Re: Emacs design and architecture. How about copy-on-write?

[Po Lu]

Ihor Radchenko <yantar92@posteo.net> writes:

> As I understand his previous explanations, global variable values are
> stored per-thread, if they were changed by that thread at some point.
> For C-defined globals, the Vname instances are re-defined to
> point to thread-local "globals" struct.
>
> I do not recall Po Lu providing the details on how the thread-local
> variables are stored.

Non-forwarded thread-local bindings are saved within a separate
association list in Lisp_Symbol, where each element ties a thread to its
local binding.  do_specbind and do_one_specbind manage this association
list by introducing new associations.  set_internal and
find_symbol_value search within this association list for a pair
matching the current thread, and set or return its cdr respectively if
it is present.

Within lispfwds, the pointer to the field itself is replaced with an
offset to a field within struct thread_state which is set to a pointer
to the matching field in `globals' or one of its local bindings.

Whenever a forwarded variable is specbound for the first time in a given
thread, the designated field within that thread's state is set to a
pointer into thread local storage, and once unbound, restored to its
initial value within globals.