|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
STRFTIME(3P) POSIX Programmer's Manual STRFTIME(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
strftime, strftime_l — convert date and time to a string
#include <time.h>
size_t strftime(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr);
size_t strftime_l(char *restrict s, size_t maxsize,
const char *restrict format, const struct tm *restrict timeptr,
locale_t locale);
For strftime(): The functionality described on this reference
page is aligned with the ISO C standard. Any conflict between the
requirements described here and the ISO C standard is
unintentional. This volume of POSIX.1‐2017 defers to the ISO C
standard.
The strftime() function shall place bytes into the array pointed
to by s as controlled by the string pointed to by format. The
format is a character string, beginning and ending in its initial
shift state, if any. The format string consists of zero or more
conversion specifications and ordinary characters.
Each conversion specification is introduced by the '%' character
after which the following appear in sequence:
* An optional flag:
0 The zero character ('0'), which specifies that the
character used as the padding character is '0',
+ The <plus-sign> character ('+'), which specifies that
the character used as the padding character is '0', and
that if and only if the field being produced consumes
more than four bytes to represent a year (for %F, %G,
or %Y) or more than two bytes to represent the year
divided by 100 (for %C) then a leading <plus-sign>
character shall be included if the year being processed
is greater than or equal to zero or a leading <hyphen-
minus> character ('-') shall be included if the year is
less than zero.
The default padding character is unspecified.
* An optional minimum field width. If the converted value,
including any leading '+' or '-' sign, has fewer bytes than
the minimum field width and the padding character is not the
NUL character, the output shall be padded on the left (after
any leading '+' or '-' sign) with the padding character.
* An optional E or O modifier.
* A terminating conversion specifier character that indicates
the type of conversion to be applied.
The results are unspecified if more than one flag character is
specified, a flag character is specified without a minimum field
width; a minimum field width is specified without a flag
character; a modifier is specified with a flag or with a minimum
field width; or if a minimum field width is specified for any
conversion specifier other than C, F, G, or Y.
All ordinary characters (including the terminating NUL character)
are copied unchanged into the array. If copying takes place
between objects that overlap, the behavior is undefined. No more
than maxsize bytes are placed into the array. Each conversion
specifier is replaced by appropriate characters as described in
the following list. The appropriate characters are determined
using the LC_TIME category of the current locale and by the
values of zero or more members of the broken-down time structure
pointed to by timeptr, as specified in brackets in the
description. If any of the specified values are outside the
normal range, the characters stored are unspecified.
The strftime_l() function shall be equivalent to the strftime()
function, except that the locale data used is from the locale
represented by locale.
Local timezone information is used as though strftime() called
tzset().
The following conversion specifiers shall be supported:
a Replaced by the locale's abbreviated weekday name.
[tm_wday]
A Replaced by the locale's full weekday name. [tm_wday]
b Replaced by the locale's abbreviated month name. [tm_mon]
B Replaced by the locale's full month name. [tm_mon]
c Replaced by the locale's appropriate date and time
representation. (See the Base Definitions volume of
POSIX.1‐2017, <time.h>.)
C Replaced by the year divided by 100 and truncated to an
integer, as a decimal number. [tm_year]
If a minimum field width is not specified, the number of
characters placed into the array pointed to by s will be
the number of digits in the year divided by 100 or two,
whichever is greater. If a minimum field width is
specified, the number of characters placed into the array
pointed to by s will be the number of digits in the year
divided by 100 or the minimum field width, whichever is
greater.
d Replaced by the day of the month as a decimal number
[01,31]. [tm_mday]
D Equivalent to %m/%d/%y. [tm_mon, tm_mday, tm_year]
e Replaced by the day of the month as a decimal number
[1,31]; a single digit is preceded by a space. [tm_mday]
F Equivalent to %+4Y-%m-%d if no flag and no minimum field
width are specified. [tm_year, tm_mon, tm_mday]
If a minimum field width of x is specified, the year
shall be output as if by the Y specifier (described
below) with whatever flag was given and a minimum field
width of x-6. If x is less than 6, the behavior shall be
as if x equalled 6.
