[PD-dev] difference between t_canvas and t_pd

[Rich E]

I can't remember, are questions related to pd's C API appropriate for
pd-list, or are they better asked here?  It seems that everyone who responds
to those questions is on this list just as much, anyway.

I have been mucking around with opening and closing pd patches using API
calls instead of pd's messaging system.  The reason for this is that I would
like to be able to manage (open/close) multiple instances of a patch, and be
able to send each copy of the patch a unique message via its $0 argument
(see http://www.mail-archive.com/pd-list@iem.at/msg41648.html).  So far, the
method has been to hack glob_evalfile to look like:

t_canvas *glob_evalfile(t_pd *ignore, t_symbol *name, t_symbol *dir)
{
    t_pd *x = 0;
/* even though binbuf_evalfile appears to take care of dspstate,
 we have to do it again here, because canvas_startdsp() assumes
 that all toplevel canvases are visible.  LATER check if this
 is still necessary -- probably not. */
    t_canvas *x_canvas = 0;
    int dspstate = canvas_suspend_dsp();
    binbuf_evalfile(name, dir);
 while ((x != s__X.s_thing) && (x = s__X.s_thing))
{
x_canvas = canvas_getcurrent();
vmess(x, gensym("pop"), "i", 1);
}
    pd_doloadbang();
    canvas_resume_dsp(dspstate);
    return x_canvas;
}

The only addition is that x_canvas is retrieved before x is 'popped' off of
pd's global stack, and is then returned, giving me a handle to close that
particular patch with something like:

canvas_menuclose(x_canvas, 0);

But I realize that in global_evalfile, t_pd and x_canvas point to the same
address space.  This leads me to ask, what is the difference between these
two pointers?  Is t_canvas a t_pd along with variables for its graphical
nature?

Mathieu also pointed to me (as I was using the first set of these) that
canvas_setcurrent() and canvas_unsetcurrent() are aliases of pd_pushsym()
and pd_popsym().  Is there a good reason to use one over the other?

Last question:  Does anyone see a way that I could get the value of x in
glob_evalfile above without changing the function?  I don't think it is a
dangerous change (and it looks like I could just hand x.gl_pd to
canvas_menuclose, thereby not really needing a t_canvas pointer), but I
would of course prefer to not change the API if possible.

Cheers,
Rich
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