[PD-dev] per-thread storage in Pd in support of pdlib - discussion?

[Miller Puckette]

To Pd dev -

For some time the good folks who brought us pdlib have been asking how
one could make it possible to run several instances of Pd in a single
address space.

The question I'd like to rais is this: would it suffice to make Pd instances
be per-thread?  This could be done by going through all the source and
modifyin global and static variables with the nonstandard __thread
leyword (gcc) or some declspec horror in Miscoroft C.  Alternatively,
one could use the C++11 standard thread_local keyword, although I believe
that's not widely implemented yet.

To do this I'd replace all globals like
static t_symbol *symhash[HASHSIZE];
with
PDSTATIC  t_symbol *symhash[HASHSIZE];

and define PDSTATIC to static (also PDGLOBAL to the empty string and
PDEXTERN to extern).  Then anyone wanting to come along and recompile pd
to work per-thread only has to redefine the three appropriately to the
compiler.

Here are the gotchas I can foresee:

1.  external objects making explicit references to global storage
(such as canvas_dspstate and cos-table in m_pd.h and much stuff in the more 
'private' header files) would have to be recompiled to run per-thread.  They'd
still work fine with vanilla Pd.

2. existing externs that create threads would break at the source level if
they use any Pd-supplied functions at all (outlet_bang(), clock_set(), 
gensym(), etc) in 'other" threads.  Again they'd still work in Pd vanilla,
just not with versions of Pd recompiled to run per-thread.

3. lots of lines of code would be touched and this might make a number of
existing patches fail to apply cleanly.

4. supposing you use this to make a VST plug-in: what would happen if some
stupid host app called all its VST plug-ins from the same thread?  (I
think it's normal for hosts always to make a new thread for every VST
plug-in instance but I don't know if this is universal).

5.  If you wanted to run two instances of Pd in the same thread this wouldn't
help.  You'd have to spawn new threads and pass control to them to get into
the alternate Pds.

Comments anyone?

thanks
Miller