full name of the day of the week according to the
current locale. (Calculated from _tmwday_.) (The specific
names used in the current locale can be obtained by calling
[nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **DAY_**{**1**–**7**} as an argument.)
**%b** The abbreviated month name according to the current locale.
(Calculated from _tmmon_.) (The specific names used in the
current locale can be obtained by calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html)
with **ABMON_**{**1**–**12**} as an argument.)
**%B** The full month name according to the current locale.
(Calculated from _tmmon_.) (The specific names used in the
current locale can be obtained by calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html)
with **MON_**{**1**–**12**} as an argument.)
**%c** The preferred date and time representation for the current
locale. (The specific format used in the current locale
can be obtained by calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **D_T_FMT** as
an argument for the **%c** conversion specification, and with
**ERA_D_T_FMT** for the **%Ec** conversion specification.) (In the
POSIX locale this is equivalent to **%a %b %e %H:%M:%S %Y**.)
**%C** The century number (year/100) as a 2-digit integer. (SU)
(The **%EC** conversion specification corresponds to the name
of the era.) (Calculated from _tmyear_.)
**%d** The day of the month as a decimal number (range 01 to 31).
(Calculated from _tmmday_.)
**%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
_tmmday_.)
**%E** Modifier: use alternative ("era-based") 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 _tmyear_, _tmyday_, and _tmwday_.)
**%g** Like **%G**, but without century, that is, with a 2-digit year
(00–99). (TZ) (Calculated from _tmyear_, _tmyday_, and
_tmwday_.)
**%h** Equivalent to **%b**. (SU)
**%H** The hour as a decimal number using a 24-hour clock (range
00 to 23). (Calculated from _tmhour_.)
**%I** The hour as a decimal number using a 12-hour clock (range
01 to 12). (Calculated from _tmhour_.)
**%j** The day of the year as a decimal number (range 001 to 366).
(Calculated from _tmyday_.)
**%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 _tmhour_.) (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 _tmhour_.) (TZ)
**%m** The month as a decimal number (range 01 to 12).
(Calculated from _tmmon_.)
**%M** The minute as a decimal number (range 00 to 59).
(Calculated from _tmmin_.)
**%n** A newline character. (SU)
**%O** Modifier: use alternative numeric symbols, 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
_tmhour_.) (The specific string representations used for
"AM" and "PM" in the current locale can be obtained by
calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **AM_STR** and **PM_STR**,
respectively.)
**%P** Like **%p** but in lowercase: "am" or "pm" or a corresponding
string for the current locale. (Calculated from _tmhour_.)
(GNU)
**%r** The time in a.m. or p.m. notation. (SU) (The specific
format used in the current locale can be obtained by
calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **T_FMT_AMPM** as an argument.)
(In the POSIX locale this is equivalent to **%I:%M:%S %p**.)
**%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 _tmsec_.)
**%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 _tmwday_.) (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
_tmyday_ and _tmwday_.)
**%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 _tmyear_, _tmyday_, and _tmwday_.)
(SU)
**%w** The day of the week as a decimal, range 0 to 6, Sunday
being 0. See also **%u**. (Calculated from _tmwday_.)
**%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 _tmyday_ and _tmwday_.)
**%x** The preferred date representation for the current locale
without the time. (The specific format used in the current
locale can be obtained by calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **D_FMT**
as an argument for the **%x** conversion specification, and
with **ERA_D_FMT** for the **%Ex** conversion specification.) (In
the POSIX locale this is equivalent to **%m/%d/%y**.)
**%X** The preferred time representation for the current locale
without the date. (The specific format used in the current
locale can be obtained by calling [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html) with **T_FMT**
as an argument for the **%X** conversion specification, and
with **ERA_T_FMT** for the **%EX** conversion specification.) (In
the POSIX locale this is equivalent to **%H:%M:%S**.)
**%y** The year as a decimal number without a century (range 00 to
99). (The **%Ey** conversion specification corresponds to the
year since the beginning of the era denoted by the **%EC**
conversion specification.) (Calculated from _tmyear_)
**%Y** The year as a decimal number including the century. (The
**%EY** conversion specification corresponds to the full
alternative year representation.) (Calculated from
_tmyear_)
**%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)](../man1/date.1.html) 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 rules governing date representation with the
**E** modifier can be obtained by supplying **ERA** as an argument to a
[nl_langinfo(3)](../man3/nl%5Flanginfo.3.html). One example of such alternative forms is the
Japanese era calendar scheme in the **ja_JP** glibc locale.
**strftime_l**() is equivalent to **strftime**(), except it uses the
specified _locale_ instead of the current locale. The behaviour is
undefined if _locale_ is invalid or **LC_GLOBAL_LOCALE**.RETURN VALUE top
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.ENVIRONMENT top
The environment variables **TZ** and **LC_TIME** are used.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌───────────────────────────┬───────────────┬────────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├───────────────────────────┼───────────────┼────────────────────┤
│ **strftime**(), **strftime_l**() │ Thread safety │ MT-Safe env locale │
└───────────────────────────┴───────────────┴────────────────────┘STANDARDS top
**strftime**()
C11, POSIX.1-2008.
**strftime_l**()
POSIX.1-2008.HISTORY top
**strftime**()
SVr4, C89.
**strftime_l**()
POSIX.1-2008.
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)](../man1/date.1.html) 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).NOTES top
ISO 8601 week dates %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 or 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 52 or 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. Similarly, the first two days of January 2011 are considered to be part of week 52 of the year 2010.
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 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.BUGS top
If the output string would exceed _max_ bytes, _[errno](../man3/errno.3.html)_ 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](../man3/errno.3.html)_ settings for
**strftime**().
Some buggy versions of [gcc(1)](../man1/gcc.1.html) 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**, as it gives the
preferred date and time representation. One meets all kinds of
strange obfuscations to circumvent this [gcc(1)](../man1/gcc.1.html) 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)](../man1/gcc.1.html) provides the _-Wno-format-y2k_ option to prevent
the warning, so that the above workaround is no longer required.EXAMPLES top
**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"Example program 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"Program source
#include <stdio.h>
#include <stdlib.h>
#include <time.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);
}SEE ALSO top
[date(1)](../man1/date.1.html), [time(2)](../man2/time.2.html), [ctime(3)](../man3/ctime.3.html), [nl_langinfo(3)](../man3/nl%5Flanginfo.3.html), [setlocale(3)](../man3/setlocale.3.html),
[sprintf(3)](../man3/sprintf.3.html), [strptime(3)](../man3/strptime.3.html)COLOPHON top
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