libc/time/strftime.3

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.\"     from: @(#)strftime.3	5.12 (Berkeley) 6/29/91
.\"	$NetBSD: strftime.3,v 1.38 2025/01/23 22:44:22 christos Exp $
.\"
.Dd October 9, 2020
.Dt STRFTIME 3
.Os
.Sh NAME
.Nm strftime ,
.Nm strftime_z
.Nd format date and time
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Ft size_t
.Fn strftime "char * restrict buf" "size_t maxsize" "const char * restrict format" "const struct tm * restrict timeptr"
.Ft size_t
.Fn strftime_z "const timezone_t tz" "char * restrict buf" "size_t maxsize" "const char * restrict format" "const struct tm * restrict timeptr"
.Sh DESCRIPTION
The
.Fn strftime
function formats the information from
.Fa timeptr
into the array pointed to by
.Fa buf
according to the string pointed to by
.Fa format .
.Pp
The
.Fa format
string consists of zero or more conversion specifications and
ordinary characters.
All ordinary characters are copied directly into the array.
A conversion specification consists of a percent sign
.Ql %
and one other character.
.Pp
No more than
.Fa maxsize
bytes will be placed into the array.
Otherwise, zero is returned.
.Pp
Each conversion specification is replaced by the characters as
follows which are then copied into the array.
The characters depend on the values of zero or more members of
.Fa timeptr
as specified by brackets in the description.
If a bracketed member name is followed by
.Dq + ,
.Nm strftime
can use the named member even though POSIX.1-2024 does not list it;
if the name is followed by
.Dq \&- ,
.Nm strftime
ignores the member even though POSIX.1-2024 lists it
which means portable code should set it.
For portability,
.Fa timeptr
should be initialized as if by a successful call to
.Xr gmtime 3 ,
.Xr localtime 3 ,
.Xr mktime 3 ,
.Xr timegm 3 ,
or similar functions.
.Bl -tag -width "xxxx"
.It Cm \&%A
is replaced by the locale's full weekday name.
.Dv [ tm_wday ]
.It Cm %a
is replaced by the locale's abbreviated weekday name.
.Dv [ tm_wday ]
.It Cm \&%B
is replaced by the locale's full month name.
.Dv [ tm_mon ]
.It Cm \&%b No or Cm \&%h
is replaced by the locale's abbreviated month name.
.RI [ tm_mon ]
.It Cm \&%C
is replaced by the century (a year divided by 100 and truncated to an integer)
as a decimal number, with at least two digits by default.
.RI [ tm_year ]
.It Cm \&%c
is replaced by the locale's appropriate date and time representation.
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_mon ,
.Dv tm_mday ,
.Dv tm_wday ,
.Dv tm_hour ,
.Dv tm_min ,
.Dv tm_sec ,
.Dv tm_gmtoff ,
.Dv tm_zone ,
.Dv tm_isdst \&-].
.It Cm \&%D
is replaced by the date in the format
.Dq Li %m/%d/%y .
Although used in the United States for current dates,
this format is ambiguous elsewhere
and for dates that might involve other centuries.
.Dv [ tm_year ,
.Dv tm_mon ,
.Dv tm_mday ]
.It Cm \&%d
is replaced by the day of the month as a decimal number [01,31].
.Dv [ tm_mday ]
.It Cm \&%e
is replaced by the day of month as a decimal number [1,31];
single digits are preceded by a blank.
.Dv [ tm_mday ]
.It Cm \&%F
is equivalent to
.Dq Li %Y-%m-%d
(the ISO 8601 date format).
.Dv [ tm_year ,
.Dv tm_mon ,
.Dv tm_mday ]
.It Cm \&%G
is replaced by the ISO 8601 year with century as a decimal number.
See also the
.Cm \&%V
conversion specification
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_wday ]
.It Cm \&%g
is replaced by the ISO 8601 year without century as a decimal number.
This is the year that includes the greater part of the week.
(Monday as the first day of a week).
See also the
.Ql \&%V
conversion specification.
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_wday ]
.It Cm \&%H
is replaced by the hour (24-hour clock) as a decimal number [00,23].
.Dv [ tm_hour ]
.It Cm \&%I
is replaced by the hour (12-hour clock) as a decimal number [01,12].
.Dv [ tm_hour ]
.It Cm \&%j
is replaced by the day of the year as a decimal number [001,366].
