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diff --git a/chrome/common/transport_dib.h b/chrome/common/transport_dib.h
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+++ b/chrome/common/transport_dib.h
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+// Copyright (c) 2006-2009 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.
+
+#ifndef CHROME_COMMON_TRANSPORT_DIB_H_
+#define CHROME_COMMON_TRANSPORT_DIB_H_
+
+#include "base/basictypes.h"
+
+#if defined(OS_WIN) || defined(OS_MACOSX)
+#include "base/shared_memory.h"
+#endif
+
+#if defined(OS_WIN)
+#include <windows.h>
+#endif
+
+// -----------------------------------------------------------------------------
+// A TransportDIB is a block of memory that is used to transport pixels
+// from the renderer process to the browser.
+// -----------------------------------------------------------------------------
+class TransportDIB {
+ public:
+  ~TransportDIB();
+
+  // Two typedefs are defined. A Handle is the type which can be sent over
+  // the wire so that the remote side can map the transport DIB. The Id typedef
+  // is sufficient to identify the transport DIB when you know that the remote
+  // side already may have it mapped.
+#if defined(OS_WIN)
+  typedef HANDLE Handle;
+  // On Windows, the Id type includes a sequence number (epoch) to solve an ABA
+  // issue:
+  //   1) Process A creates a transport DIB with HANDLE=1 and sends to B.
+  //   2) Process B maps the transport DIB and caches 1 -> DIB.
+  //   3) Process A closes the transport DIB and creates a new one. The new DIB
+  //      is also assigned HANDLE=1.
+  //   4) Process A sends the Handle to B, but B incorrectly believes that it
+  //      already has it cached.
+  struct HandleAndSequenceNum {
+    HandleAndSequenceNum()
+        : handle(NULL),
+          sequence_num(0) {
+    }
+
+    HandleAndSequenceNum(HANDLE h, uint32 seq_num)
+        : handle(h),
+          sequence_num(seq_num) {
+    }
+
+    bool operator< (const HandleAndSequenceNum& other) const {
+      if (other.handle < handle)
+        return true;
+      if (other.sequence_num < sequence_num)
+        return true;
+      return false;
+    }
+
+    HANDLE handle;
+    uint32 sequence_num;
+  };
+  typedef HandleAndSequenceNum Id;
+#elif defined(OS_MACOSX)
+  typedef base::SharedMemoryHandle Handle;
+  // On Mac, the inode number of the backing file is used as an id.
+  typedef base::SharedMemoryId Id;
+#elif defined(OS_LINUX)
+  typedef int Handle;  // These two ints are SysV IPC shared memory keys
+  typedef int Id;
+#endif
+
+  // Create a new TransportDIB
+  //   size: the minimum size, in bytes
+  //   epoch: Windows only: a global counter. See comment above.
+  //   returns: NULL on failure
+  static TransportDIB* Create(size_t size, uint32 sequence_num);
+
+  // Map the referenced transport DIB. Returns NULL on failure.
+  static TransportDIB* Map(Handle transport_dib);
+
+  // Return a pointer to the shared memory
+  void* memory() const;
+
+  // Return the maximum size of the shared memory. This is not the amount of
+  // data which is valid, you have to know that via other means, this is simply
+  // the maximum amount that /could/ be valid.
+  size_t size() const { return size_; }
+
+  // Return the identifier which can be used to refer to this shared memory
+  // on the wire.
+  Id id() const;
+
+  // Return a handle to the underlying shared memory. This can be sent over the
+  // wire to give this transport DIB to another process.
+  Handle handle() const;
+
+ private:
+  TransportDIB();
+#if defined(OS_WIN) || defined(OS_MACOSX)
+  explicit TransportDIB(base::SharedMemoryHandle dib);
+  base::SharedMemory shared_memory_;
+  uint32 sequence_num_;
+#elif defined(OS_LINUX)
+  int key_;  // SysV shared memory id
+  void* address_;  // mapped address
+#endif
+  size_t size_;  // length, in bytes
+};
+
+class MessageLoop;
+
+#endif  // CHROME_COMMON_TRANSPORT_DIB_H_