Handout

kernel/sync/semaphore.h

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+#pragma once
+#include "../object/queue.h"
+#include "../types.h"
+
+/*! \file
+ *  \brief \ref Semaphore for synchronization of threads.
+ */
+
+/*!
+ * \defgroup ipc Inter-Process Communication
+ * \brief Communication between threads
+ */
+
+// Forward declarations to break cyclic includes
+struct Vault;
+class Thread;
+
+/*! \brief Semaphore used for synchronization of threads.
+ *  \ingroup ipc
+ *
+ *  The class Semaphore implements the concept of counting semaphores.
+ *  Waiting threads are parked inside a \ref Queue.
+ */
+class Semaphore {
+	// Prevent copies and assignments
+	Semaphore(const Semaphore&) = delete;
+	Semaphore& operator=(const Semaphore&) = delete;
+	unsigned counter;
+	Queue<Thread> waiting;
+
+   public:
+	/*! \brief Constructor; initialized the counter with provided value `c`
+	 *  \param c Initial counter value
+	 */
+	explicit Semaphore(unsigned c = 0) : counter(c) {}
+
+	/*! \brief Wait for access to the critical area.
+	 *
+	 *  Enter/decrement/wait operation: If the counter is greater than 0, then
+	 * it is decremented by one. Otherwise the calling thread will be enqueued
+	 *  to wait until it can do that.
+	 *
+	 */
+	void p(Vault& vault);
+
+	/*! \brief Leave the critical area.
+	 *
+	 *  Leave/increment operation: If there are threads waiting, wake the
+	 *  first one; otherwise increment the counter by one.
+	 *
+	 */
+	void v(Vault& vault);
+};