Make IPC_MESSAGE_EXPORT more robust.

Currently, files that want to export ipc messages currently do

  #undef IPC_MESSAGE_EXPORT
  #define IPC_MESSAGE_EXPORT  CONTENT_EXPORT

at the top, and files that don't want to export ipc messages just do nothing. This is problematic if a cc file does

  #include "exported_messages.h"
  #include "not_exported_messages.h"

because the second header file picks up the #define from the first file and declares all its messages as exported. In other translation units, where not_exported_messages.h is #included without another header above it, the messages will get default visibility – so the same class ends up with different visibilities in different translation units.

Instead, let ipc_message_macros.h #undef IPC_MESSAGE_EXPORT outside of the include guard, so that all files that don't set the define see it as defined to nothing. (Idea from jam@)

BUG=90078
TEST=No linker errors about IPC messages when doing components build on mac. (Other linker errors remain for now.)
TBR=brettw

Review URL: http://codereview.chromium.org/9425006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@122689 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 9 insertions, 6 deletions

diff --git a/ipc/ipc_message_macros.h b/ipc/ipc_message_macros.h
index 783108e..f37c05b 100644
--- a/ipc/ipc_message_macros.h
+++ b/ipc/ipc_message_macros.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -173,6 +173,14 @@
 //     ViewHostMsg_SyncMessageName::WriteReplyParams(reply_msg, out1, out2);
 //     Send(reply_msg);
 
+// Files that want to export their ipc messages should do
+//   #undef IPC_MESSAGE_EXPORT
+//   #define IPC_MESSAGE_EXPORT VISIBILITY_MACRO
+// after including this header, but before using any of the macros below.
+// (This needs to be before the include guard.)
+#undef IPC_MESSAGE_EXPORT
+#define IPC_MESSAGE_EXPORT
+
 #ifndef IPC_IPC_MESSAGE_MACROS_H_
 #define IPC_IPC_MESSAGE_MACROS_H_
 
@@ -184,11 +192,6 @@
 #include "ipc/ipc_message_utils_impl.h"
 #endif
 
-// Override this to force message classes to be exported.
-#ifndef IPC_MESSAGE_EXPORT
-#define IPC_MESSAGE_EXPORT
-#endif
-
 // Macros for defining structs.  May be subsequently redefined.
 #define IPC_STRUCT_BEGIN(struct_name) \
   IPC_STRUCT_BEGIN_WITH_PARENT(struct_name, IPC::NoParams)