Re: strftime(3) oddities with %s, %z

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> That's from POSIX, but should be more or less identical to what the C
> standard says (I don't have a copy of, any version of, the C standard
> to verify that, but it is generally possible from POSIX to tell when
> something is intended to agree with C, and when things are being
> modified).

I have a very late draft of C99 (not quite the standard, but I'm told
that, to a first approximation, it's identical).  I'll include the
whole strftime section below, but, of immediate relevance:

       %z  is replaced by the  offset  from  UTC  in  the  ISO 8601
           format ``-0430'' (meaning 4 hours 30 minutes behind UTC,
           west of Greenwich), or by no characters if no time  zone
           is determinable.  [tm_isdst]
       %Z  is   replaced   by   the  locale's  time  zone  name  or
           abbreviation, or by no characters if  no  time  zone  is
           determinable.  [tm_isdst]

This is underspecified.  It speaks of _the_ offset, as though there
were only one, but does not specify whether it's referring to the local
timezone configured for the system, the timezone of the struct tm's
information, getenv("TZ") (C99 does specify getenv(), though it is
mostly implementation-defined), or what.

I do note that there is nothing at all saying that %z's offset has to
bear any relation to the string %Z produces, except possibly by
implication because each one is allowed to examine only tm_isdst.

It also adds

       [#7]  In  the  "C" locale, [...]
       and the replacement strings  for  the  following  specifiers
       are:

[...]
       %Z  implementation-defined.

which, of course, is wide open for interpretation, innovation, and/or
further specification by the likes of POSIX.

> So, from just this, it is clear (I believe) that %Z is intended to
> print the local timezone name or abbreviation, always - with the only
> variation being whether the standard or summer time name/abbr is
> printed (which is based upon tm_isdst).

Intent is difficult to judge, absent someone involved in its creation
chiming in.  But the (C) spec is that %Z produces the locale's timezone
name, unless you're in the C locale, in which case it's up to the
implementation - regardless of things such as where the struct tm came
from.  It's not clear to me whether the implementation-defined C locale
%Z output may be defined to be based on other fields of the struct tm.

Here's the full strftime section (from here to my signature).

       7.23.3.5  The strftime function

       Synopsis

       [#1]

               #include <time.h>
               size_t strftime(char * restrict s,
                       size_t maxsize,
                       const char * restrict format,
                       const struct tm * restrict timeptr);

       Description

       [#2]  The strftime function places characters into the array
       pointed to by s as controlled by the string  pointed  to  by
       format.  The format shall be a multibyte character sequence,
       beginning and ending in its initial shift state.  The format
       string  consists  of  zero or more conversion specifiers and
       ordinary  multibyte  characters.   A  conversion   specifier
       consists  of  a  % character, possibly followed by an E or O
       modifier  character  (described  below),   followed   by   a
       character  that  determines  the  behavior of the conversion
       specifier.  All ordinary multibyte characters (including the
       terminating  null  character)  are copied unchanged into the
       array.  If copying takes place between objects that overlap,
       the  behavior is undefined.  No more than maxsize characters
       are placed into the array.

       [#3] Each conversion specifier is  replaced  by  appropriate
       characters   as   described  in  the  following  list.   The
       appropriate characters  are  determined  using  the  LC_TIME
       category  of the current locale and by the values of zero or
       more members of the broken-down time structure pointed to by
       timeptr,  as  specified  in brackets in the description.  If
       any of the specified values is outside the normal range, the
       characters stored are unspecified.

