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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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