Re: Problems with default _POSIX_C_SOURCE in features.h ??

[Chris Hall <glibc at gmch dot uk>]

On 09/04/2019 14:09, Adhemerval Zanella wrote:
> On 01/04/2019 07:11, Chris Hall wrote:
> > On 29/03/2019 17:54, Adhemerval Zanella wrote:
...
> > > My understanding is _POSIX_C_SOURCE and _XOPEN_SOURCE are tests are
> > > orthogonal and it will enable the most recent features you enable.
> > > The  -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=600 will set to higher
> > > standard you set, which is _POSIX_C_SOURCE=200809L from 
> > > _DEFAULT_SOURCE.
...
> Now, I am not sure which would be best course of action on glibc for such
> unspecified combinations. Current practice is to follow your description to
> conditionality older XSI extension if where the case.

I think we are agreed that _POSIX_C_SOURCE and _XOPEN_SOURCE are not 
orthogonal.

...
> So my understanding is glibc is not promoting the -D_DEFAULT_SOURCE
> -D_XOPEN_SOURCE=600 to _XOPEN_SOURCE=700 and it is indeed behaving 
> as the manual states which is only setting -D_POSIX_C_SOURCE=200809L.

I think we are agreed that -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=600 select 
an undefined combination of _POSIX_C_SOURCE and _XOPEN_SOURCE.

Setting "the 2008 edition of POSIX" is indeed what the manual says.

So asking for -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=600 is to ask for 
something which is undefined.

However, glibc goes to some effort to support the selection of older 
_POSIX_C_SOURCE and/or _XOPEN_SOURCE.  But *not* with __USE_MISC and 
*not* with __USE_GNU.  I have failed to discover why that is -- though 
that is probably because I am too stupid.

...
> > Except that _DEFAULT_SOURCE is (apparently) the only way of 
> > turning on __USE_MISC.

> The _GNU_SOURCE, which also defines _DEFAULT_SOURCE, also 
> defines __USE_MISC.

True.  But I don't want GNU extensions -- so I was ignoring that.  I 
apologise for not being precise.

...
> > Perhaps:
> >
> >    1. _DEFAULT_SOURCE is intended to exclude XSI altogether.
...
> That's not the idea, _DEFAULT_SOURCE does not *include* the 
> XSI extension but also does not *exclude* them.  It is still
> possible to include the XSI extension if is is the intention.

OK, so ...

> >    2. _DEFAULT_SOURCE is intended to not include XSI, but
> >       _XOPEN_SOURCE may be used to add same.
> >
> >       The documentation does not rule this out.

... -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=700 is an expected/supported 
combination (currently).

Mind you, when _POSIX_C_SOURCE=202101 (say) arrives and _DEFAULT_SOURCE 
selects it, -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=700 will then be an 
undefined/unsupported combination :-(

So, to correctly select XSI with _DEFAULT_SOURCE, I have to work on my 
configure script, to check (a) what version of _POSIX_C_SOURCE will be 
selected by _DEFAULT_SOURCE, and (b) what -D_XOPEN_SOURCE=??? is 
required to (correctly) enable XSI.  Hurrah !

> >       If _XOPEN_SOURCE is defined, I would force it to 700,
> >       to also be consistent with _GNU_SOURCE and be unambiguous.

> Again, I do not think it would be safer or yield much gains to
> force either _DEFAULT_SOURCE or _POSIX_C_SOURCE=200809L to set
> _XOPEN_SOURCE to 700.  Since it is undefined by standard and 
> has been defined by glibc, it will most likely crate 
> incompatibilities if we change it and it is not easy to 
> create a compatibility layer for such cases.

Sorry, I don't understand this.

My point is: if I ask for XSI, by -D_DEFAULT_SOURCE -D_XOPEN_SOURCE=xxx, 
I think it would be helpful if _DEFAULT_SOURCE selected the latest and 
greatest _XOPEN_SOURCE, to go with the latest and greatest 
_POSIX_C_SOURCE which _DEFAULT_SOURCE  is also selecting...

...so that, when _POSIX_C_SOURCE=202101 (say) arrives, _DEFAULT_SOURCE 
would then set the corresponding _XOPEN_SOURCE=800.

I agree that anybody currently asking for (say) -D_DEFAULT_SOURCE 
-D_XOPEN_SOURCE=600 might be surprised by the new result.  Though I 
think we are agreed that what they currently get is undefined :-(

Chris