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NAME

       strftime - format date and time

SYNOPSIS

       #include <time.h>

       size_t strftime(char *s, size_t max, const char *format,
                       const struct tm *tm);

DESCRIPTION

       The  strftime()  function  formats  the  broken-down  time  tm  according  to  the  format
       specification format and places the result in the character array s of size max.

       The format specification is a null-terminated string and  may  contain  special  character
       sequences called conversion specifications, each of which is introduced by a '%' character
       and terminated by some other character known as a  conversion  specifier  character.   All
       other character sequences are ordinary character sequences.

       The  characters  of  ordinary  character  sequences  (including  the null byte) are copied
       verbatim from format to s.  However,  the  characters  of  conversion  specifications  are
       replaced 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 date and time representation for the current locale.

       %C     The century number (year/100) as a 2-digit integer. (SU)

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

       %D     Equivalent  to %m/%d/%y.  (Yecch—for Americans only.  Americans should note that in
              other countries %d/%m/%y is  rather  common.   This  means  that  in  international
              context this format is ambiguous and should not be used.) (SU)

       %e     Like  %d,  the day of the month as a decimal number, but a leading zero is replaced
              by a space. (SU)

       %E     Modifier: use alternative format, see below. (SU)

       %F     Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)

       %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 %V).  This has the same
              format and value as %Y, except that if the ISO week number belongs to the  previous
              or next year, that year is used instead. (TZ)

       %g     Like %G, but without century, that is, with a 2-digit year (00-99). (TZ)

       %h     Equivalent to %b.  (SU)

       %H     The hour as a decimal number using a 24-hour clock (range 00 to 23).

       %I     The hour as a decimal number using a 12-hour clock (range 01 to 12).

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

       %k     The  hour  (24-hour  clock)  as a decimal number (range 0 to 23); single digits are
              preceded by a blank.  (See also %H.)  (TZ)

       %l     The hour (12-hour clock) as a decimal number (range 1 to  12);  single  digits  are
              preceded by a blank.  (See also %I.)  (TZ)

       %m     The month as a decimal number (range 01 to 12).

       %M     The minute as a decimal number (range 00 to 59).

       %n     A newline character. (SU)

       %O     Modifier: use alternative format, see below. (SU)

       %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     Like %p but in lowercase: "am" or "pm" or a corresponding string  for  the  current
              locale. (GNU)

       %r     The  time  in  a.m.  or  p.m.  notation.  In the POSIX locale this is equivalent to
              %I:%M:%S %p.  (SU)

       %R     The time in 24-hour notation (%H:%M). (SU) For a version including the seconds, see
              %T below.

       %s     The number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). (TZ)

       %S     The  second  as a decimal number (range 00 to 60).  (The range is up to 60 to allow
              for occasional leap seconds.)

       %t     A tab character. (SU)

       %T     The time in 24-hour notation (%H:%M:%S). (SU)

       %u     The day of the week as a decimal, range 1 to 7, Monday being 1.  See also %w.  (SU)

       %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 %V and %W.

       %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 %U and %W.  (SU)

       %w     The day of the week as a decimal, range 0 to 6, Sunday being 0.  See also %u.

       %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.

       %x     The preferred date representation for the current locale without the time.

       %X     The preferred time representation for the current locale without the date.

       %y     The year as a decimal number without a century (range 00 to 99).

       %Y     The year as a decimal number including the century.

       %z     The +hhmm or -hhmm numeric timezone (that is, the hour and minute offset from UTC).
              (SU)

       %Z     The timezone or name or abbreviation.

       %+     The date and time in date(1) format. (TZ) (Not supported in glibc2.)

       %%     A literal '%' character.

       Some  conversion  specifications  can  be  modified  by preceding the conversion specifier
       character by the E or O 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  %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe, %OH, %OI, %Om, %OM,
       %OS, %Ou, %OU, %OV, %Ow, %OW,  %Oy,  where  the  effect  of  the  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.

       The broken-down time structure tm is defined in <time.h>.  See also ctime(3).

RETURN VALUE

       The strftime() function returns the number of  characters  placed  in  the  array  s,  not
       including  the  terminating null byte, provided the string, including the terminating null
       byte, fits.  Otherwise, it returns 0, and the contents of the array is  undefined.   (This
       behavior applies since at least libc 4.4.4; very old versions of libc, such as libc 4.4.1,
       would return max if the array was too small.)

       Note that the return value 0 does not necessarily indicate an error; for example, in  many
       locales %p yields an empty string.

ENVIRONMENT

       The environment variables TZ and LC_TIME are used.

CONFORMING TO

       SVr4,  C89, C99.  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
       %+ 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 date(1) several extensions that
       could apply to strftime() as well.  The %F conversion is in C99 and POSIX.1-2001.

       In SUSv2, the %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).

NOTES

   ISO 8601 Week Dates
       %G, %g, and %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  (%V)  of  the year 2009 (%G) ; week 01 of ISO 8601 year 2010 starts on Monday, 4
       January 2010.

   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.)  Between the
       '%' character and the conversion specifier character, an optional flag and field width may
       be specified.  (These precede the E or O modifiers, if present.)

       The following flag characters are permitted:

       _      (underscore) Pad a numeric result string with spaces.

       -      (dash) Do not pad a numeric result string.

       0      Pad  a  numeric result string with zeros even if the conversion specifier character
              uses space-padding by default.

       ^      Convert alphabetic characters in result string to upper case.

       #      Swap the case of the result string.  (This flag only works with certain  conversion
              specifier characters, and of these, it is only really useful with %Z.)

       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.

BUGS

       Some buggy versions of gcc(1) complain about the use of %c: warning: `%c' yields only last
       2 digits of year in some locales.  Of course programmers are  encouraged  to  use  %c,  it
       gives  the  preferred  date  and  time  representation.   One  meets  all kinds of strange
       obfuscations to circumvent this gcc(1) problem.  A relatively  clean  one  is  to  add  an
       intermediate function

           size_t
           my_strftime(char *s, size_t max, const char *fmt,
                       const struct tm *tm)
           {
               return strftime(s, max, fmt, tm);
           }

       Nowadays,  gcc(1)  provides the -Wno-format-y2k option to prevent the warning, so that the
       above workaround is no longer required.

EXAMPLES

       RFC 2822-compliant date format (with an English locale for %a and %b)

         "%a, %d %b %Y %T %z"

       RFC 822-compliant date format (with an English locale for %a and %b)

         "%a, %d %b %y %T %z"

   Example Program
       The program below can be used to experiment with strftime().

       Some examples of the result string produced by the glibc implementation of strftime()  are
       as follows:

           $ ./a.out '%m'
           Result string is "11"
           $ ./a.out '%5m'
           Result string is "00011"
           $ ./a.out '%_5m'
           Result string is "   11"

       Here's the program source:

       #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);
       }

SEE ALSO

       date(1), time(2), ctime(3), setlocale(3), sprintf(3), strptime(3)

COLOPHON

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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.