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diff --git a/src/libstrongswan/threading/semaphore.h b/src/libstrongswan/threading/semaphore.h
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+++ b/src/libstrongswan/threading/semaphore.h
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+/*
+ * Copyright (C) 2011 Tobias Brunner
+ * Hochschule fuer Technik Rapperswil
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
+ *
+ * This program 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 General Public License
+ * for more details.
+ */
+
+/**
+ * @defgroup semaphore semaphore
+ * @{ @ingroup threading
+ */
+
+#ifndef THREADING_SEMAPHORE_H_
+#define THREADING_SEMAPHORE_H_
+
+typedef struct semaphore_t semaphore_t;
+
+/**
+ * A semaphore is basically an integer whose value is never allowed to be
+ * lower than 0.  Two operations can be performed on it: increment the
+ * value by one, and decrement the value by one.  If the value is currently
+ * zero, then the decrement operation will blcok until the value becomes
+ * greater than zero.
+ */
+struct semaphore_t {
+
+	/**
+	 * Decrease the value by one, if it is greater than zero. Otherwise the
+	 * current thread is blocked and it waits until the value increases.
+	 */
+	void (*wait)(semaphore_t *this);
+
+	/**
+	 * Decrease the value by one, if it is greater than zero. Otherwise the
+	 * current thread is blocked and it waits until the value increases, or the
+	 * call times out.
+	 *
+	 * @param timeout		timeout im ms
+	 * @return				TRUE if timed out, FALSE otherwise
+	 */
+	bool (*timed_wait)(semaphore_t *this, u_int timeout);
+
+	/**
+	 * Decrease the value by one, if it is greater than zero. Otherwise the
+	 * current thread is blocked and it waits until the value increases, or the
+	 * call times out.
+	 *
+	 * The passed timeval should be calculated based on the time_monotonic()
+	 * function.
+	 *
+	 * @param tv			absolute time until timeout
+	 * @return				TRUE if timed out, FALSE otherwise
+	 */
+	bool (*timed_wait_abs)(semaphore_t *this, timeval_t tv);
+
+	/**
+	 * Increase the value by one. If the value becomes greater than zero, then
+	 * another thread waiting will be woken up.
+	 */
+	void (*post)(semaphore_t *this);
+
+	/**
+	 * Destroy a semaphore and free its resources.
+	 */
+	void (*destroy)(semaphore_t *this);
+};
+
+/**
+ * Create a semaphore instance.
+ *
+ * @param value		initial value (typically 0)
+ * @return			semaphore instance
+ */
+semaphore_t *semaphore_create(u_int value);
+
+#endif /** THREADING_SEMAPHORE_H_ @} */
+