man3/strftime.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\"
.\" 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 (faith@cs.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.
.\"
.TH STRFTIME 3  2017-09-15 "GNU" "Linux Programmer's Manual"
.SH NAME
strftime \- format date and time
.SH SYNOPSIS
.nf
.B #include <time.h>
.PP
.BI "size_t strftime(char *" s ", size_t " max ", const char *" format ,
.BI "                const struct tm *" tm );
.fi
.SH DESCRIPTION
The
.BR strftime ()
function formats the broken-down time
.I tm
according to the format specification
.I format
and places the
result in the character array
.I s
of size
.IR max .
The broken-down time structure
.I tm
is defined in
.IR <time.h> .
See also
.BR ctime (3).
.\" FIXME . POSIX says: Local timezone information is used as though
.\" strftime() called tzset().  But this doesn't appear to be the case
.PP
The format specification is a null-terminated string and may contain
special character sequences called
.IR "conversion specifications",
each of which is introduced by a \(aq%\(aq character and terminated by
some other character known as a
.IR "conversion specifier character".
All other character sequences are
.IR "ordinary character sequences".
.PP
The characters of ordinary character sequences (including the null byte)
are copied verbatim from
.I format
to
.IR s .
However, the characters
of conversion specifications are replaced as shown in the list below.
In this list, the field(s) employed from the
.I tm
structure are also shown.
.TP
.B %a
The abbreviated name of the day of the week according to the current locale.
(Calculated from
.IR tm_wday .)
.TP
.B %A
The full name of the day of the week according to the current locale.
(Calculated from
.IR tm_wday .)
.TP
.B %b
The abbreviated month name according to the current locale.
(Calculated from
.IR tm_mon .)
.TP
.B %B
The full month name according to the current locale.
(Calculated from
.IR tm_mon .)
.TP
.B %c
The preferred date and time representation for the current locale.
.TP
.B %C
The century number (year/100) as a 2-digit integer. (SU)
(Calculated from
.IR tm_year .)
.TP
.B %d
The day of the month as a decimal number (range 01 to 31).
(Calculated from
.IR tm_mday .)
.TP
.B %D
Equivalent to
.BR %m/%d/%y .
(Yecch\(emfor Americans only.
Americans should note that in other countries
.B %d/%m/%y
is rather common.
This means that in international context this format is
ambiguous and should not be used.) (SU)
.TP
.B %e
Like
.BR %d ,
the day of the month as a decimal number, but a leading
zero is replaced by a space. (SU)
(Calculated from
.IR tm_mday .)
.TP
.B %E
Modifier: use alternative format, see below. (SU)
.TP
.B %F
Equivalent to
.B %Y-%m-%d
(the ISO\ 8601 date format). (C99)
.TP
.B %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
.BR %V ).
This has the same format and value as
.BR %Y ,
except that if the ISO week number belongs to the previous or next year,
that year is used instead. (TZ)
(Calculated from
.IR tm_year ,
.IR tm_yday ,
and
.IR tm_wday .)
.TP
.B %g
Like
.BR %G ,
but without century, that is, with a 2-digit year (00\(en99). (TZ)
(Calculated from
.IR tm_year ,
.IR tm_yday ,
and
.IR tm_wday .)
.TP
.B %h
Equivalent to
.BR %b .
(SU)
.TP
.B %H
The hour as a decimal number using a 24-hour clock (range 00 to 23).
(Calculated from
.IR tm_hour .)
.TP
.B %I
The hour as a decimal number using a 12-hour clock (range 01 to 12).
(Calculated from
.IR tm_hour .)
.TP
.B %j
The day of the year as a decimal number (range 001 to 366).
(Calculated from
.IR tm_yday .)
.TP
.B %k
The hour (24-hour clock) as a decimal number (range 0 to 23);
single digits are preceded by a blank.
(See also
.BR %H .)
(Calculated from
.IR tm_hour .)