       %a  is  replaced  by  the locale's abbreviated weekday name.
           [tm_wday]
       %A  is  replaced  by  the  locale's   full   weekday   name.
           [tm_wday]
       %b  is  replaced  by  the  locale's  abbreviated month name.
           [tm_mon]
       %B  is replaced by the locale's full month name.  [tm_mon]
       %c  is replaced by the locale's appropriate  date  and  time
           representation.  [all specified in 7.23.1]
       %C  is  replaced by the year divided by 100 and truncated to
           an integer, as a decimal number (00-99).  [tm_year]
       %d  is replaced by the day of the month as a decimal  number
           (01-31).  [tm_mday]
       %D  is   equivalent   to  ``%m/%d/%y''.   [tm_mon,  tm_mday,
           tm_year]
       %e  is replaced by the day of the month as a decimal  number
           (1-31);   a   single  digit  is  preceded  by  a  space.
           [tm_mday]
       %F  is  equivalent  to  ``%Y-%m-%d''  (the   ISO 8601   date
           format).  [tm_year, tm_mon, tm_mday]
       %g  is  replaced by the last 2 digits of the week-based year
           (see below) as  a  decimal  number  (00-99).   [tm_year,
           tm_wday, tm_yday]
       %G  is  replaced  by  the  week-based  year (see below) as a
           decimal  number  (e.g.,   1997).    [tm_year,   tm_wday,
           tm_yday]
       %h  is equivalent to ``%b''.  [tm_mon]
       %H  is  replaced  by  the  hour (24-hour clock) as a decimal
           number (00-23).  [tm_hour]
       %I  is replaced by the hour (12-hour  clock)  as  a  decimal
           number (01-12).  [tm_hour]
       %j  is  replaced  by the day of the year as a decimal number
           (001-366).  [tm_yday]
       %m  is replaced by the month as a  decimal  number  (01-12).
           [tm_mon]
       %M  is  replaced  by the minute as a decimal number (00-59).
           [tm_min]
       %n  is replaced by a new-line character.
       %p  is replaced by the  locale's  equivalent  of  the  AM/PM
           designations associated with a 12-hour clock.  [tm_hour]
       %r  is  replaced  by  the  locale's  12-hour   clock   time.
           [tm_hour, tm_min, tm_sec]
       %R  is equivalent to ``%H:%M''.  [tm_hour, tm_min]
       %S  is  replaced  by the second as a decimal number (00-60).
           [tm_sec]
       %t  is replaced by a horizontal-tab character.
       %T  is  equivalent  to  ``%H:%M:%S''  (the   ISO 8601   time
           format).  [tm_hour, tm_min, tm_sec]
       %u  is  replaced by the ISO 8601 weekday as a decimal number
           (1-7), where Monday is 1.  [tm_wday]
       %U  is replaced by the week number of the  year  (the  first
           Sunday  as  the first day of week 1) as a decimal number
           (00-53).  [tm_year, tm_wday, tm_yday]
       %V  is replaced by the ISO 8601 week number (see below) as a
           decimal number (01-53).  [tm_year, tm_wday, tm_yday]
       %w  is  replaced  by  the weekday as a decimal number (0-6),
           where Sunday is 0.  [tm_wday]
       %W  is replaced by the week number of the  year  (the  first
           Monday  as  the first day of week 1) as a decimal number
           (00-53).  [tm_year, tm_wday, tm_yday]
       %x  is   replaced   by   the   locale's   appropriate   date
           representation.  [all specified in 7.23.1]
       %X  is   replaced   by   the   locale's   appropriate   time
           representation.  [all specified in 7.23.1]
       %y  is replaced by the last  2  digits  of  the  year  as  a
           decimal number (00-99).  [tm_year]
       %Y  is  replaced  by  the  year  as  a decimal number (e.g.,
           1997).  [tm_year]
       %z  is replaced by the  offset  from  UTC  in  the  ISO 8601
           format ``-0430'' (meaning 4 hours 30 minutes behind UTC,
           west of Greenwich), or by no characters if no time  zone
           is determinable.  [tm_isdst]
       %Z  is   replaced   by   the  locale's  time  zone  name  or
           abbreviation, or by no characters if  no  time  zone  is
           determinable.  [tm_isdst]
       %%  is replaced by %.

