[Bug libstdc++/89591] How can thread.joinable() reliably work if the pthread_t id is not initialized?

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89591

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Alexandre Bique from comment #0)
> Hi,
> 
> I've been reading the <thread> header and I've found that the thread id of
> std::thread is not initialized, I hope that I'm wrong and I missed something.

It's initialized here:

>       id() noexcept : _M_thread() { }


> So I wonder how std::thread.joinable() can work?
> 
> On top of that, pthread_t has no "invalid thread id" or "uninitialized
> thread id" value. So I wonder how the whole logic can work.

Right. We assume that a value-initialized pthread_t will not be used for any
valid thread. This certainly isn't guaranteed by POSIX.

> To me if your only attribute is pthread_t, you can't know figure anything
> about the thread, and even if the thread exists, there is no guarantee that
> the current std::thread object created it: if you create a object:
> std::thread t; then t.id will be uninitialized, so could very well point to
> an existing thread right?

No, because it's not uninitialized.

In practice pthread_t is either an integer or a pointer in all implementations
I know of, and the zero value is not used for running threads.


> 
> Regards,
> Alex
> 
> 
> 
> 
> From the header:
> 
> thread() noexcept = default;
> 
> and also:
> 
>     typedef __gthread_t			native_handle_type;
> 
>     /// thread::id
>     class id
>     {
>       native_handle_type	_M_thread;
> 
>     public:

> 
>       explicit
>       id(native_handle_type __id) : _M_thread(__id) { }
> 
>     private:
>       friend class thread;
>       friend class hash<thread::id>;
> 
>       friend bool
>       operator==(thread::id __x, thread::id __y) noexcept;
> 
>       friend bool
>       operator<(thread::id __x, thread::id __y) noexcept;
> 
>       template<class _CharT, class _Traits>
> 	friend basic_ostream<_CharT, _Traits>&
> 	operator<<(basic_ostream<_CharT, _Traits>& __out, thread::id __id);
>     };
> 
>   private:
>     id				_M_id;
> 
> 
> 
> and then later:
> 
> typedef pthread_t __gthread_t;