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man:strftime

STRFTIME(3) Linux Programmer's Manual STRFTIME(3)

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 charac-
     ter 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.  (Calcu-
            lated 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) (Calcu-
            lated 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 ambigu-
            ous 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 deci-
            mal number.  The 4-digit year corresponding to the ISO week num-
            ber  (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.)  (Calcu-
            lated 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.)  (Calcu-
            lated 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 conver-
     sion  specifier  character  by  the E or O modifier to indicate that an
     alternative format should be used.  If the alternative format or speci-
     fication 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 for-
     mat 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).
     +-----------+---------------+--------------------+
     |Interface  | Attribute     | Value              |
     +-----------+---------------+--------------------+
     |strftime() | Thread safety | MT-Safe env locale |
     +-----------+---------------+--------------------+

CONFORMING TO

     SVr4, C89, C99.  There are strict inclusions between the set of conver-
     sions given in ANSI C (unmarked), those given in the Single UNIX Speci-
     fication  (marked  SU), those given in Olson's timezone package (marked
     TZ), and those given in glibc (marked GNU), except that %+ is not  sup-
     ported  in  glibc2.   On  the other hand glibc2 has several more exten-
     sions.  POSIX.1 only refers to ANSI C; POSIX.2 describes under  date(1)
     several extensions that could apply to strftime() as well.  The %F con-
     version 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  conver-
     sion specifier character, an optional flag and field width may be spec-
     ified.  (These precede the E or O modifiers, if present.)
     The following flag characters are permitted:
     _      (underscore) Pad a numeric result string with spaces.
  1. (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 pro-
     grammers are encouraged to use %c, it gives the preferred date and time
     representation.  One meets all kinds of strange obfuscations to circum-
     vent this gcc(1) problem.  A relatively clean one is to add an interme-
     diate 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

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

GNU 2017-09-15 STRFTIME(3)

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