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

       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 shown in the list below.  In this list, the  field(s)  employed  from  the  tm
       structure are also shown.

       %a     The  abbreviated  name  of  the  day  of  the week according to the current locale.
              (Calculated from tm_wday.)

       %A     The full name of the day of the week according to the current locale.   (Calculated
              from tm_wday.)

       %b     The  abbreviated  month  name  according  to  the current locale.  (Calculated from
              tm_mon.)

       %B     The full month name according to the current locale.  (Calculated from tm_mon.)

       %c     The preferred date and time representation for the current locale.

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

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

       %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) (Calculated from tm_mday.)

       %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) (Calculated from tm_year, tm_yday,
              and tm_wday.)

       %g     Like %G,  but  without  century,  that  is,  with  a  2-digit  year  (00–99).  (TZ)
              (Calculated from tm_year, tm_yday, and tm_wday.)

       %h     Equivalent to %b.  (SU)

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

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

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

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

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

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

       %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".  (Calculated
              from tm_hour.)

       %P     Like %p but in lowercase: "am" or "pm" or a corresponding string  for  the  current
              locale.  (Calculated from tm_hour.)  (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)
              (Calculated from mktime(tm).)

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

       %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.
              (Calculated from tm_wday.)  (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.
              (Calculated from tm_yday and tm_wday.)

       %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.  (Calculated from tm_year, tm_yday, and tm_wday.)  (SU)

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

       %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.  (Calculated  from  tm_yday  and
              tm_wday.)

       %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).  (Calculated from
              tm_year)

       %Y     The year as a decimal number including the century.  (Calculated from tm_year)

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

       %Z     The timezone 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.

RETURN VALUE

       Provided that the result string, including the terminating null byte, does not exceed  max
       bytes, strftime() returns the number of bytes (excluding the terminating null byte) placed
       in the array s.  If the length of the result string (including the terminating null  byte)
       would  exceed  max  bytes,  then  strftime()  returns 0, and the contents of the array are
       undefined.

       Note that the return value 0 does not necessarily indicate an error.  For example, in many
       locales  %p  yields  an empty string.  An empty format string will likewise yield an empty
       string.

ENVIRONMENT

       The environment variables TZ and LC_TIME are used.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────┬───────────────┬────────────────────┐
       │InterfaceAttributeValue              │
       ├───────────┼───────────────┼────────────────────┤
       │strftime() │ Thread safety │ MT-Safe env locale │
       └───────────┴───────────────┴────────────────────┘

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 uppercase.

       #      Swap  the case of the result string.  (This flag works only 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

       If the output string would exceed max bytes, errno is not set.  This makes  it  impossible
       to  distinguish this error case from cases where the format string legitimately produces a
       zero-length  output  string.   POSIX.1-2001  does  not  specify  any  errno  settings  for
       strftime().

       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.

EXAMPLE

       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"

   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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