api.h, api_lib.c, api_msg.c, sockets.c, opt.h: add SO_RCVBUF option for UDP & RAW netconn. You need to set LWIP_SO_RCVBUF=1 in your lwipopts.h (it's disabled by default). Netconn API users can use the netconn_recv_bufsize macro to access it. This is a first release which have to be improve for TCP. Note it used the netconn::recv_avail which need to be more "thread-safe" (note there is already the problem for FIONREAD with lwip_ioctl/ioctlsocket).

src/include/lwip/api.h

@@ -114,6 +114,9 @@ struct netconn {
 #if LWIP_SO_RCVTIMEO
   int recv_timeout;
 #endif /* LWIP_SO_RCVTIMEO */
+#if LWIP_SO_RCVBUF
+  int recv_bufsize;
+#endif /* LWIP_SO_RCVBUF */
   u16_t recv_avail;
   /** TCP: when data passed to netconn_write doesn't fit into the send buffer,
       this temporarily stores the message. */
@@ -176,7 +179,8 @@ err_t             netconn_join_leave_group (struct netconn *conn,
                                             enum netconn_igmp join_or_leave);
 #endif /* LWIP_IGMP */
 
-#define netconn_err(conn)         ((conn)->err)
+#define netconn_err(conn)          ((conn)->err)
+#define netconn_recv_bufsize(conn) ((conn)->recv_bufsize)
 
 #ifdef __cplusplus
 }

src/include/lwip/opt.h

@@ -927,6 +927,13 @@
 #define LWIP_SO_RCVTIMEO                0
 #endif
 
+/**
+ * LWIP_SO_RCVBUF==1: Enable SO_RCVBUF processing.
+ */
+#ifndef LWIP_SO_RCVBUF
+#define LWIP_SO_RCVBUF                  0
+#endif
+
 /**
  * SO_REUSE==1: Enable SO_REUSEADDR and SO_REUSEPORT options. DO NOT USE!
  */