class.c

void
rb_include_module(klass, module)
    VALUE klass, module;
{
    VALUE p, c;
    int changed = 0;

    rb_frozen_class_p(klass);
    if (!OBJ_TAINTED(klass)) {
	rb_secure(4);
    }
    
    if (TYPE(module) != T_MODULE) {
	Check_Type(module, T_MODULE);
    }

    OBJ_INFECT(klass, module);
    c = klass;
    while (module) {
	int superclass_seen = Qfalse;

	if (RCLASS(klass)->m_tbl == RCLASS(module)->m_tbl)
	    rb_raise(rb_eArgError, "cyclic include detected");
	/* ignore if the module included already in superclasses */
	for (p = RCLASS(klass)->super; p; p = RCLASS(p)->super) {
	    switch (BUILTIN_TYPE(p)) {
	      case T_ICLASS:
		if (RCLASS(p)->m_tbl == RCLASS(module)->m_tbl) {
		    if (!superclass_seen) {
			c = p;	/* move insertion point */
		    }
		    goto skip;
		}
		break;
	      case T_CLASS:
		superclass_seen = Qtrue;
		break;
	    }
	}
	c = RCLASS(c)->super = include_class_new(module, RCLASS(c)->super);
	changed = 1;
      skip:
	module = RCLASS(module)->super;
    }
    if (changed) rb_clear_cache();
}

eval.c

void
rb_clear_cache()
{
   struct cache_entry *ent, *end;

    if (!ruby_running) return;
    ent = cache; end = ent + CACHE_SIZE;
    while (ent < end) {
	ent->mid = 0;
	ent++;
    }
}

void
rb_extend_object(obj, module)
    VALUE obj, module;
{
    rb_include_module(rb_singleton_class(obj), module);
}