What does it mean to include pthread.h with -std=c11?

[Florian Weimer]

* Zack Weinberg:

>> I think there are no two sides here.  #include <pthread.h> must work
>> in ISO C mode.  Everything else is just very wrong.
>
> I tend to agree, but we need a definition of "work".
>
> I've always assumed that the intention was something like: if you
> request strict conformance and then include a header that was added to
> ISO C and/or POSIX at a conformance level above the one you picked,
> you get the set of definitions that that header had as of the oldest
> supported standard that included it.  (So, for instance, if you put
> -std=c99 on the command line, don't define any _*_SOURCE macros, and
> then include <threads.h>, you get what C11 specifies for that header,
> and no more.)  This is what I had in mind when I wrote
> check-installed-headers.sh.  I'm not aware of any cases where this
> policy can't be made to work, but I haven't investigated it too
> closely.

That makes a lot of sense to me.

A now-hypothetical example: C add lambda expressions and a future
version of POSIX enables them for use with pthread_create.  When you
build in C11 mode (say) but with a POSIX version that includes this
feature, this specific feature for pthread_create has to be left out.

But these are really fringe cases.  Bug 25271 is just an internal
inconsistency in the header itself, which should be easy enough to
fix.