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(libc.info)Formatting Calendar Time


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Formatting Calendar Time
------------------------

   The functions described in this section format calendar time values
as strings.  These functions are declared in the header file `time.h'.

 - Function: char * asctime (const struct tm *BROKENTIME)
     The `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.)

 - Function: char * asctime_r (const struct tm *BROKENTIME, char
          *BUFFER)
     This function is similar to `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'.

 - Function: char * ctime (const time_t *TIME)
     The `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.
     Note: Time Zone Functions.

 - Function: char * ctime_r (const time_t *TIME, char *BUFFER)
     This function is similar to `ctime', but places the result in the
     string pointed to by BUFFER.  It is equivalent to (written using
     gcc extensions, Note: Statement Exprs.):

          ({ 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'.

 - Function: size_t strftime (char *S, size_t SIZE, const char
          *TEMPLATE, const struct tm *BROKENTIME)
     This function is similar to the `sprintf' function (Note:
     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 (Note:
     Locales).

     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:

    `_'
          The number is padded with spaces.

    `-'
          The number is not padded at all.

    `0'
          The number is padded with zeros even if the format specifies
          padding with spaces.

    `^'
          The output uses uppercase characters, but only if this is
          possible (Note: Case Conversion).

     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'
          Use the locale's alternate representation for date and time.
          This modifier applies to the `%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'
          Use the locale's alternate numeric symbols for numbers.  This
          modifier applies only to numeric format specifiers.

     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'
          The abbreviated weekday name according to the current locale.

    `%A'
          The full weekday name according to the current locale.

    `%b'
          The abbreviated month name according to the current locale.

    `%B'
          The full month name according to the current locale.

    `%c'
          The preferred calendar time representation for the current
          locale.

    `%C'
          The century of the year.  This is equivalent to the greatest
          integer not greater than the year divided by 100.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%d'
          The day of the month as a decimal number (range `01' through
          `31').

    `%D'
          The date using the format `%m/%d/%y'.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%e'
          The day of the month like with `%d', but padded with blank
          (range ` 1' through `31').

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%F'
          The date using the format `%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'
          The year corresponding to the ISO week number, but without
          the century (range `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'
          The year corresponding to the ISO week number.  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 but was previously
          available as a GNU extension.

    `%h'
          The abbreviated month name according to the current locale.
          The action is the same as for `%b'.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%H'
          The hour as a decimal number, using a 24-hour clock (range
          `00' through `23').

    `%I'
          The hour as a decimal number, using a 12-hour clock (range
          `01' through `12').

    `%j'
          The day of the year as a decimal number (range `001' through
          `366').

    `%k'
          The hour as a decimal number, using a 24-hour clock like
          `%H', but padded with blank (range ` 0' through `23').

          This format is a GNU extension.

    `%l'
          The hour as a decimal number, using a 12-hour clock like
          `%I', but padded with blank (range ` 1' through `12').

          This format is a GNU extension.

    `%m'
          The month as a decimal number (range `01' through `12').

    `%M'
          The minute as a decimal number (range `00' through `59').

    `%n'
          A single `\n' (newline) character.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%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'.

    `%P'
          Either `am' or `pm', according to the given time value; or the
          corresponding strings for the current locale, printed in
          lowercase characters.  Noon is treated as `pm' and midnight
          as `am'.

          This format was introduced in ISO C99 but was previously
          available as a GNU extension.

    `%r'
          The complete calendar time using the AM/PM format of the
          current locale.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%R'
          The hour and minute in decimal numbers using the format
          `%H:%M'.

          This format was introduced in ISO C99 but was previously
          available as a GNU extension.

    `%s'
          The number of seconds since the epoch, i.e., since 1970-01-01
          00:00:00 UTC.  Leap seconds are not counted unless leap
          second support is available.

          This format is a GNU extension.

    `%S'
          The seconds as a decimal number (range `00' through `60').

    `%t'
          A single `\t' (tabulator) character.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%T'
          The time of day using decimal numbers using the format
          `%H:%M:%S'.

          This format is a POSIX.2 extension.

    `%u'
          The day of the week as a decimal number (range `1' through
          `7'), Monday being `1'.

          This format is a POSIX.2 extension and also appears in
          ISO C99.

    `%U'
          The week number of the current year as a decimal number
          (range `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'
          The ISO 8601:1988 week number as a decimal number (range `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'
          The day of the week as a decimal number (range `0' through
          `6'), Sunday being `0'.

    `%W'
          The week number of the current year as a decimal number
          (range `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'
          The preferred date representation for the current locale.

    `%X'
          The preferred time of day representation for the current
          locale.

    `%y'
          The year without a century as a decimal number (range `00'
          through `99').  This is equivalent to the year modulo 100.

    `%Y'
          The year as a decimal number, using the Gregorian calendar.
          Years before the year `1' are numbered `0', `-1', and so on.

    `%z'
          RFC 822/ISO 8601:1988 style numeric time zone (e.g., `-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 time zone abbreviation (empty if the time zone can't be
          determined).

    `%%'
          A literal `%' character.

     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 Note: Time Functions Example.

 - Function: size_t wcsftime (wchar_t *S, size_t SIZE, const wchar_t
          *TEMPLATE, const struct tm *BROKENTIME)
     The `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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