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smob gc protection, and inheritance
From: |
Doug Evans |
Subject: |
smob gc protection, and inheritance |
Date: |
Tue, 03 Sep 2013 12:27:51 -0700 |
Hi.
I have a few questions about smobs:
1) Suppose I have some C code that creates a smob and its containing
SCM, but does not always expose the SCM to Scheme.
E.g.
struct foo_object
{
int bar;
SCM baz;
}
static SCM
make_foo_smob (void)
{
struct foo_object *foo_smob = (struct foo_object *)
scm_gc_malloc (sizeof (struct foo_object), "foo");
SCM foo_scm;
foo_smob->bar = -1;
foo_smob->baz = SCM_BOOL_F;
foo_scm = scm_new_smob (foo_smob_tag, (scm_t_bits) foo_smob);
return foo_scm;
}
If the caller stores foo_smob in the heap somewhere, and not foo_scm,
is that enough to prevent the object from being garbage collected?
Or should the caller be saving foo_scm?
e.g.
void
caller (struct another_struct *s)
{
SCM foo_scm = make_foo_smob ();
#ifdef SAVE_foo_smob
struct foo_object *foo_smob = (struct foo_object *) SCM_SMOB_DATA (foo_scm);
s->xyz = foo_smob;
#endif
#ifdef SAVE_foo_scm
s->xyz = foo_scm;
#endif
}
IOW, if I use SAVE_foo_smob, will my object get accidentally gc'd?
2) Is it possible to inherit, e.g., with goops, a smob?
IOW, can I extend a smob through inheritance?
Or must I store the smob in a class, and provide accessors?
[kinda like the "is a" vs "has a" relationship]
3) The docs aren't as clear as they could be on whether the "smob"
free function needs to scm_gc_free all results of calls to scm_gc_malloc
made when constructing the smob. IIUC, this is not necessary.
However, why does the image example do this?
ref: libguile-smobs.texi:
size_t
free_image (SCM image_smob)
{
struct image *image = (struct image *) SCM_SMOB_DATA (image_smob);
scm_gc_free (image->pixels,
image->width * image->height,
"image pixels");
scm_gc_free (image, sizeof (struct image), "image"); <<<<<<<<<<<<
return 0;
}
TIA
- smob gc protection, and inheritance,
Doug Evans <=