If the minimum field width is specified to be 10, and the
year is four digits long, then the output string produced
will match the ISO 8601:2004 standard subclause 4.1.2.2
complete representation, extended format date
representation of a specific day. If a + flag is
specified, a minimum field width of x is specified, and
x-7 bytes are sufficient to hold the digits of the year
(not including any needed sign character), then the
output will match the ISO 8601:2004 standard subclause
4.1.2.4 complete representation, expanded format date
representation of a specific day.
g Replaced by the last 2 digits of the week-based year (see
below) as a decimal number [00,99]. [tm_year, tm_wday,
tm_yday]
G Replaced by the week-based year (see below) as a decimal
number (for example, 1977). [tm_year, tm_wday, tm_yday]
If a minimum field width is specified, the number of
characters placed into the array pointed to by s will be
the number of digits and leading sign characters (if any)
in the year, or the minimum field width, whichever is
greater.
h Equivalent to %b. [tm_mon]
H Replaced by the hour (24-hour clock) as a decimal number
[00,23]. [tm_hour]
I Replaced by the hour (12-hour clock) as a decimal number
[01,12]. [tm_hour]
j Replaced by the day of the year as a decimal number
[001,366]. [tm_yday]
m Replaced by the month as a decimal number [01,12].
[tm_mon]
M Replaced by the minute as a decimal number [00,59].
[tm_min]
n Replaced by a <newline>.
p Replaced by the locale's equivalent of either a.m. or
p.m. [tm_hour]
r Replaced by the time in a.m. and p.m. notation; in the
POSIX locale this shall be equivalent to %I:%M:%S %p.
[tm_hour, tm_min, tm_sec]
R Replaced by the time in 24-hour notation (%H:%M).
[tm_hour, tm_min]
S Replaced by the second as a decimal number [00,60].
[tm_sec]
t Replaced by a <tab>.
T Replaced by the time (%H:%M:%S). [tm_hour, tm_min,
tm_sec]
u Replaced by the weekday as a decimal number [1,7], with 1
representing Monday. [tm_wday]
U Replaced by the week number of the year as a decimal
number [00,53]. The first Sunday of January is the first
day of week 1; days in the new year before this are in
week 0. [tm_year, tm_wday, tm_yday]
V Replaced by the week number of the year (Monday as the
first day of the week) as a decimal number [01,53]. If
the week containing 1 January has four or more days in
the new year, then it is considered week 1. Otherwise,
it is the last week of the previous year, and the next
week is week 1. Both January 4th and the first Thursday
of January are always in week 1. [tm_year, tm_wday,
tm_yday]
w Replaced by the weekday as a decimal number [0,6], with 0
representing Sunday. [tm_wday]
W Replaced by the week number of the year as a decimal
number [00,53]. The first Monday of January is the first
day of week 1; days in the new year before this are in
week 0. [tm_year, tm_wday, tm_yday]
x Replaced by the locale's appropriate date representation.
(See the Base Definitions volume of POSIX.1‐2017,
<time.h>.)
X Replaced by the locale's appropriate time representation.
(See the Base Definitions volume of POSIX.1‐2017,
<time.h>.)
y Replaced by the last two digits of the year as a decimal
number [00,99]. [tm_year]
Y Replaced by the year as a decimal number (for example,
1997). [tm_year]
If a minimum field width is specified, the number of
characters placed into the array pointed to by s will be
the number of digits and leading sign characters (if any)
in the year, or the minimum field width, whichever is
greater.
z Replaced by the offset from UTC in the ISO 8601:2004
standard format (+hhmm or -hhmm), or by no characters if
no timezone is determinable. For example, "-0430" means 4
hours 30 minutes behind UTC (west of Greenwich). If
tm_isdst is zero, the standard time offset is used. If
tm_isdst is greater than zero, the daylight savings time
offset is used. If tm_isdst is negative, no characters
are returned. [tm_isdst]
Z Replaced by the timezone name or abbreviation, or by no
bytes if no timezone information exists. [tm_isdst]
% Replaced by %.
If a conversion specification does not correspond to any of the
above, the behavior is undefined.
If a struct tm broken-down time structure is created by
localtime() or localtime_r(), or modified by mktime(), and the
value of TZ is subsequently modified, the results of the %Z and
%z strftime() conversion specifiers are undefined, when
strftime() is called with such a broken-down time structure.
If a struct tm broken-down time structure is created or modified
by gmtime() or gmtime_r(), it is unspecified whether the result
of the %Z and %z conversion specifiers shall refer to UTC or the
current local timezone, when strftime() is called with such a
broken-down time structure.
Modified Conversion Specifiers
Some conversion specifiers can be modified by the E or O modifier
characters to indicate that an alternative format or
specification should be used rather than the one normally used by
the unmodified conversion specifier. If the alternative format or
specification does not exist for the current locale (see ERA in
the Base Definitions volume of POSIX.1‐2017, Section 7.3.5,
LC_TIME), the behavior shall be as if the unmodified conversion
specification were used.