(TZ)
.TP
.B %l
The hour (12-hour clock) as a decimal number (range 1 to 12);
single digits are preceded by a blank.
(See also
.BR %I .)
(Calculated from
.IR tm_hour .)
(TZ)
.TP
.B %m
The month as a decimal number (range 01 to 12).
(Calculated from
.IR tm_mon .)
.TP
.B %M
The minute as a decimal number (range 00 to 59).
(Calculated from
.IR tm_min .)
.TP
.B %n
A newline character. (SU)
.TP
.B %O
Modifier: use alternative format, see below. (SU)
.TP
.B %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
.IR tm_hour .)
.TP
.B %P
Like
.B %p
but in lowercase: "am" or "pm" or a corresponding
string for the current locale.
(Calculated from
.IR tm_hour .)
(GNU)
.TP
.B %r
The time in a.m. or p.m. notation.
In the POSIX locale this is equivalent to
.BR "%I:%M:%S %p" .
(SU)
.TP
.B %R
The time in 24-hour notation
.RB ( %H:%M ).
(SU)
For a version including the seconds, see
.B %T
below.
.TP
.B %s
The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). (TZ)
(Calculated from
.IR mktime(tm) .)
.TP
.B %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
.IR tm_sec .)
.TP
.B %t
A tab character. (SU)
.TP
.B %T
The time in 24-hour notation
.RB ( %H:%M:%S ).
(SU)
.TP
.B %u
The day of the week as a decimal, range 1 to 7, Monday being 1.
See also
.BR %w .
(Calculated from
.IR tm_wday .)
(SU)
.TP
.B %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
.B %V
and
.BR %W .
(Calculated from
.IR tm_yday
and
.IR tm_wday .)
.TP
.B %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
.B %U
and
.BR %W .
(Calculated from
.IR tm_year ,
.IR tm_yday ,
and
.IR tm_wday .)
(SU)
.TP
.B %w
The day of the week as a decimal, range 0 to 6, Sunday being 0.
See also
.BR %u .
(Calculated from
.IR tm_wday .)
.TP
.B %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
.IR tm_yday
and
.IR tm_wday .)
.TP
.B %x
The preferred date representation for the current locale without the time.
.TP
.B %X
The preferred time representation for the current locale without the date.
.TP
.B %y
The year as a decimal number without a century (range 00 to 99).
(Calculated from
.IR tm_year )
.TP
.B %Y
The year as a decimal number including the century.
(Calculated from
.IR tm_year )
.TP
.B %z
The
.I +hhmm
or
.I -hhmm
numeric timezone (that is, the hour and minute offset from UTC). (SU)
.TP
.B %Z
The timezone name or abbreviation.
.TP
.B %+
.\" Nov 05 -- Not in Linux/glibc, but is in some BSDs (according to
.\" their man pages)
The date and time in
.BR date (1)
format. (TZ)
(Not supported in glibc2.)
.TP
.B %%
A literal \(aq%\(aq character.
.PP
Some conversion specifications can be modified by preceding the
conversion specifier character by the
.B E
or
.B O
.I 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
.BR %Ec ,
.BR %EC ,
.BR %Ex ,
.BR %EX ,
.BR %Ey ,
.BR %EY ,
.BR %Od ,
.BR %Oe ,
.BR %OH ,
.BR %OI ,
.BR %Om ,
.BR %OM ,
.BR %OS ,
.BR %Ou ,
.BR %OU ,
.BR %OV ,
.BR %Ow ,
.BR %OW ,
.BR %Oy ,
where the effect of the
.B 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.
.SH RETURN VALUE
Provided that the result string,
including the terminating null byte, does not exceed
.I max
bytes,
.BR strftime ()
returns the number of bytes (excluding the terminating null byte)
placed in the array
.IR s .
If the length of the result string (including the terminating null byte)
would exceed
.I max
bytes, then
.BR 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
.\" .I max
.\" if the array was too small.)