       [#4]  Some  conversion  specifiers  can  be  modified by the
       inclusion of an E or O modifier  character  to  indicate  an
       alternative  format  or  specification.   If the alternative
       format or specification  does  not  exist  for  the  current
       locale, the modifier is ignored.

       %Ec is  replaced  by  the locale's alternative date and time
           representation.
       %EC is replaced by the name of the base year (period) in the
           locale's alternative representation.
       %Ex is   replaced   by   the   locale's   alternative   date
           representation.
       %EX is   replaced   by   the   locale's   alternative   time
           representation.
       %Ey is  replaced  by  the offset from %EC (year only) in the
           locale's alternative representation.
       %EY is  replaced  by  the  locale's  full  alternative  year
           representation.
       %Od is  replaced by the day of the month, using the locale's
           alternative  numeric  symbols  (filled  as  needed  with
           leading  zeros,  or  with  leading spaces if there is no
           alternative symbol for zero).
       %Oe is replaced by the day of the month, using the  locale's
           alternative  numeric  symbols  (filled  as  needed  with
           leading spaces).
       %OH is replaced by  the  hour  (24-hour  clock),  using  the
           locale's alternative numeric symbols.
       %OI is  replaced  by  the  hour  (12-hour  clock), using the
           locale's alternative numeric symbols.
       %Om is replaced by the month, using the locale's alternative
           numeric symbols.
       %OM is   replaced   by   the  minutes,  using  the  locale's
           alternative numeric symbols.
       %OS is  replaced  by  the  seconds,   using   the   locale's
           alternative numeric symbols.
       %Ou is  replaced  by the ISO 8601 weekday as a number in the
           locale's alternative representation, where Monday is  1.
       %OU is  replaced  by  the  week  number,  using the locale's
           alternative numeric symbols.
       %OV is replaced by  the  ISO 8601  week  number,  using  the
           locale's alternative numeric symbols.
       %Ow is  replaced  by  the  weekday  as  a  number, using the
           locale's alternative numeric symbols.
       %OW is replaced by the week number of the  year,  using  the
           locale's alternative numeric symbols.
       %Oy is  replaced by the last 2 digits of the year, using the
           locale's alternative numeric symbols.

       [#5] %g, %G, and %V give values according  to  the  ISO 8601
       week-based  year.   In  this system, weeks begin on a Monday
       and week 1 of the year is the  week  that  includes  January
       4th, which is also the week that includes the first Thursday
       of the year, and is also the first  week  that  contains  at
       least four days in the year.  If the first Monday of January
       is the 2nd, 3rd, or 4th, the preceding days are part of  the
       last  week  of  the  preceding  year; thus, for Saturday 2nd
       January 1999, %G is replaced by 1998 and %V is  replaced  by
       53.  If December 29th, 30th, or 31st is a Monday, it and any
       following days are part of week 1  of  the  following  year.
       Thus, for Tuesday 30th December 1997, %G is replaced by 1998
       and %V is replaced by 1.

       [#6] If a conversion specifier is not one of the above,  the
       behavior is undefined.

       [#7]  In  the  "C" locale, the E and O modifiers are ignored
       and the replacement strings  for  the  following  specifiers
       are:

       %a  the first three characters of %A.
       %A  one of ``Sunday'', ``Monday'', ... , ``Saturday''.
       %b  the first three characters of %B.
       %B  one of ``January'', ``February'', ... , ``December''.
       %c  equivalent to ``%A %B %d %T %Y''.
       %p  one of ``am'' or ``pm''.
       %r  equivalent to ``%I:%M:%S %p''.
       %x  equivalent to ``%A %B %d %Y''.
       %X  equivalent to %T.
       %Z  implementation-defined.

       Returns

       [#8]  If  the total number of resulting characters including
       the terminating null character is not more than maxsize, the
       strftime  function  returns  the number of characters placed
       into the array pointed to by s not including the terminating
       null   character.   Otherwise,  zero  is  returned  and  the
       contents of the array are indeterminate.

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