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<section id="threads.concepts" last-revision="$Date: 2005/10/16 14:37:34 $">
	<title>Concepts</title>

	<section id="threads.concepts.mutexes">
		<title>Mutexes</title>

		<note>Certain changes to the mutexes and lock concepts are
		currently under discussion. In particular, the combination of
		the multiple lock concepts into a single lock concept
		is likely, and the combination of the multiple mutex
		concepts into a single mutex concept is also possible.</note>
		
		<para>A mutex (short for mutual-exclusion) object is used to serialize
		access to a resource shared between multiple threads. The
		<link linkend="threads.concepts.Mutex">Mutex</link> concept, with
		<link linkend="threads.concepts.TryMutex">TryMutex</link> and
		<link linkend="threads.concepts.TimedMutex">TimedMutex</link> refinements,
		formalize the requirements. A model that implements Mutex and its
		refinements has two states: <emphasis role="bold">locked</emphasis> and
		<emphasis role="bold">unlocked</emphasis>. Before using a shared resource, a
		thread locks a &Boost.Threads; mutex object
		(an object whose type is a model of
		<link linkend="threads.concepts.Mutex">Mutex</link> or one of it's
		refinements), ensuring
		<link linkend="threads.glossary.thread-safe">thread-safe</link> access to
		the shared resource. When use of the shared resource is complete, the thread
		unlocks the mutex object, allowing another thread to acquire the lock and
		use the shared resource.</para>
		<para>Traditional C thread APIs, like POSIX threads or the Windows thread
		APIs, expose functions to lock and unlock a mutex object. This is dangerous
		since it's easy to forget to unlock a locked mutex. When the flow of control
		is complex, with multiple return points, the likelihood of forgetting to
		unlock a mutex object becomes even greater. When exceptions are thrown,
		it becomes nearly impossible to ensure that the mutex object is unlocked
		properly when using these traditional API's. The result is
		<link linkend="threads.glossary.deadlock">deadlock</link>.</para>
		<para>Many C++ threading libraries use a pattern known as <emphasis>Scoped
		Locking</emphasis> &cite.SchmidtStalRohnertBuschmann; to free the programmer from
		the need to explicitly lock and unlock mutex objects. With this pattern, a
		<link linkend="threads.concepts.lock-concepts">Lock</link> concept is employed where
		the lock object's constructor locks the associated mutex object and the
		destructor automatically does the unlocking. The
		&Boost.Threads; library takes this pattern to
		the extreme in that Lock concepts are the only way to lock and unlock a
		mutex object: lock and unlock functions are not exposed by any
		&Boost.Threads; mutex objects. This helps to
		ensure safe usage patterns, especially when code throws exceptions.</para>

		<section id="threads.concepts.locking-strategies">
			<title>Locking Strategies</title>
			
			<para>Every mutex object follows one of several locking strategies. These
			strategies define the semantics for the locking operation when the calling
			thread already owns a lock on the mutex object.</para>
			
			<section id="threads.concepts.recursive-locking-strategy">
				<title>Recursive Locking Strategy</title>
			
				<para>With a recursive locking strategy, when a thread attempts to acquire
				a lock on the mutex object for which it already owns a lock, the operation
				is successful. Note the distinction between a thread, which may have
				multiple locks outstanding on a recursive mutex object, and a lock object,
				which even for a recursive mutex object cannot have any of its lock
				functions called multiple times without first calling unlock.</para>
				
				<para>Internally a lock count is maintained and the owning thread must
				unlock the mutex object the same number of times that it locked it before
				the mutex object's state returns to unlocked. Since mutex objects in
				&Boost.Threads; expose locking
				functionality only through lock concepts, a thread will always unlock a
				mutex object the same number of times that it locked it. This helps to
				eliminate a whole set of errors typically found in traditional C style
				thread APIs.</para>
				
				<para>Classes <classname>boost::recursive_mutex</classname>,
				<classname>boost::recursive_try_mutex</classname> and
				<classname>boost::recursive_timed_mutex</classname> use this locking
				strategy.</para>
			</section>
			
			<section id="threads.concepts.checked-locking-strategy">
				<title>Checked Locking Strategy</title>
				
				<para>With a checked locking strategy, when a thread attempts to acquire a
				lock on the mutex object for which the thread already owns a lock, the
				operation will fail with some sort of error indication. Further, attempts
				by a thread to unlock a mutex object that was not locked by the thread
				will also return some sort of error indication. In
				&Boost.Threads;, an exception of type
				<classname>boost::lock_error</classname> 
				would be thrown in these cases.</para>
				
				<para>&Boost.Threads; does not currently
				provide any mutex objects that use this strategy.</para>
			</section>
			
			<section id="threads.concepts.unchecked-locking-strategy">
				<title>Unchecked Locking Strategy</title>
				
				<para>With an unchecked locking strategy, when a thread attempts to acquire
				a lock on a mutex object for which the thread already owns a lock the
				operation will
				<link linkend="threads.glossary.deadlock">deadlock</link>. In general
				this locking strategy is less safe than a checked or recursive strategy,
				but it's also a faster strategy and so is employed by many libraries.</para>
				
				<para>&Boost.Threads; does not currently
				provide any mutex objects that use this strategy.</para>
			</section>
			
			<section id="threads.concepts.unspecified-locking-strategy">
				<title>Unspecified Locking Strategy</title>
				