.Dv [ tm_yday ]
.It Cm \&%k
is replaced by the hour (24-hour clock) as a decimal number [0,23];
single digits are preceded by a blank.
.Dv [ tm_hour ]
.It Cm \&%l
is replaced by the hour (12-hour clock) as a decimal number [1,12];
single digits are preceded by a blank.
.Dv [ tm_hour ]
.It Cm \&%M
is replaced by the minute as a decimal number [00,59].
.Dv [ tm_min ]
.It Cm %m
is replaced by the month as a decimal number [01,12].
.Dv [ tm_mon ]
.It Cm %n
is replaced by a newline.
.It Cm %p
is replaced by the locale's equivalent of either
.Dq Tn AM
or
.Dq Tn PM .
.Dv [ tm_hour ]
.It Cm \&%R
is replaced by the time in the format
.Dq Li %H:%M .
.Dv [ tm_hour ,
.Dv tm_min ]
.It Cm \&%r
is replaced by the locale's representation of 12-hour clock time
using AM/PM notation.
.It Cm \&%S
is replaced by the second as a decimal number [00,60].
The range of
seconds is [00-60] instead of [00-59] to allow for the periodic occurrence
of leap seconds.
.Dv [ tm_sec ]
.It Cm \&%s
is replaced by the number of seconds since the Epoch (see
.Xr ctime 3 ) .
Although %s is reliable in this implementation,
it can have glitches on other platforms
(notably obsolescent platforms lacking
.Fa tm_gmtoff
or where
.Tp time_t
is no wider than int), and POSIX allows
.Nm strftime
to set
.Dv errno
to
.Dv EINVAL
or
.Dv EOVERFLOW
and return 0 if the number of seconds would be negative or out of range for
.Tp time_t .
Portable code should therefore format a
.Tp time_t
value directly via something like
.Xr snprintf 3
instead of via
.Xr localtime 3
followed by
.Nm strftime
with "%s".
.Dv [ tm_year ,
.Dv tm_mon ,
.Dv tm_mday ,
.Dv tm_hour ,
.Dv tm_min ,
.Dv tm_sec ,
.Dv tm_gmtoff +,
.Dv tm_isdst \&-].
.It Cm \&%T
is replaced by the time in the format
.Dq Li %H:%M:%S .
.Dv [ tm_hour ,
.Dv tm_min ,
.Dv tm_sec ]
.It Cm \&%t
is replaced by a tab.
.It Cm \&%U
is replaced by the week number of the year (Sunday as the first day of
the week) as a decimal number [00,53].
.Dv [ tm_wday ,
.Dv tm_yday ,
.Dv tm_year \&-]
.It Cm \&%u
is replaced by the weekday (Monday as the first day of the week)
as a decimal number [1,7].
.Dv [ tm_wday ]
.It Cm \&%V
is replaced by the week number of the year (Monday as the first day of
the week) as a decimal number [01,53]. According to ISO 8601 the week
containing January 1 is week 1 if it has four or more days in the new year,
otherwise it is week 53 of the previous year, and the next week is week 1.
The year is given by the
.Ql \&%G
conversion specification.
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_wday ]
.It Cm \&%v
is replaced by the date in the format
.Dq Li %e-%b-%Y .
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_wday ]
.It Cm \&%W
is replaced by the week number of the year (Monday as the first day of
the week) as a decimal number [00,53].
.Dv [ tm_yday ,
.Dv tm_wday ]
.It Cm \&%w
is replaced by the weekday (Sunday as the first day of the week)
as a decimal number [0,6].
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_wday ]
.It Cm \&%X
is replaced by the locale's appropriate time representation.
.Dv [ tm_year \&-,
.Dv tm_yday \&-,
.Dv tm_mon \&-,
.Dv tm_mday \&-,
.Dv tm_wday \&-,
.Dv tm_hour ,
.Dv tm_min ,
.Dv tm_sec ,
.Dv tm_gmtoff ,
.Dv tm_zone ,
.Dv tm_isdst \&-].
.It Cm \&%x
is replaced by the locale's appropriate date representation.
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_mon ,
.Dv tm_mday ,
.Dv tm_wday ,
.Dv tm_hour \&-,
.Dv tm_min \&-,
.Dv tm_sec \&-,
.Dv tm_gmtoff \&-,
.Dv tm_zone \&-,
.Dv tm_isdst \&-].