%Ec Replaced by the locale's alternative appropriate date and
time representation.
%EC Replaced by the name of the base year (period) in the
locale's alternative representation.
%Ex Replaced by the locale's alternative date representation.
%EX Replaced by the locale's alternative time representation.
%Ey Replaced by the offset from %EC (year only) in the
locale's alternative representation.
%EY Replaced by the full alternative year representation.
%Od Replaced by the day of the month, using the locale's
alternative numeric symbols, filled as needed with
leading zeros if there is any alternative symbol for
zero; otherwise, with leading <space> characters.
%Oe Replaced by the day of the month, using the locale's
alternative numeric symbols, filled as needed with
leading <space> characters.
%OH Replaced by the hour (24-hour clock) using the locale's
alternative numeric symbols.
%OI Replaced by the hour (12-hour clock) using the locale's
alternative numeric symbols.
%Om Replaced by the month using the locale's alternative
numeric symbols.
%OM Replaced by the minutes using the locale's alternative
numeric symbols.
%OS Replaced by the seconds using the locale's alternative
numeric symbols.
%Ou Replaced by the weekday as a number in the locale's
alternative representation (Monday=1).
%OU Replaced by the week number of the year (Sunday as the
first day of the week, rules corresponding to %U) using
the locale's alternative numeric symbols.
%OV Replaced by the week number of the year (Monday as the
first day of the week, rules corresponding to %V) using
the locale's alternative numeric symbols.
%Ow Replaced by the number of the weekday (Sunday=0) using
the locale's alternative numeric symbols.
%OW Replaced by the week number of the year (Monday as the
first day of the week) using the locale's alternative
numeric symbols.
%Oy Replaced by the year (offset from %C) using the locale's
alternative numeric symbols.
%g, %G, and %V give values according to the ISO 8601:2004
standard week-based year. In this system, weeks begin on a Monday
and week 1 of the year is the week that includes January 4th,
which is also the week that includes the first Thursday of the
year, and is also the first week that contains at least four days
in the year. If the first Monday of January is the 2nd, 3rd, or
4th, the preceding days are part of the last week of the
preceding year; thus, for Saturday 2nd January 1999, %G is
replaced by 1998 and %V is replaced by 53. If December 29th,
30th, or 31st is a Monday, it and any following days are part of
week 1 of the following year. Thus, for Tuesday 30th December
1997, %G is replaced by 1998 and %V is replaced by 01.
If a conversion specifier is not one of the above, the behavior
is undefined.
The behavior is undefined if the locale argument to strftime_l()
is the special locale object LC_GLOBAL_LOCALE or is not a valid
locale object handle.
If the total number of resulting bytes including the terminating
null byte is not more than maxsize, these functions shall return
the number of bytes placed into the array pointed to by s, not
including the terminating NUL character. Otherwise, 0 shall be
returned and the contents of the array are unspecified.
No errors are defined.
The following sections are informative.
Getting a Localized Date String
The following example first sets the locale to the user's
default. The locale information will be used in the nl_langinfo()
and strftime() functions. The nl_langinfo() function returns the
localized date string which specifies how the date is laid out.
The strftime() function takes this information and, using the tm
structure for values, places the date and time information into
datestring.
#include <time.h>
#include <locale.h>
#include <langinfo.h>
...
struct tm *tm;
char datestring[256];
...
setlocale (LC_ALL, "");
...
strftime (datestring, sizeof(datestring), nl_langinfo (D_T_FMT), tm);
...
The range of values for %S is [00,60] rather than [00,59] to
allow for the occasional leap second.
Some of the conversion specifications are duplicates of others.
They are included for compatibility with nl_cxtime() and
nl_ascxtime(), which were published in Issue 2.
The %C, %F, %G, and %Y format specifiers in strftime() always
print full values, but the strptime() %C, %F, and %Y format
specifiers only scan two digits (assumed to be the first two
digits of a four-digit year) for %C and four digits (assumed to
be the entire (four-digit) year) for %F and %Y. This mimics the
behavior of printf() and scanf(); that is:
printf("%2d", x = 1000);
prints "1000", but:
scanf(%2d", &x);
when given "1000" as input will only store 10 in x).
