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diff --git a/src/semaphore.hpp b/src/semaphore.hpp
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+/*
+    Copyright (c) 2007-2011 iMatix Corporation
+    Copyright (c) 2007-2011 Other contributors as noted in the AUTHORS file
+
+    This file is part of 0MQ.
+
+    0MQ is free software; you can redistribute it and/or modify it under
+    the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation; either version 3 of the License, or
+    (at your option) any later version.
+
+    0MQ is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef __ZMQ_SEMAPHORE_HPP_INCLUDED__
+#define __ZMQ_SEMAPHORE_HPP_INCLUDED__
+
+#include "platform.hpp"
+#include "err.hpp"
+
+#if defined ZMQ_HAVE_WINDOWS
+#include "windows.hpp"
+#elif defined ZMQ_HAVE_OPENVMS
+#include <pthread.h>
+#else
+#include <semaphore.h>
+#endif
+
+namespace zmq
+{
+    //  Simple semaphore. Only single thread may be waiting at any given time.
+    //  Also, the semaphore may not be posted before the previous post
+    //  was matched by corresponding wait and the waiting thread was
+    //  released.
+
+#if defined ZMQ_HAVE_WINDOWS
+
+    //  On Windows platform simple semaphore is implemeted using event object.
+
+    class semaphore_t
+    { 
+    public:
+
+        //  Initialise the semaphore.
+        inline semaphore_t ()
+        {
+            ev = CreateEvent (NULL, FALSE, FALSE, NULL);
+            win_assert (ev != NULL);
+        }
+
+        //  Destroy the semaphore.
+        inline ~semaphore_t ()
+        {
+            int rc = CloseHandle (ev);
+            win_assert (rc != 0);    
+        }
+
+        //  Wait for the semaphore.
+        inline void wait ()
+        {
+            DWORD rc = WaitForSingleObject (ev, INFINITE);
+            win_assert (rc != WAIT_FAILED);
+        }
+
+        //  Post the semaphore.
+        inline void post ()
+        {
+            int rc = SetEvent (ev);
+            win_assert (rc != 0);
+        }
+
+    private:
+
+        HANDLE ev;
+
+        semaphore_t (const semaphore_t&);
+        const semaphore_t &operator = (const semaphore_t&);
+    };
+
+#elif defined ZMQ_HAVE_LINUX || defined ZMQ_HAVE_OSX || defined ZMQ_HAVE_OPENVMS
+
+    //  On platforms that allow for double locking of a mutex from the same
+    //  thread, simple semaphore is implemented using mutex, as it is more
+    //  efficient than full-blown semaphore.
+
+    //  Note that OS-level semaphore is not implemented on OSX, so the below
+    //  code is not only optimisation, it's necessary to make 0MQ work on OSX.
+
+    class semaphore_t
+    { 
+    public:
+
+        //  Initialise the semaphore.
+        inline semaphore_t ()
+        {
+            int rc = pthread_mutex_init (&mutex, NULL);
+            posix_assert (rc);
+            rc = pthread_mutex_lock (&mutex);
+            posix_assert (rc);
+        }
+
+        //  Destroy the semaphore.
+        inline ~semaphore_t ()
+        {
+            int rc = pthread_mutex_unlock (&mutex);
+            posix_assert (rc);
+            rc = pthread_mutex_destroy (&mutex);
+            posix_assert (rc);
+        }
+
+        //  Wait for the semaphore.
+        inline void wait ()
+        {
+             int rc = pthread_mutex_lock (&mutex);
+             posix_assert (rc);
+        }
+
+        //  Post the semaphore.
+        inline void post ()
+        {
+            int rc = pthread_mutex_unlock (&mutex);
+            posix_assert (rc);
+        }
+
+    private:
+
+        pthread_mutex_t mutex;
+
+        semaphore_t (const semaphore_t&);
+        const semaphore_t &operator = (const semaphore_t&);
+    };
+
+#else
+
+    //  Default implementation maps simple semaphore to POSIX semaphore.
+
+    class semaphore_t
+    { 
+    public:
+
+        //  Initialise the semaphore.
+        inline semaphore_t ()
+        {
+             int rc = sem_init (&sem, 0, 0);
+             errno_assert (rc != -1);
+        }
+
+        //  Destroy the semaphore.
+        inline ~semaphore_t ()
+        {
+             int rc = sem_destroy (&sem);
+             errno_assert (rc != -1);
+        }
+
+        //  Wait for the semaphore.
+        inline void wait ()
+        {
+             int rc = sem_wait (&sem);
+             errno_assert (rc != -1);
+        }
+
+        //  Post the semaphore.
+        inline void post ()
+        {
+            int rc = sem_post (&sem);
+            errno_assert (rc != -1);
+        }
+
+    private:
+
+        //  Underlying system semaphore object.
+        sem_t sem;
+
+        semaphore_t (const semaphore_t&);
+        const semaphore_t &operator = (const semaphore_t&);
+    };
+
+#endif
+
+}
+
+#endif
+