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diff --git a/lib/asio/detail/macos_fenced_block.hpp b/lib/asio/detail/macos_fenced_block.hpp
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+//
+// detail/macos_fenced_block.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2018 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_DETAIL_MACOS_FENCED_BLOCK_HPP
+#define ASIO_DETAIL_MACOS_FENCED_BLOCK_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+
+#if defined(__MACH__) && defined(__APPLE__)
+
+#include <libkern/OSAtomic.h>
+#include "asio/detail/noncopyable.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+class macos_fenced_block
+  : private noncopyable
+{
+public:
+  enum half_t { half };
+  enum full_t { full };
+
+  // Constructor for a half fenced block.
+  explicit macos_fenced_block(half_t)
+  {
+  }
+
+  // Constructor for a full fenced block.
+  explicit macos_fenced_block(full_t)
+  {
+    OSMemoryBarrier();
+  }
+
+  // Destructor.
+  ~macos_fenced_block()
+  {
+    OSMemoryBarrier();
+  }
+};
+
+} // namespace detail
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // defined(__MACH__) && defined(__APPLE__)
+
+#endif // ASIO_DETAIL_MACOS_FENCED_BLOCK_HPP