Applications using extended ranges of years must be sure that the
number of digits specified for scanning years with strptime()
matches the number of digits that will actually be present in the
input stream. Historic implementations of the %Y conversion
specification (with no flags and no minimum field width) produced
different output formats. Some always produced at least four
digits (with 0 fill for years from 0 through 999) while others
only produced the number of digits present in the year (with no
fill and no padding). These two forms can be produced with the
'0' flag and a minimum field width options using the conversions
specifications %04Y and %01Y, respectively.
In the past, the C and POSIX standards specified that %F produced
an ISO 8601:2004 standard date format, but didn't specify which
one. For years in the range [0001,9999], POSIX.1‐2008 requires
that the output produced match the ISO 8601:2004 standard
complete representation extended format (YYYY-MM-DD) and for
years outside of this range produce output that matches the
ISO 8601:2004 standard expanded representation extended format
(<+/-><Underline>Y</Underline>YYYY-MM-DD). To fully meet
ISO 8601:2004 standard requirements, the producer and consumer
must agree on a date format that has a specific number of bytes
reserved to hold the characters used to represent the years that
is sufficiently large to hold all values that will be shared. For
example, the %+13F conversion specification will produce output
matching the format "<+/->YYYYYY-MM-DD" (a leading '+' or '-'
sign; a six-digit, 0-filled year; a '-'; a two-digit, leading
0-filled month; another '-'; and the two-digit, leading 0-filled
day within the month).
Note that if the year being printed is greater than 9999, the
resulting string from the unadorned %F conversion specifications
will not conform to the ISO 8601:2004 standard extended format,
complete representation for a date and will instead be an
extended format, expanded representation (presumably without the
required agreement between the date's producer and consumer).
In the C or POSIX locale, the E and O modifiers are ignored and
the replacement strings for the following specifiers are:
%a The first three characters of %A.
%A One of Sunday, Monday, ..., Saturday.
%b The first three characters of %B.
%B One of January, February, ..., December.
%c Equivalent to %a %b %e %T %Y.
%p One of AM or PM.
%r Equivalent to %I:%M:%S %p.
%x Equivalent to %m/%d/%y.
%X Equivalent to %T.
%Z Implementation-defined.
The %Y conversion specification to strftime() was frequently
assumed to be a four-digit year, but the ISO C standard does not
specify that %Y is restricted to any subset of allowed values
from the tm_year field. Similarly, the %C conversion
specification was assumed to be a two-digit field and the first
part of the output from the %F conversion specification was
assumed to be a four-digit field. With tm_year being a signed 32
or more-bit int and with many current implementations supporting
64-bit time_t types in one or more programming environments,
these assumptions are clearly wrong.
POSIX.1‐2008 now allows the format specifications %0xC, %0xF,
%0xG, and %0xY (where 'x' is a string of decimal digits used to
specify printing and scanning of a string of x decimal digits)
with leading zero fill characters. Allowing applications to set
the field width enables them to agree on the number of digits to
be printed and scanned in the ISO 8601:2004 standard expanded
representation of a year (for %F, %G, and %Y) or all but the last
two digits of the year (for %C). This is based on a feature in
some versions of GNU libc's strftime(). The GNU version allows
specifying space, zero, or no-fill characters in strftime()
format strings, but does not allow any flags to be specified in
strptime() format strings. These implementations also allow these
flags to be specified for any numeric field. POSIX.1‐2008 only
requires the zero fill flag ('0') and only requires that it be
recognized when processing %C, %F, %G, and %Y specifications when
a minimum field width is also specified. The '0' flag is the only
flag needed to produce and scan the ISO 8601:2004 standard year
fields using the extended format forms. POSIX.1‐2008 also allows
applications to specify the same flag and field width specifiers
to be used in both strftime() and strptime() format strings for
symmetry. Systems may provide other flag characters and may
accept flags in conjunction with conversion specifiers other than
%C, %F, %G, and %Y; but portable applications cannot depend on
such extensions.
POSIX.1‐2008 now also allows the format specifications %+xC,
%+xF, %+xG, and %+xY (where 'x' is a string of decimal digits
used to specify printing and scanning of a string of 'x' decimal
digits) with leading zero fill characters and a leading '+' sign
character if the year being converted is more than four digits or
a minimum field width is specified that allows room for more than
four digits for the year. This allows date providers and
consumers to agree on a specific number of digits to represent a
year as required by the ISO 8601:2004 standard expanded
representation formats. The expanded representation formats all
require the year to begin with a leading '+' or '-' sign. (All
of these specifiers can also provide a leading '-' sign for
negative years. Since negative years and the year 0 don't fit
well with the Gregorian or Julian calendars, the normal ranges of
dates start with year 1. The ISO C standard allows tm_year to
assume values corresponding to years before year 1, but the use
of such years provided unspecified results.)
