diff --git a/ChangeLog b/ChangeLog
index 6cd56f68a3..b6f4a9479d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Sep 13 19:56:50 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/socket/rubysocket.h (__DARWIN_ALIGNBYTES): workaround of a
+	  bug in system header of darwin 9.  [ruby-core:32341]
+
 Mon Sep 13 18:11:55 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (try_do): fix typo.  a patch from Peter Weldon
diff --git a/ext/socket/extconf.rb b/ext/socket/extconf.rb
index 7c31030d72..a6bc9bb46c 100644
--- a/ext/socket/extconf.rb
+++ b/ext/socket/extconf.rb
@@ -117,8 +117,7 @@ if !have_macro("IPPROTO_IPV6", headers) && have_const("IPPROTO_IPV6", headers)
   }
 end
 
-if (have_func("sendmsg") | have_func("recvmsg")) && /64-darwin/ !~ RUBY_PLATFORM
-  # CMSG_ macros are broken on 64bit darwin, because of use of __DARWIN_ALIGN.
+if have_func("sendmsg") | have_func("recvmsg")
   have_struct_member('struct msghdr', 'msg_control', ['sys/types.h', 'sys/socket.h'])
   have_struct_member('struct msghdr', 'msg_accrights', ['sys/types.h', 'sys/socket.h'])
 end
diff --git a/ext/socket/rubysocket.h b/ext/socket/rubysocket.h
index 356e6f3b1a..60ac25fbbd 100644
--- a/ext/socket/rubysocket.h
+++ b/ext/socket/rubysocket.h
@@ -138,6 +138,16 @@ struct sockaddr_storage {
 };
 #endif
 
+#if defined __APPLE__ && defined __MACH__
+/*
+ * CMSG_ macros are broken on 64bit darwin, because __DARWIN_ALIGN
+ * aligns up to __darwin_size_t which is 64bit, but CMSG_DATA is
+ * 32bit-aligned.
+ */
+#undef __DARWIN_ALIGNBYTES
+#define __DARWIN_ALIGNBYTES (sizeof(unsigned int) - 1)
+#endif
+
 #if defined(_AIX)
 #ifndef CMSG_SPACE
 # define CMSG_SPACE(len) (_CMSG_ALIGN(sizeof(struct cmsghdr)) + _CMSG_ALIGN(len))