Re: [PATCH 1/8] nptl: Add C11 threads thrd_* functions

[Florian Weimer <fweimer at redhat dot com>]

On 11/15/2017 12:22 PM, Adhemerval Zanella wrote:
> On 15/11/2017 06:09, Florian Weimer wrote:
> > On 09/22/2017 10:26 PM, Adhemerval Zanella wrote:
> > > This patch adds the thrd_* definitions from C11 threads (ISO/IEC
> > > 9899:2011),
> > > more specifically thrd_create, thrd_curent, rhd_detach, thrd_equal,
> > > thrd_exit, thrd_join, thrd_sleep, thrd_yield, and required types.
> > >
> > > Mostly of the definitions are composed based on POSIX conterparts,
> > > such as
> > > thrd_t (using pthread_t).
> >
> > I have skimmed the C11 discussion of these types, and C11 says that
> > mtx_t et al. are “identifiers”.  To me, this suggests they are
> > handles, just like pthread_t in the POSIX threads implementation.

> My understanding is "identifiers" follow the definition from C11 6.2.1
> which does not exclude a tag/struct to represent it.

I don't think “identifier” is used in this sense here.

The standard says (7.2.6.1/4):

“
The types are
\begin{quote}
  \verb|cnd_t|
\end{quote}
which is a complete object type that holds an identifier for a condition 
variable;
”

The “holds an identifier” suggests to me that this use of the term 
“identifier” does not refer to the C language construct.  The 
definitions in ISO/IEC 2382:2015(en) do not seem to be applicable, either.

The POSIX model assumes that the identity of a condition variable is 
determined by its address.  This means that pthread_cond_t cannot hold 
an identifier, it is the thing itself, which means, among other things, 
that you cannot make a copy (e.g. using memcpy) and the copy will behave 
in the same way as the original, accessing the same condition variable.

If something is just an identifier for something else, it strongly 
suggests to me that the underlying object is separate, and I can use a 
bit-wise identical identifier to reference the identical underlying object.

Thanks,
Florian