objc_msgSend的定义是什么？

IMP _class_lookupMethodAndLoadCache (Class cls, SEL sel) { ... if (methodPC == NULL) { //! 这里指定消息转发入口 // Class and superclasses do not respond -- use forwarding smt = malloc_zone_malloc (_objc_create_zone(), sizeof(struct objc_method)); smt->method_name = sel; smt->method_types = ""; smt->method_imp = &_objc_msgForward; _cache_fill (cls, smt, sel); methodPC = &_objc_msgForward;
}

... }