The functions described in this section format calendar time values as strings. These functions are declared in the header file `time.h'.
asctime function converts the broken-down time value that
brokentime points to into a string in a standard format:
"Tue May 21 13:46:22 1991\n"
The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.
The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.
The return value points to a statically allocated string, which might be
overwritten by subsequent calls to asctime or ctime.
(But no other library function overwrites the contents of this
string.)
asctime but instead of placing the
result in a static buffer it writes the string in the buffer pointed to
by the parameter buffer. This buffer should have room
for at least 26 bytes, including the terminating null.
If no error occurred the function returns a pointer to the string the
result was written into, i.e., it returns buffer. Otherwise
return NULL.
ctime function is similar to asctime, except that you
specify the calendar time argument as a time_t simple time value
rather than in broken-down local time format. It is equivalent to
asctime (localtime (time))
ctime sets the variable tzname, because localtime
does so. See section Functions and Variables for Time Zones.
ctime, but places the result in the
string pointed to by buffer. It is equivalent to (written using
gcc extensions, see section `Statement Exprs' in Porting and Using gcc):
({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })
If no error occurred the function returns a pointer to the string the
result was written into, i.e., it returns buffer. Otherwise
return NULL.
sprintf function (see section Formatted Input), but the conversion specifications that can appear in the format
template template are specialized for printing components of the date
and time brokentime according to the locale currently specified for
time conversion (see section Locales and Internationalization).
Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers:
_
-
0
^
The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them.
Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size.
An optional modifier can follow the optional flag and width specification. The modifiers, which are POSIX.2 extensions, are:
E
%c, %C, %x, %X,
%y and %Y format specifiers. In a Japanese locale, for
example, %Ex might yield a date format based on the Japanese
Emperors' reigns.
O
If the format supports the modifier but no alternate representation is available, it is ignored.
The conversion specifier ends with a format specifier taken from the following list. The whole `%' sequence is replaced in the output string as follows:
%a
%A
%b
%B
%c
%C
%d
01 through 31).
%D
%m/%d/%y.
This format is a POSIX.2 extension and also appears in ISO C99.
%e
%d, but padded with blank (range
1 through 31).
This format is a POSIX.2 extension and also appears in ISO C99.
%F
%Y-%m-%d. This is the form specified
in the ISO 8601 standard and is the preferred form for all uses.
This format is a ISO C99 extension.
%g
00 through 99). This has the same format and value
as %y, except that if the ISO week number (see %V) belongs
to the previous or next year, that year is used instead.
This format was introduced in ISO C99.
%G
%Y, except that if the ISO week number (see
%V) belongs to the previous or next year, that year is used
instead.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%h
%b.
This format is a POSIX.2 extension and also appears in ISO C99.
%H
00 through
23).
%I
01 through
12).
%j
001 through 366).
%k
%H, but
padded with blank (range 0 through 23).
This format is a GNU extension.
%l
%I, but
padded with blank (range 1 through 12).
This format is a GNU extension.
%m
01 through 12).
%M
00 through 59).
%n
%p
%P
%r
%R
%H:%M.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%s
%S
00 through 60).
%t
%T
%H:%M:%S.
This format is a POSIX.2 extension.
%u
1 through
7), Monday being 1.
This format is a POSIX.2 extension and also appears in ISO C99.
%U
00
through 53), starting with the first Sunday as the first day of
the first week. Days preceding the first Sunday in the year are
considered to be in week 00.
%V
01
through 53). ISO weeks start with Monday and end with Sunday.
Week 01 of a year is the first week which has the majority of its
days in that year; this is equivalent to the week containing the year's
first Thursday, and it is also equivalent to the week containing January
4. Week 01 of a year can contain days from the previous year.
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.
This format is a POSIX.2 extension and also appears in ISO C99.
%w
0 through
6), Sunday being 0.
%W
00
through 53), starting with the first Monday as the first day of
the first week. All days preceding the first Monday in the year are
considered to be in week 00.
%x
%X
%y
00 through
99). This is equivalent to the year modulo 100.
%Y
1 are numbered 0, -1, and so on.
%z
-0600 or +0100), or nothing if no time zone is
determinable.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
A full RFC 822 timestamp is generated by the format
`"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent
`"%a, %d %b %Y %T %z"').
%Z
%%
The size parameter can be used to specify the maximum number of
characters to be stored in the array s, including the terminating
null character. If the formatted time requires more than size
characters, strftime returns zero and the contents of the array
s are undefined. Otherwise the return value indicates the
number of characters placed in the array s, not including the
terminating null character.
Warning: This convention for the return value which is prescribed
in ISO C can lead to problems in some situations. For certain
format strings and certain locales the output really can be the empty
string and this cannot be discovered by testing the return value only.
E.g., in most locales the AM/PM time format is not supported (most of
the world uses the 24 hour time representation). In such locales
"%p" will return the empty string, i.e., the return value is
zero. To detect situations like this something similar to the following
code should be used:
buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
{
/* Something went wrong in the strftime call. */
...
}
If s is a null pointer, strftime does not actually write
anything, but instead returns the number of characters it would have written.
According to POSIX.1 every call to strftime implies a call to
tzset. So the contents of the environment variable TZ
is examined before any output is produced.
For an example of strftime, see section Time Functions Example.
wcsftime function is equivalent to the strftime
function with the difference that it operates on wide character
strings. The buffer where the result is stored, pointed to by s,
must be an array of wide characters. The parameter size which
specifies the size of the output buffer gives the number of wide
character, not the number of bytes.
Also the format string template is a wide character string. Since
all characters needed to specify the format string are in the basic
character set it is portably possible to write format strings in the C
source code using the L"..." notation. The parameter
brokentime has the same meaning as in the strftime call.
The wcsftime function supports the same flags, modifiers, and
format specifiers as the strftime function.
The return value of wcsftime is the number of wide characters
stored in s. When more characters would have to be written than
can be placed in the buffer s the return value is zero, with the
same problems indicated in the strftime documentation.
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