.It Cm \&%Y
is replaced by the year with century as a decimal number.
.Dv [ tm_year ]
.It Cm \&%y
is replaced by the year without century as a decimal number [00,99].
.Dv [ tm_year ]
.It Cm \&%Z
is replaced by the time zone abbreviation,
or the empty string if this is not determinable.
.Dv [ tm_zone ,
.Dv tm_isdst \&-]
.It Cm \&%z
is replaced by the offset from the Prime Meridian in the format
+HHMM or -HHMM (ISO 8601) as appropriate, with positive values representing
locations east of Greenwich, or by the empty string if this is
not determinable.
The numeric time zone abbreviation \&-0000 is used when the time is
Universal Time
but local time is indeterminate; by convention this is used for
locations while uninhabited, and corresponds to a zero offset when the
time zone abbreviation begins with
.Dq Li [-] .
.It Cm %+
is replaced by locale's date and time in
.Xr date 1
format.
On
.Nx
currently this only works for the C locale.
.Dv [ tm_year ,
.Dv tm_yday ,
.Dv tm_mon ,
.Dv tm_mday ,
.Dv tm_wday ,
.Dv tm_hour ,
.Dv tm_min ,
.Dv tm_sec ,
.Dv tm_gmtoff ,
.Dv tm_zone ]
.It Cm %-*
GNU libc extension.
Do not do any padding when performing numerical outputs.
.It Cm %_*
GNU libc extension.
Explicitly specify space for padding.
.It Cm %0*
GNU libc extension.
Explicitly specify zero for padding.
.It Cm %%
is replaced by as single
.Ql % .
.El
.Pp
As a side effect,
.Nm strftime
also behaves as if
.Xr tzset 3
were called.
This is for compatibility with older platforms, as required by POSIX;
it is not needed for
.Nm strftime
's
own use.
.Pp
The
.Fn strftime_z
function is similar to
.Fn strftime ,
but it also takes a
.Ft "const timezone_t"
.Fa tz
argument.
.Sh RETURN VALUES
If the conversion is successful,
.Nm
returns the number of bytes placed into the array, not counting the
terminating
.Dv NUL ;
.Va errno
is unchanged if the returned value is zero.
Otherwise,
.Va errno
is set to indicate the error, zero is returned,
and the array contents are unspecified.
.Sh ERRORS
This function fails if:
.Bl -tag -width Er
.It Bq Er ERANGE
The specified file offset is invalid.
The total number of resulting bytes, including the terminating
.Dv NUL
character, is more than
.Fa maxsize .
.It Bq Er EOVERFLOW
The format includes an
.Cm \&%s
conversion and the number of seconds since the Epoch cannot be represented
in a
.Ft time_t .
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr printf 1 ,
.Xr ctime 3 ,
.Xr printf 3 ,
.Xr strptime 3 ,
.Xr tm 3
.Sh STANDARDS
The
.Fn strftime
function
conforms to
.St -isoC-99 .
The
.Ql \&%C ,
.Ql \&%D ,
.Ql \&%e ,
.Ql \&%g ,
.Ql \&%G ,
.Ql \&%h ,
.Ql \&%k ,
.Ql \&%l ,
.Ql \&%n ,
.Ql \&%r ,
.Ql \&%R ,
.Ql \&%s ,
.Ql \&%t ,
.Ql \&%T ,
.Ql \&%u ,
.Ql \&%V ,
and
.Ql \&%v
conversion specifications are extensions.
.Pp
Use of the ISO 8601 conversions may produce non-intuitive results.
Week 01 of a year is per definition the first week which has the Thursday
in this year, which is equivalent to the week which contains the fourth
day of January.
In other words, the first week of a new year is the week which has the
majority of its days in the new year.
Week 01 might also contain days from the previous year and the week
before week 01 of a year is the last week (52 or 53) of the previous
year even if it contains days from the new year.
A week starts with Monday (day 1) and ends with Sunday (day 7).
For example, the first week of the year 1997 lasts from
1996-12-30 to 1997-01-05.
.Sh BUGS
There is no conversion specification for the phase of the moon.
.Pp
A return value of zero does not necessarily indicate an error.
If the resulting string is an empty string, the result value is
zero and it is not possible to distinguish between success and error.
For example, in many locales
.Cm \&%p
yields an empty string.
This problem can be avoided by inserting an extra space at the
beginning of the format string and then skipping over it or removing
it from the result.