On 2024-08-23 08:25, Alejandro Colomar wrote:
Hi Brian,
On Fri, Aug 23, 2024 at 08:04:20AM GMT, Brian Inglis wrote:
AFAICS from 9899 1998 Draft to 2023, the *mktime* wording has been essentially:
I find the above confusing. What is 9899 1998? The draft is for
ISO/IEC 9899:2024. There's no 1998 in the name.
Sorry for the typo - from the original *1988* Draft from ANSI X3J11 88-090
1988-05-13 of ANSI/ISO/IEC 9899:1990 through 2023.
"The original values of the tm_wday and tm_yday components of the structure
are ignored, and the original values of the other components are not
restricted to the ranges indicated above. On successful completion, the
values of the tm_wday and tm_yday components of the structure are set
appropriately,"
The text you've quoted is there since ISO C89.
<https://port70.net/~nsz/c/c89/c89-draft.html#4.12.2.3>
And that is not guarantee enough. It says that wday and yday are
ignored (for the purposes of determining the return value). And it says
that on success it modifies them. But that text is silent about what
happens on error.
It states they are set on successful completion, and implied left alone if not.
It is C23 in 7.19.2.3p3 which provides a novel guarantee, that those
^^^^^^^^^^
7.29.2.3.p3
See "WD" https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=415
fields won't be modified on a failed call.
so the recommendation has been to do something like (pointless example):
time_t tt = time(&tt);
struct tm *tm = localtime(&tt);
tm->tm_wday = tm->tm_yday = -1;
if ((tt = mktime(tm)) == -1 && tm->tm_wday == -1 && tm->tm_yday == -1)
This conditional, just like the one in the POSIX 2024 standard, is
redundant. Reading tm_wday is enough for determining a failure.
Otherwise, what would you expect if tm_yday is 4 but tm_wday is -1? Half an error? :)
Checking both are still untouched guarantees an error!
If one is set, it's a QoI bug, not an error ;^>
--
Take care. Thanks, Brian Inglis Calgary, Alberta, Canada
La perfection est atteinte Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer but when there is no more to cut
-- Antoine de Saint-Exupéry
