That's a great news !

I don't understand what you mean by " If this were (pd_this) were made per-thread". Perhaps that's the point I didn't understand when we were speaking about it at the Pd convention. How can we ensure that an object, a patch or whatever uses the right pd_instance ?

Here a concrete example of my problem, if you load a new patch (canvas) in a specific instance, to ensure that the canvas is added to the canvas list of this instance, you need:

1 - Lock a global lock

2 - Set the right instance

3 - Load the patch

4 - Unlock the global lock

I don't see how we can remove this global lock without changing the functions' prototypes that use pd_this (by adding a instance's pointer as argument and removing pd_this).

Date: Sun, 1 Jan 2017 15:31:51 -0800
From: Miller Puckette <msp@ucsd.edu>
To: pd-list@iem.at
Subject: [PD] threadsafe multi-instance Pd?
Message-ID: <20170101233151.GE21861@fuzz.localdomain>
Content-Type: text/plain; charset=us-ascii

To Pd List,

Here's my promised followup mail on Pd thread-ability (the hoped for
ability to call Pd instances, via pdlib, from separate threads concurrently).

Peter Brinkmann made a suggestion during the Pd convention round-table
discussion that I'll paraphrase here. There is a "pd_this" variable in
m_pd.c, pointing to the current Pd instance. If this were made per-thread,
then it should be possible to run different instances on different threads
simultaneously; the only protection needed would be that each individual
instance should be protected by pdlib with its own lock.

(There would also have to be a global lock to protect pd_init(), which need
only be called as setup time).

But there's a snag, because the symbol table is global. It wouldn't help to
make this per-thread, since calls to pd instances might migrate from thread to
thread. Instead, we could do 1 of these 2 things:

1 (Peter's idea) : make gensym(), pd_bind(), and pd_unbind() threadsafe using
a lock. Access via gensym() could be nonexclusive, as could locking out
pd_bind() and pd_unbind() during accesses to s->s_thing (we'd have to hunt
down everywhere in the code this is done).

I think there's a complication: what if you pass a message to an object via
a symbol's s_thing that's in a different instance from the current one
(pd_this) in the thread of the caller? I don't know an easy way to determine
what pd instance an arbitrary object belongs to.

OR:

2. (another possibility): Go back and make the symbol table be per-instance. I
tried this earlier and got stumped because classes, which are global to all Pd
instances, contain a list of selectors (symbols) and their associated messages.
If the symbol address changes because we switch to a new Pd instance, messages
(like #N print) no longer are associated with the method (print_new()), so
nothing works.

I think it's impractical to make classes per-instance, but what about making
each class maintain a separate list of messages for each Pd instance. When
a new Pd instance is created, we'd go find all the classes, and add a new
message list to each of them for the new instance.

Then whenever one passes a message (assuming it's not one of s_bang, s_float,
s_symbol, s_list, or s_anything, which are usually handled by a faster
mechanism), the message passing functions pd_typedmess(), getfn(), etc., would
have to look up the message list associated with the current pd instance, and
then look down the list of selectors/methods as before.

Anyone see anything fatally wrong with this, and/or can option (1) be made
workable and is it better?

cheers
Miller