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[Python-ideas] New PyThread_tss_ C-API for CPython

Nick Coghlan ncoghlan at gmail.com
Wed Dec 21 11:07:07 EST 2016 
On 21 December 2016 at 20:01, Erik Bray <erik.m.bray at gmail.com> wrote:

> On Wed, Dec 21, 2016 at 2:10 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> > Option 2: Similar to option 1, but using a custom type alias, rather than
> > using a C99 bool directly
> >
> > The closest API we have to these semantics at the moment would be
> > PyGILState_Ensure, so the following API naming might work for option 2:
> >
> >     Py_ensure_t
> >     Py_ENSURE_NEEDS_INIT
> >     Py_ENSURE_INITIALIZED
> >
> > Respectively, these would just be aliases for bool, false, and true.
> >
> > And then modify the proposed PyThread_tss_create and PyThread_tss_delete
> > APIs to accept a "Py_ensure_t *init_flag" in addition to their current
> > arguments.
>
> That all sounds good--between the two option 2 looks a bit more explicit.
>
> Though what about this?  Rather than adding another type, the original
> proposal could be changed slightly so that Py_tss_t *is* partially
> defined as a struct consisting of a bool, with whatever the native TLS
> key is.   E.g.
>
> typedef struct {
>     bool init_flag;
>     #if defined(_POSIX_THREADS)
>     pthreat_key_t key;
>     #elif defined (NT_THREADS)
>     DWORD key;
>     /* etc... */
> } Py_tss_t;
>
> Then it's just taking Masayuki's original patch, with the global bool
> variables, and formalizing that by combining the initialized flag with
> the key, and requiring the semantics you described above for
> PyThread_tss_create/delete.
>
> For Python's purposes it seems like this might be good enough, with
> the more general purpose pthread_once-like functionality not required.
>

Aye, I also thought of that approach, but talked myself out of it since
there's no definable default value for pthread_key_t. However, C99 partial
initialisation may deal with that for us (by zeroing the memory without
actually assigning a typed value to it), and if it does, I agree it would
be better to handle the initialisation flag automatically rather than
requiring callers to do it.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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