sockets.c, ip.h, api.h, tcp.h: declare a "struct ip_pcb" which only contains IP_PCB. Add in the netconn's "pcb" union a "struct ip_pcb *ip;" (no size change). Use this new field to access to common pcb fields (ttl, tos, so_options, etc...). Enable to access to these fields with LWIP_TCP=0.

src/include/ipv4/lwip/ip.h

@@ -90,6 +90,10 @@ err_t ip_output_if(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
   /* link layer address resolution hint */ \
   IP_PCB_ADDRHINT
 
+struct ip_pcb {
+/* Common members of all PCB types */
+  IP_PCB;
+};
 
 /*
  * Option flags per-socket. These are the same like SO_XXX.

src/include/lwip/api.h

@@ -98,6 +98,7 @@ struct netconn {
   enum netconn_type type;
   enum netconn_state state;
   union {
+    struct ip_pcb  *ip;
     struct tcp_pcb *tcp;
     struct udp_pcb *udp;
     struct raw_pcb *raw;

src/include/lwip/tcp.h

@@ -34,7 +34,7 @@
 
 #include "lwip/opt.h"
 
-/*#if LWIP_TCP*/ /* don't build if not configured for use in lwipopts.h */
+#if LWIP_TCP /* don't build if not configured for use in lwipopts.h */
 
 #include "lwip/sys.h"
 #include "lwip/mem.h"
@@ -592,6 +592,6 @@ extern struct tcp_pcb *tcp_tmp_pcb;      /* Only used for temporary storage. */
 }
 #endif
 
-/*#endif*/ /* LWIP_TCP */
+#endif /* LWIP_TCP */
 
 #endif /* __LWIP_TCP_H__ */