.PP
Note that the return value 0 does not necessarily indicate an error.
For example, in many locales
.B %p
yields an empty string.
An empty
.I format
string will likewise yield an empty string.
.SH ENVIRONMENT
The environment variables
.B TZ
and
.B LC_TIME
are used.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR strftime ()
T}	Thread safety	MT-Safe env locale
.TE
.SH CONFORMING TO
SVr4, C89, C99.
.\" FIXME strftime() is in POSIX.1-2001 and POSIX.1-2008, but the details
.\" in the standards changed across versions. Investigate and
.\" write up.
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
.B %+
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
.BR date (1)
several extensions that could apply to
.BR strftime ()
as well.
The
.B %F
conversion is in C99 and POSIX.1-2001.
.PP
In SUSv2, the
.B %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).
.SH NOTES
.SS ISO 8601 week dates
.BR %G ,
.BR %g ,
and
.BR %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
.RB ( %V )
of the year 2009
.RB ( %G );
week 01 of ISO\ 8601 year 2010 starts on Monday, 4 January 2010.
.SS 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.)
.\" HP-UX and Tru64 also have features like this.
Between the \(aq%\(aq character and the conversion specifier character,
an optional
.I flag
and field
.I width
may be specified.
(These precede the
.B E
or
.B O
modifiers, if present.)
.PP
The following flag characters are permitted:
.TP
.B _
(underscore)
Pad a numeric result string with spaces.
.TP
.B \-
(dash)
Do not pad a numeric result string.
.TP
.B 0
Pad a numeric result string with zeros even if the conversion
specifier character uses space-padding by default.
.TP
.B ^
Convert alphabetic characters in result string to uppercase.
.TP
.B #
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
.BR %Z .)
.PP
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.
.SH BUGS
If the output string would exceed
.I max
bytes,
.I errno
is
.I not
set.
This makes it impossible to distinguish this error case from cases where the
.I format
string legitimately produces a zero-length output string.
POSIX.1-2001
does
.I not
specify any
.I errno
settings for
.BR strftime ().
.PP
Some buggy versions of
.BR gcc (1)
complain about the use of
.BR %c :
.IR "warning: `%c' yields only last 2 digits of year in some locales" .
Of course programmers are encouraged to use
.BR %c ,
it gives the preferred date and time representation.
One meets all kinds of strange obfuscations
to circumvent this
.BR gcc (1)
problem.
A relatively clean one is to add an
intermediate function
.PP
.in +4n
.EX
size_t
my_strftime(char *s, size_t max, const char *fmt,
            const struct tm *tm)
{
    return strftime(s, max, fmt, tm);
}
.EE
.in
.PP
Nowadays,
.BR gcc (1)
provides the
.IR \-Wno\-format\-y2k
option to prevent the warning,
so that the above workaround is no longer required.
.SH EXAMPLE
.BR "RFC\ 2822-compliant date format"
(with an English locale for %a and %b)
.PP
.in +2n
"%a,\ %d\ %b\ %Y\ %T\ %z"
.PP
.BR "RFC\ 822-compliant date format"
(with an English locale for %a and %b)
.PP
.in +2n
"%a,\ %d\ %b\ %y\ %T\ %z"
.SS Example program
The program below can be used to experiment with
.BR strftime ().
.PP
Some examples of the result string produced by the glibc implementation of
.BR strftime ()
are as follows:
.PP
.in +4n
.EX
.RB "$" " ./a.out \(aq%m\(aq"
Result string is "11"
.RB "$" " ./a.out \(aq%5m\(aq"
Result string is "00011"
.RB "$" " ./a.out \(aq%_5m\(aq"
Result string is "   11"
.EE
.in
.SS Program source
\&
.EX
#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);
}
.EE
.SH SEE ALSO
.BR date (1),
.BR time (2),
.BR ctime (3),
.BR setlocale (3),
.BR sprintf (3),
.BR strptime (3)