Re: SMOB tutorial for Scwm and Guile

[Greg Harvey <Greg.Harvey@thezone.net>]

Greg Badros <gjb@cs.washington.edu> writes:

> 
> MAKING AN INSTANCE OF A SMOB
> 
> So, after registering our SMOB, the Guile interpreter knows how to deal
> with SMOBs of that type, but we still have not seen how to create a new
> object of a SMOB class.  We did, however, see what a SMOB object is
> stored internally in an earlier section.  In fact, Guile has no special
> help for constructing SMOB instances--- you simply write a primitive
> procedure that builds a CONS cell with the appropriate id field and a
> pointer to just C structure, and return that SCM object.  Since this is
> error-prone, Scwm provides a helper macro, SCWM_NEWCELL_SMOB:
> 
> #define SCWM_NEWCELL_SMOB(ANSWER,ID,PSMOB) \
>    do { \
>      SCM_NEWCELL((ANSWER)); \
>      SCM_SETCAR((ANSWER),(ID)); \
>      SCM_SETCDR((ANSWER),(SCM) (PSMOB)); \
>    } while (0)

This isn't all that safe, unless you change the gc slightly;
this would require putting SCM_SETCDR first (though if things go
wrong, either way is going to give you, at best, a fairly cryptic
error). 


> MARK
> 
> The Guile Scheme interpreter uses a simple mark and sweep garbage
> collection algorithm.  GC algorithms are beyond the scope of this
> document--- all you must understand is that you must recursively mark
> any SCM object you embed in a SMOB's C structure in that SMOB's mark
> function.  For color, we had a single SCM value embedded in the
> scwm_color struct: the color's name (as a Scheme string, not a C
> string).   Thus, color's mark function must mark that object to ensure
> that it does not get freed prematurely by Guile's garbage collector:
> 
> SCM mark_color(SCM obj) {
>   scmw_color *psc = COLOR(obj);
>   SCM_SETGC8MARK(obj);
    ^^^

This isn't needed, since the gc marks the object itself before it
calls the smob mark function. Also, marking really should go thru
scm_gc_mark(obj), instead of the mark macros (I really need to write
some more complete gc docs).

-- 
Greg