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+/*
+ * The MIT License
+ *
+ * Copyright (C) 2016 Alexander Saprykin <saprykin.spb@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * 'Software'), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+ * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+ * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+ * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+/**
+ * @file prwlock.h
+ * @brief Read-write lock
+ * @author Alexander Saprykin
+ *
+ * A read-write lock is a synchronization primitive which allows access to a
+ * critical section to not only the one thread, but instead it splits all
+ * threads into the two groups:
+ * - reader threads which perform only the reading operations without any shared
+ * data modifications;
+ * - writer threads which may perform the shared data modifitcations as well as
+ * its reading.
+ *
+ * When there are only the reader threads inside a critical section it is called
+ * a shared lock - actually you do not need any locking mechanism and all the
+ * threads share the lock. In this situation an arbitrary number of reader
+ * threads can perform shared data reading.
+ *
+ * The situation changes when a writer thread requests access to the same
+ * critical section. It will wait until all the current readers finish
+ * executing the critical section before acquiring the lock in exclusive manner:
+ * no one else can access the critical section until the writer finishes with
+ * it. Even another writer thread will have to wait for the lock to be released
+ * by the first writer before entering the critical section.
+ *
+ * To prevent writer startvation usually writers are in favor over readers,
+ * which is actually implementation dependent, though most operating systems try
+ * to follow this rule.
+ *
+ * A read-write lock is usually used when the writing operations are not
+ * performed too frequently, or when the number of reader threads is a way more
+ * than writer ones.
+ *
+ * A reader enters a critical section with p_rwlock_reader_lock() or
+ * p_rwlock_reader_trylock() and exits with p_rwlock_reader_unlock().
+ *
+ * A writer enters the critical section with p_rwlock_writer_lock() or
+ * p_rwlock_writer_trylock() and exits with p_rwlock_writer_unlock().
+ */
+
+#if !defined (PLIBSYS_H_INSIDE) && !defined (PLIBSYS_COMPILATION)
+#  error "Header files shouldn't be included directly, consider using <plibsys.h> instead."
+#endif
+
+#ifndef PLIBSYS_HEADER_PRWLOCK_H
+#define PLIBSYS_HEADER_PRWLOCK_H
+
+#include <pmacros.h>
+#include <ptypes.h>
+
+P_BEGIN_DECLS
+
+/** Read-write lock opaque data structure. */
+typedef struct PRWLock_ PRWLock;
+
+/**
+ * @brief Creates a new #PRWLock object.
+ * @return Pointer to a newly created #PRWLock object.
+ * @since 0.0.1
+ */
+P_LIB_API PRWLock *	p_rwlock_new		(void);
+
+/**
+ * @brief Locks a read-write lock for reading.
+ * @param lock #PRWLock to lock.
+ * @return TRUE in case of success, FALSE otherwise.
+ * @since 0.0.1
+ * @warning Do not lock a read-write lock recursively - it may lead to an
+ * application deadlock (implementation dependent).
+ *
+ * Forces the calling thread to sleep until the @a lock becomes available for
+ * locking.
+ */
+P_LIB_API pboolean	p_rwlock_reader_lock	(PRWLock *lock);
+
+/**
+ * @brief Tries to lock a read-write lock for reading immediately.
+ * @param lock #PRWLock to lock.
+ * @return TRUE in case of success, FALSE otherwise.
+ * @since 0.0.1
+ * @warning Do not lock a read-write lock recursively - it may lead to an
+ * application deadlock (implementation dependent).
+ *
+ * Tries to lock the @a lock and returns immediately if it is not available for
+ * locking.
+ */
+P_LIB_API pboolean	p_rwlock_reader_trylock	(PRWLock *lock);
+
+/**
+ * @brief Releases a locked for reading read-write lock.
+ * @param lock #PRWLock to release.
+ * @return TRUE in case of success, FALSE otherwise.
+ * @since 0.0.1
+ * @warning Do not use this function on non-locked read-write locks - behavior
+ * may be unpredictable.
+ * @warning Do not use this function to unlock a read-write lock which was
+ * locked for writing.
+ *
+ * If the @a lock was previously locked for reading then it becomes unlocked.
+ *
+ * It's implementation dependent whether only the same thread can lock and
+ * unlock the same read-write lock.
+ */
+P_LIB_API pboolean	p_rwlock_reader_unlock	(PRWLock *lock);
+
+/**
+ * @brief Locks a read-write lock for writing.
+ * @param lock #PRWLock to lock.
+ * @return TRUE in case of success, FALSE otherwise.
+ * @since 0.0.1
+ * @warning Do not lock a read-write lock recursively - it may lead to an
+ * application deadlock (implementation dependent).
+ *
+ * Forces the calling thread to sleep until the @a lock becomes available for
+ * locking.
+ */
+P_LIB_API pboolean	p_rwlock_writer_lock	(PRWLock *lock);
+
+/**
+ * @brief Tries to lock a read-write lock immediately.
+ * @param lock #PRWLock to lock.
+ * @return TRUE in case of success, FALSE otherwise.
+ * @since 0.0.1
+ * @warning Do not lock a read-write lock recursively - it may lead to an
+ * application deadlock (implementation dependent).
+ *
+ * Tries to lock the @a lock and returns immediately if it is not available for
+ * locking.
+ */
+P_LIB_API pboolean	p_rwlock_writer_trylock	(PRWLock *lock);
+
+/**
+ * @brief Releases a locked for writing read-write lock.
+ * @param lock #PRWLock to release.
+ * @return TRUE in case of success, FALSE otherwise.
+ * @since 0.0.1
+ * @warning Do not use this function on non-locked read-write locks - behavior
+ * may be unpredictable.
+ * @warning Do not use this function to unlock a read-write lock which was
+ * locked for reading.
+ *
+ * If the @a lock was previously locked for writing then it becomes unlocked.
+ *
+ * It's implementation dependent whether only the same thread can lock and
+ * unlock the same read-write lock.
+ */
+P_LIB_API pboolean	p_rwlock_writer_unlock	(PRWLock *lock);
+
+/**
+ * @brief Frees a #PRWLock object.
+ * @param lock #PRWLock to free.
+ * @since 0.0.1
+ * @warning It doesn't unlock the @a lock before freeing memory, so you should
+ * do it manually.
+ */
+P_LIB_API void		p_rwlock_free		(PRWLock *lock);
+
+P_END_DECLS
+
+#endif /* PLIBSYS_HEADER_PRWLOCK_H */