diff --git a/ext/etc/.document b/ext/etc/.document
index 2b0d7eb..9bbea23 100644
--- a/ext/etc/.document
+++ b/ext/etc/.document
@@ -1 +1,2 @@
 etc.c
+constdefs.h
diff --git a/ext/etc/mkconstants.rb b/ext/etc/mkconstants.rb
index a752d64..c8ebb67 100644
--- a/ext/etc/mkconstants.rb
+++ b/ext/etc/mkconstants.rb
@@ -35,6 +35,12 @@ def c_str(str)
 
 opt.parse!
 
+CONST_PREFIXES = {
+  'SC' => 'for Etc.sysconf; See <tt>man sysconf</tt>',
+  'CS' => 'for Etc.confstr; See <tt>man constr</tt>',
+  'PC' => 'for IO#pathconf; See <tt>man fpathconf</tt>',
+}
+
 h = {}
 COMMENTS = {}
 
@@ -49,6 +55,13 @@ def c_str(str)
     next
   end
   h[name] = default_value
+  if additional = CONST_PREFIXES[name[/\A_([A-Z]+)_/, 1]]
+    if comment&.match(/\w\z/)
+      comment << " " << additional
+    else
+      (comment ||= String.new) << " " << additional.sub(/\A\w/) {$&.upcase}
+    end
+  end
   COMMENTS[name] = comment if comment
 }
 DEFS = h.to_a
@@ -123,6 +136,9 @@ def each_name(pat)
 static void
 init_constants(VALUE mod)
 {
+#if 0
+    mod = rb_define_module("Etc");
+#endif
 <%= gen_const_defs %>
 }
 EOS