Re: save-excursion and multi-thread?

[Tassilo Horn]

Qiantan Hong <qhong@mit.edu> writes:

> I tried
> (make-thread
>  (lambda ()
>    (save-excursion (goto-char (point-min)) (sleep-for 1))))
> In *scratch*.
>
> It seems that after the thread yields (because of sleep-for) it
> doesn’t restore point position, and after 1 second cursor is reseted
> to the original position,

The `sleep-for' is part of the thread's function, and also contained in
the `save-excursion'.  So how could it be different?

> discarding any of my movements during this 1 second.
>
> Is this how save-excursion supposed to work?
> It’s inconsistent with the behavior of special variable + dynamic
> bindings, which works currently under multi-thread.

How are those related?

> What’s the supposed way to have “thread-local” movements?

The thing is, right now there are threads in emacs but no actual
parallelism.  There's ever only one thread active and emacs switches
between the existing threads at certain conditions, e.g., when one
thread waits for input or a condition variable.  I guess another one
might be `sleep-for' which would obviously make a lot of sense.

Oh, ok, I think now I got your question.  If `point' was a special
variable, and let-bindings of special variables in a thread are local to
this thread, then you would naturally have thread-local movements.

Right now, point is a field in the buffer data structure, so there's
just one per buffer shared between all threads.  I think as long as your
thread doesn't use `sit-for', `sleep-for', or reads input, you can be
sure it won't be interrupted.  If it does, you cannot assume point is
still there where your thread moved it.

Bye,
Tassilo