diff --git a/include/nuttx/net/ioctl.h b/include/nuttx/net/ioctl.h
index afaa1df13ac..3442698a4e7 100644
--- a/include/nuttx/net/ioctl.h
+++ b/include/nuttx/net/ioctl.h
@@ -169,6 +169,11 @@
 #define SIOCGMIIREG      _SIOC(0x004d) /* Get a MII register via MDIO */
 #define SIOCSMIIREG      _SIOC(0x004e) /* Set a MII register via MDIO */
 
+/* Unix domain sockets ******************************************************/
+
+#define SIOCINQ          _SIOC(0x004f) /* Returns the amount of queued unread
+                                        * data in the receive */
+
 /****************************************************************************
  * Type Definitions
  ****************************************************************************/
diff --git a/include/sys/un.h b/include/sys/un.h
index a9eb1cd30b2..eb431780c95 100644
--- a/include/sys/un.h
+++ b/include/sys/un.h
@@ -74,6 +74,17 @@ struct sockaddr_un
   char sun_path[UNIX_PATH_MAX];  /* pathname */
 };
 
+/* There are three types of addresses:
+ *
+ * 1. pathname:  sun_path holds a null terminated string.  The allocated
+ *    size may be variable:  sizeof(sa_family_t) + strlen(pathname) + 1
+ * 2. unnamed:  A unix socket that is not bound to any name.  This case
+ *    there is no path.  The allocated size may be sizeof(sa_family_t)
+ * 3. abstract. The abstract path is destinguished because the pathname
+ *    consists of only the NUL terminator.  The allocated size is then
+ *    sizeof(s_family_t) + 1.
+ */
+
 /****************************************************************************
  * Public Function Prototypes
  ****************************************************************************/