[tz] Question about bug seen in OpenBSD and FreeBSD related to tzname

[Andras Farkas]

I noticed something in the NEWS file and in the code after sending this email.
I see that in tzcode, along with USG_COMPAT for the XSI-required
(optional in POSIX) daylight and timezone, there is HAVE_TZNAME for
tzname, which is required by POSIX (and hence also in POSIX+XSI)
Thus, the macros already exist for this separation of standards and
optional/required stuff.

On Sat, Nov 23, 2019 at 1:27 AM Andras Farkas <deepbluemistake at gmail.com> wrote:
>
> On Fri, Nov 22, 2019 at 9:30 PM Paul Eggert <eggert at cs.ucla.edu> wrote:
> > What happens if you compile with -DHAVE_POSIX_DECLS=0? Arguably it
> > should be set that way if FreeBSD does not declare the POSIX-required
> > 'daylight' and 'timezone'.
> With CFLAGS set to -DHAVE_POSIX_DECLS=0 alone, things compile and date
> runs A-OK.
> With CFLAGS set to '-DHAVE_POSIX_DECLS=0 -DUSG_COMPAT=1' the
> compilation fails, albeit in a different way from with -DUSG_COMPAT=1
> alone.  Should I send both sorts of failures as text files?
> > I see now that POSIX marks 'daylight' and 'timezone' with "XSI", meaning
> > it's optional and present only if the X/Open Systems Interfaces option
> > is supported, so perhaps tzcode should have a different macro for
> > identifiers that are XSI rather than core POSIX.
> Correct.  The standard is separated into core POSIX, and POSIX+XSI
> (known combined as SUS, UNIX, X/Open, and a bunch of other names)
> The read-only (or intended to be read-only in your C program, and set
> by the implementation, i.e. the OS) C macros are:
> _POSIX_VERSION
> _POSIX2_VERSION
> _XOPEN_VERSION
> The macros intended to be set within a C program's source are:
> _POSIX_C_SOURCE
> _XOPEN_SOURCE
> In all these cases, if a XOPEN macro is set to something, you can
> already assume a POSIX value too, since POSIX is a subset of XOPEN,
> and XOPEN a superset of POSIX.
> Hence, daylight and timezone might be considered XSI-required rather
> than POSIX-required, when using those words specifically.
> A quick grep shows tzcode does already uses _POSIX_VERSION and _XOPEN_VERSION
> (I wonder if USG_COMPAT could be replaced entirely by POSIX and XOPEN
> macros, but I'll have to look more deeply into that tonight before
> saying what I think about that)
>
> As a historical note, I think more XSI extensions to POSIX are
> SystemV-biased than BSD-biased, with a lot of them coming from the
> SVID.  (though there are a couple BSD-biased XSI extensions, too)
> That's just a note and not a judgment or anything. uwu