 !IP_FRAG_USES_STATIC_BUF && !LWIP_NETIF_TX_SINGLE_PBUF /** A custom pbuf that holds a reference to another pbuf, which is freed  * when this custom pbuf is freed. This is used to create a custom PBUF_REF  * that points into the original pbuf. */ struct pbuf_custom_ref {   /** 'base class' */   struct pbuf_custom pc;   /** pointer to the original pbuf that is referenced */   struct pbuf *original; }; #endif /* !IP_FRAG_USES_STATIC_BUF && !LWIP_NETIF_TX_SINGLE_PBUF 