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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.)  (The specific names used in the current locale  can  be
              obtained by calling nl_langinfo(3) with ABDAY_{17} as an argument.)

       %A     The  full name of the day of the week according to the current locale.  (Calculated
              from tm_wday.)  (The specific names used in the current locale can be  obtained  by
              calling nl_langinfo(3) with DAY_{17} as an argument.)

       %b     The  abbreviated  month  name  according  to  the current locale.  (Calculated from
              tm_mon.)  (The specific names used in the current locale can be obtained by calling
              nl_langinfo(3) with ABMON_{112} as an argument.)

       %B     The  full  month  name  according to the current locale.  (Calculated from tm_mon.)
              (The specific names  used  in  the  current  locale  can  be  obtained  by  calling
              nl_langinfo(3) with MON_{112} as an argument.)

       %c     The  preferred  date and time representation for the current locale.  (The specific
              format used in the current locale can be obtained by  calling  nl_langinfo(3)  with
              D_T_FMT  as  an  argument for the %c conversion specification, and with ERA_D_T_FMT
              for the %Ec conversion specification.)  (In the POSIX locale this is equivalent  to
              %a %b %e %H:%M:%S %Y.)

       %C     The  century  number  (year/100)  as  a  2-digit  integer. (SU) (The %EC conversion
              specification corresponds to the name of the era.)  (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 ("era-based") 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 numeric symbols, 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.)  (The specific string representations used for "AM" and "PM" in  the
              current  locale  can  be obtained by calling nl_langinfo(3) with AM_STR and PM_STR,
              respectively.)

       %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.  (SU) (The specific format used in the current
              locale can be obtained by calling nl_langinfo(3) with T_FMT_AMPM as  an  argument.)
              (In the POSIX locale this is equivalent to %I:%M:%S %p.)

       %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.   (The
              specific   format   used   in  the  current  locale  can  be  obtained  by  calling
              nl_langinfo(3) with D_FMT as an argument for the %x conversion  specification,  and
              with ERA_D_FMT for the %Ex conversion specification.)  (In the POSIX locale this is
              equivalent to %m/%d/%y.)

       %X     The preferred time representation for the current locale without  the  date.   (The
              specific   format   used   in  the  current  locale  can  be  obtained  by  calling
              nl_langinfo(3) with T_FMT as an argument for the %X conversion  specification,  and
              with ERA_T_FMT for the %EX conversion specification.)  (In the POSIX locale this is
              equivalent to %H:%M:%S.)

       %y     The year as a decimal number  without  a  century  (range  00  to  99).   (The  %Ey
              conversion  specification  corresponds  to  the year since the beginning of the era
              denoted by the %EC conversion specification.)  (Calculated from tm_year)

       %Y     The  year  as  a  decimal  number  including  the  century.   (The  %EY  conversion
              specification   corresponds   to   the   full   alternative  year  representation.)
              (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.  The rules governing date representation with
       the  E  modifier can be obtained by supplying ERA as an argument to a nl_langinfo(3).  One
       example of such alternative forms is the Japanese era calendar scheme in the  ja_JP  glibc
       locale.

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  or
       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 52 or 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.  Similarly, the first two days of January 2011 are considered to be part  of
       week 52 of the year 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,  as  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"

   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), nl_langinfo(3), setlocale(3), sprintf(3), strptime(3)

COLOPHON

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