Re: Emacs and Gnome Canvas

[Eli Zaretskii]

> From: Óscar Fuentes <address@hidden>
> Date: Fri, 16 Jul 2010 23:21:19 +0200
> 
> > How can a display engine has its memory management isolated, if it
> > needs to call Lisp primitives to access Lisp objects and operate on
> > them?  Just a random example: when you bump into a buffer position
> > that is covered by a text property or overlay, you need to call a
> > function that returns that property's value, which requires a call
> > such as
> >
> >   prop = get_char_property_and_overlay (make_number (position->charpos),
> >                                     Qdisplay, object, &overlay);
> >
> > The call to make_number here creates a Lisp object.
> > get_char_property_and_overlay is one of the few basic APIs the display
> > engine calls to find what are the text properties at a specific buffer
> > or string position.
> 
> For this especific example, I see no problem. The display backend will
> obtain `prop' as you say, gather the needed information and dispose
> `prop' immediately, following whatever Emacs protocol is required.

`prop' is a Lisp object.  (So are `object' and `overlay'.)  Lisp
objects don't need to be disposed of, they are garbage-collected by
the GC.  But GC, too, might need to be examined for possible
incompatibilities with C++ memory management.

As another example, consider constructing the mode line.  The
mode-line spec is nowadays full of Lisp strings and text properties,
and the code which handles that in the current engine calls
make_string quite a lot.  make_string creates a Lisp string and
triggers all kinds of behind-the-scenes reshuffling of string and
buffer text that a normal C/C++ program does not expect.

Anyway, what I tried to show was that manipulating Lisp objects is an
integral part of any Emacs display engine; you cannot hope to sidestep
it.  Therefore, it is important to have a plan and infrastructure for
dealing with it, because otherwise you will need to change your
implementation each time you bump into an example you didn't think
about.

> I want to use Qt.

What for?