diff --git a/src/select.c b/src/select.c
index fafaa62..da5e37e 100644
--- a/src/select.c
+++ b/src/select.c
@@ -40,6 +40,9 @@ int select_open(lua_State *L) {
     lua_pushstring(L, "_SETSIZE");
     lua_pushnumber(L, FD_SETSIZE);
     lua_rawset(L, -3);
+    lua_pushstring(L, "_SOCKETINVALID");  // for use with getfd() and setfd()
+    lua_pushnumber(L, SOCKET_INVALID);
+    lua_rawset(L, -3);
 #if LUA_VERSION_NUM > 501 && !defined(LUA_COMPAT_MODULE)
     luaL_setfuncs(L, func, 0);
 #else
diff --git a/src/udp.c b/src/udp.c
index 12e320a..a6ddf2f 100644
--- a/src/udp.c
+++ b/src/udp.c
@@ -120,6 +120,9 @@ int udp_open(lua_State *L)
     auxiliar_add2group(L, "udp{connected}",   "select{able}");
     auxiliar_add2group(L, "udp{unconnected}", "select{able}");
     /* define library functions */
+    lua_pushstring(L, "_UDPDATAGRAMSIZE");    // max send/receive size
+    lua_pushnumber(L, UDP_DATAGRAMSIZE);
+    lua_rawset(L, -3);
 #if LUA_VERSION_NUM > 501 && !defined(LUA_COMPAT_MODULE)
     luaL_setfuncs(L, func, 0);
 #else