Hi Vincent,

On Fri, Aug 23, 2024 at 03:54:49PM GMT, Vincent Lefevre wrote:

On 2024-08-23 15:12:16 +0200, Alejandro Colomar wrote:

quoted

Looking at the WG14 document logs, it seems it was added in n3147:
<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3147.txt>

Thanks for the reference. Additional details can be found
in CD2 ballot at

  https://open-std.org/JTC1/SC22/WG14/www/docs/n3148.doc

It's interesting that WG14 claims that they're not aware of any existing
implementations that would modify tm_wday on failure.

Although it's weird, because WG14 attributes that claim to the Austin
Group, and

which references the POSIX bug

  https://austingroupbugs.net/view.php?id=1614

I don't see any discussion about tm_wday in that Austin Group bug.  :|
Maybe it happened in a mailing list or elsewhere.  Anyway, let's trust
that claim.  (If any implementation does not conform, at least it should
be feasible to fix that implementation to conform.)

where I can see at the end:

  On page 1332 line 44348 section mktime(), change:

    if (mktime(&time_str) == -1)

  to:

    time_str.tm_wday = -1;
    if (mktime(&time_str) == (time_t)-1 && time_str.tm_wday == -1)

Yep, they do that change, although I haven't been able to find a
discussion that justifies that change.  :|  (Anyway, I think the change
is good.  I'm only perplex not finding the justification.)

This is the test I suggested: a check that mktime() returns -1,

I think that test suggested by POSIX is bogus (redundant).  If mktime(3)
has failed, tm_wday is unchanged.  If it has succeeded, tm_wday must be
changed.  Thus, the return value is meaningless for the purpose of
determining if it has failed.

and since it can be a valid value, a second test on tm_wday
(where the input, which is ignored, has an invalid value such
as -1 here, or INT_MAX in your case; note that -1 may be more
efficient with some processors, and shorter to write).

I didn't use -1 because I thought some weird weeks might contain 8 days
(for some of those weird timezone adjustments), and that that might
cause wday -1 to actually exist.  Thus I chose INT_MIN, which is far far
away from valid values, to represent an error.  However, I assume that
they were careful enough to make sure that such weeks do not exist, so
yes, -1 will be better.  (And if it's not, we'll blame Austin.)  :)

Cheers,
Alex

-- 
<https://www.alejandro-colomar.es/>