Some earlier version of this standard specified that applications
wanting to use strptime() to scan dates and times printed by
strftime() should provide non-digit characters between fields to
separate years from months and days. It also supported %F to
print and scan the ISO 8601:2004 standard extended format,
complete representation date for years 1 through 9999 (i.e.,
YYYY-MM-DD). However, many applications were written to print
(using strftime()) and scan (using strptime()) dates written
using the basic format complete representation (four-digit years)
and truncated representation (two-digit years) specified by the
ISO 8601:2004 standard representation of dates and times which do
not have any separation characters between fields. The
ISO 8601:2004 standard also specifies basic format expanded
representation where the creator and consumer of these fields
agree beforehand to represent years as leading zero-filled
strings of an agreed length of more than four digits to represent
a year (again with no separation characters when year, month, and
day are all displayed). Applications producing and consuming
expanded representations are encouraged to use the '+' flag and
an appropriate maximum field width to scan the year including the
leading sign. Note that even without the '+' flag, years less
than zero may be represented with a leading <hyphen-minus> for
%F, %G, and %Y conversion specifications. Using negative years
results in unspecified behavior.
If a format specification %+xF with the field width x greater
than 11 is specified and the width is large enough to display the
full year, the output string produced will match the
ISO 8601:2004 standard subclause 4.1.2.4 expanded representation,
extended format date representation for a specific day. (For
years in the range [1,99999], %+12F is sufficient for an agreed
five-digit year with a leading sign using the ISO 8601:2004
standard expanded representation, extended format for a specific
day "<+/->YYYYY-MM-DD".) Note also that years less than 0 may
produce a leading <hyphen-minus> character ('-') when using %Y or
%C whether or not the '0' or '+' flags are used.
The difference between the '0' flag and the '+' flag is whether
the leading '+' character will be provided for years >9999 as
required for the ISO 8601:2004 standard extended representation
format containing a year. For example:
┌────────┬──────────────────────────┬─────────────┬────────────┐
│ │ │ strftime() │ strptime() │
│ Year │ Conversion Specification │ Output │ Scan Back │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 1970 │ %Y │ 1970 │ 1970 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 1970 │ %+4Y │ 1970 │ 1970 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 27 │ %Y │ 27 or 0027 │ 27 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 270 │ %Y │ 270 or 0270 │ 270 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 270 │ %+4Y │ 0270 │ 270 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 17 │ %C%y │ 0017 │ 17 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 270 │ %C%y │ 0270 │ 270 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 12345 │ %Y │ 12345 │ 1234* │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 12345 │ %+4Y │ +12345 │ 123* │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 12345 │ %05Y │ 12345 │ 12345 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 270 │ %+5Y or %+3C%y │ +0270 │ 270 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 12345 │ %+5Y or %+3C%y │ +12345 │ 1234* │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 12345 │ %06Y or %04C%y │ 012345 │ 12345 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 12345 │ %+6Y or %+4C%y │ +12345 │ 12345 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 123456 │ %08Y or %06C%y │ 00123456 │ 123456 │
├────────┼──────────────────────────┼─────────────┼────────────┤
│ 123456 │ %+8Y or %+6C%y │ +0123456 │ 123456 │
└────────┴──────────────────────────┴─────────────┴────────────┘
In the cases above marked with a * in the strptime() scan back
field, the implied or specified number of characters scanned by
strptime() was less than the number of characters output by
strftime() using the same format; so the remaining digits of the
year were dropped when the output date produced by strftime() was
scanned back in by strptime().
None.
asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p),
gmtime(3p), localtime(3p), mktime(3p), strptime(3p), time(3p),
tzset(3p), uselocale(3p), utime(3p)
The Base Definitions volume of POSIX.1‐2017, Section 7.3.5,
LC_TIME, time.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any
discrepancy between this version and the original IEEE and The
Open Group Standard, the original IEEE and The Open Group
Standard is the referee document. The original Standard can be
obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 STRFTIME(3P)
Pages that refer to this page: langinfo.h(0p), time.h(0p), date(1p), asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p), gmtime(3p), localeconv(3p), localtime(3p), mktime(3p), strptime(3p), time(3p), tzset(3p), wcsftime(3p)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|