Re: [PATCH 1/9] interface additions for the C thread implementation

[Rich Felker <dalias@libc.org>]

On Sun, Sep 07, 2014 at 04:45:01PM +0200, Jens Gustedt wrote:
> Am Sonntag, den 07.09.2014, 07:32 -0400 schrieb Rich Felker:
> > On Sun, Sep 07, 2014 at 01:16:43PM +0200, Jens Gustedt wrote:
> > > Am Sonntag, den 07.09.2014, 14:05 +0400 schrieb Alexander Monakov:
> > > > On Sun, 7 Sep 2014, Jens Gustedt wrote:
> > > > > > > For the C++ API/ABI, these also are different types, now, with type names
> > > > > > > (that are used for name mangling, e.g) as listed above.
> > > > > > > 
> > > > > > > Somebody better versed in C++ could perhaps contribute code that
> > > > > > > overloads the comparison and assignment operators such that a compilation
> > > > > > > that tries to compare or copy these types fails.
> > > > > > 
> > > > > > I'm not sure what you meant by this last paragraph.
> > > > > 
> > > > > AFAIR in C++ there are ways to inhibit usage of copy assignment by
> > > > > declaring some "operator=" function that is never defined. But my C++
> > > > > has really become rusty.
> > > > 
> > > > There's no need to do that since those are unrelated structs, and therefore no
> > > > operator== and operator= are available in the first place.  You also can't do
> > > > that in C (but in C++ you get an error rather than a warning when trying
> > > > to assign pointers).
> > > 
> > > This is not about assignment between different types and also not for
> > > pointers but for the struct themselves.
> > > 
> > > With the current C threads version the following is a priori allowed,
> > > but shouldn't:
> > > 
> > > mtx_t a, b;
> > > mtx_init(&a, mtx_plain);
> > > b = a;
> > > 
> > > This "works" in C and in C++.
> > > 
> > > The corresponding code in pthreads would be UB.
> > 
> > I'm not clear on whether the assignment is well-defined in pthreads,
> > but actually attempting to use the mutex (by passing it to any of the
> > pthread_mutex_* functions) would be UB. The same should be true for
> > C11 threads; if not, it's a defect.
> 
> It is.

It is a defect? Or..?

> sure, we all (should) know that, but the average user wouldn't
> 
> > I don't think the committee intended to forbid any of the above types
> > of implementation; on the contrary it seems they went out of their way
> > to support crazy types of implementations, e.g. by omitting
> > initializers.
> 
> No, unfortunately for the later, the lack of a definition for default
> initialization and initializers seems to be intentional. There are
> people on the committee who defend the interdiction of statically
> initialized mutexes, seemingly because some oldish windows thread
> implementation didn't have it.

That's what I mean. By refusing to support static initialization of
mutexes, they seem to be supporting the possibility of implementations
for which static initialization is impractical, much like some of the
crazy ideas I mentioned above.

Rich