define_finalizer

static VALUE
define_final(int argc, VALUE *argv, VALUE os)
{
    rb_objspace_t *objspace = &rb_objspace;
    VALUE obj, block, table;
    st_data_t data;

    rb_scan_args(argc, argv, "11", &obj, &block);
    rb_check_frozen(obj);
    if (argc == 1) {
        block = rb_block_proc();
    }
    else if (!rb_respond_to(block, rb_intern("call"))) {
        rb_raise(rb_eArgError, "wrong type argument %s (should be callable)",
                 rb_obj_classname(block));
    }
    if (!FL_ABLE(obj)) {
        rb_raise(rb_eArgError, "cannot define finalizer for %s",
                 rb_obj_classname(obj));
    }
    RBASIC(obj)->flags |= FL_FINALIZE;

    block = rb_ary_new3(2, INT2FIX(rb_safe_level()), block);
    OBJ_FREEZE(block);

    if (st_lookup(finalizer_table, obj, &data)) {
        table = (VALUE)data;
        rb_ary_push(table, block);
    }
    else {
        table = rb_ary_new3(1, block);
        RBASIC(table)->klass = 0;
        st_add_direct(finalizer_table, obj, table);
    }
    return block;
}