diff --git a/ChangeLog b/ChangeLog
index c524462331..9b7b0cd2e5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Jan  9 15:40:07 2009  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* ext/socket/mkconstants.rb: define macro if default_value is passed.
+
 Fri Jan  9 13:20:08 2009  Hidetoshi NAGAI  <nagai@ai.kyutech.ac.jp>
 
 	* ext/tk/lib/tk.rb: rescue abnormal Encoding.locale_charmap value.
diff --git a/ext/socket/mkconstants.rb b/ext/socket/mkconstants.rb
index 20df7b598d..23993f99fa 100644
--- a/ext/socket/mkconstants.rb
+++ b/ext/socket/mkconstants.rb
@@ -197,13 +197,17 @@ init_constants(VALUE mConst)
 %   if guard
 #if <%=guard%>
 %   end
-#if defined(<%=name%>)
-    <%=define%>(<%=c_str name%>, <%=name%>);
 %   if default_value
-#else
-    <%=define%>(<%=c_str name%>, <%=default_value%>);
-%   end
+#ifndef <%=name%>
+#define <%=name%> <%=default_value%>
 #endif
+%   else
+#if defined(<%=name%>)
+%   end
+    <%=define%>(<%=c_str name%>, <%=name%>);
+%   unless default_value
+#endif
+%   end
 %   if guard
 #endif
 %   end