strftime(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ENVIRONMENT | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

       strftime - format date and time

SYNOPSIS         top

       #include <time.h>

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

DESCRIPTION         top

       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         top

       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         top

       The environment variables TZ and LC_TIME are used.

ATTRIBUTES         top

       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         top

       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         top

   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         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.12 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                            2021-03-22                    STRFTIME(3)

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