				<para>With an unspecified locking strategy, when a thread attempts to
				acquire a lock on a mutex object for which the thread already owns a lock
				the operation results in 
				<link linkend="threads.glossary.undefined-behavior">undefined behavior</link>.
				</para>
				
				<para>In general a mutex object with an unspecified locking strategy is
				unsafe, and it requires programmer discipline to use the mutex object
				properly. However, this strategy allows an implementation to be as fast as
				possible with no restrictions on its implementation. This is especially
				true for portable implementations that wrap the native threading support
				of a platform. For this reason, the classes
				<classname>boost::mutex</classname>,
				<classname>boost::try_mutex</classname> and
				<classname>boost::timed_mutex</classname> use this locking strategy
				despite the lack of safety.</para>
			</section>
		</section>
		
		<section id="threads.concepts.sheduling-policies">
			<title>Scheduling Policies</title>
			
			<para>Every mutex object follows one of several scheduling policies. These
			policies define the semantics when the mutex object is unlocked and there is
			more than one thread waiting to acquire a lock. In other words, the policy
			defines which waiting thread shall acquire the lock.</para>
			
			<section id="threads.concepts.FIFO-scheduling-policy">
				<title>FIFO Scheduling Policy</title>
				
				<para>With a FIFO ("First In First Out") scheduling policy, threads waiting 
				for the lock will acquire it in a first-come-first-served order.
				This can help prevent a high priority thread from starving lower priority
				threads that are also waiting on the mutex object's lock.</para>
			</section>
			
			<section id="threads.concepts.priority-driven-scheduling-policy">
				<title>Priority Driven Policy</title>
				
				<para>With a Priority Driven scheduling policy, the thread with the
				highest priority acquires the lock. Note that this means that low-priority
				threads may never acquire the lock if the mutex object has high contention
				and there is always at least one high-priority thread waiting. This is
				known as thread starvation. When multiple threads of the same priority are
				waiting on the mutex object's lock one of the other scheduling priorities
				will determine which thread shall acquire the lock.</para>
			</section>
			
			<section id="threads.concepts.unspecified-scheduling-policy">
				<title>Unspecified Policy</title>
				
				<para>The mutex object does not specify a scheduling policy. In order to
				ensure portability, all &Boost.Threads;
				mutex objects use an unspecified scheduling policy.</para>
			</section>
		</section>
		
		<section id="threads.concepts.mutex-concepts">
			<title>Mutex Concepts</title>
			
			<section id="threads.concepts.Mutex">
				<title>Mutex Concept</title>
				
				<para>A Mutex object has two states: locked and unlocked. Mutex object
				state can only be determined by a lock object meeting the
				appropriate lock concept requirements
				and constructed for the Mutex object.</para>
				
				<para>A Mutex is
				<ulink url="../../libs/utility/utility.htm#Class%20noncopyable">
				NonCopyable</ulink>.</para>
				<para>For a Mutex type <code>M</code>
				and an object <code>m</code> of that type, 
				the following expressions must be well-formed
				and have the indicated effects.</para>
				
				<table>
					<title>Mutex Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry>M m;</entry>
								<entry><para>Constructs a mutex object m.</para>
								<para>Postcondition: m is unlocked.</para></entry>
							</row>
							<row>
								<entry>(&amp;m)-&gt;~M();</entry>
								<entry>Precondition: m is unlocked. Destroys a mutex object
								m.</entry>
							</row>
							<row>
								<entry>M::scoped_lock</entry>
								<entry>A model of
								<link linkend="threads.concepts.ScopedLock">ScopedLock</link>
								</entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.TryMutex">
				<title>TryMutex Concept</title>
				
				<para>A TryMutex is a refinement of
				<link linkend="threads.concepts.Mutex">Mutex</link>. 
				For a TryMutex type <code>M</code>
				and an object <code>m</code> of that type, 
				the following expressions must be well-formed 
				and have the indicated effects.</para>
				
				<table>
					<title>TryMutex Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry>M::scoped_try_lock</entry>
								<entry>A model of
								<link linkend="threads.concepts.ScopedTryLock">ScopedTryLock</link>
								</entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.TimedMutex">
				<title>TimedMutex Concept</title>
				
				<para>A TimedMutex is a refinement of
				<link linkend="threads.concepts.TryMutex">TryMutex</link>. 
				For a TimedMutex type <code>M</code>
				and an object <code>m</code> of that type, 
				the following expressions must be well-formed
				and have the indicated effects.</para>
				
				<table>
					<title>TimedMutex Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry>M::scoped_timed_lock</entry>
								<entry>A model of
								<link
								linkend="threads.concepts.ScopedTimedLock">ScopedTimedLock</link>
								</entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		</section>
		
		<section id="threads.concepts.mutex-models">
			<title>Mutex Models</title>
			
			<para>&Boost.Threads; currently supplies six models of
			<link linkend="threads.concepts.Mutex">Mutex</link>
			and its refinements.</para>
			
			<table>
				<title>Mutex Models</title>
				
				<tgroup cols="3">
					<thead>
						<row>
							<entry>Concept</entry>
							<entry>Refines</entry>
							<entry>Models</entry>
						</row>
					</thead>
					
					<tbody>
						<row>
							<entry><link linkend="threads.concepts.Mutex">Mutex</link></entry>
							<entry></entry>
							<entry>
							<para><classname>boost::mutex</classname></para>
							<para><classname>boost::recursive_mutex</classname></para>
							</entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.TryMutex">TryMutex</link></entry>
							<entry><link linkend="threads.concepts.Mutex">Mutex</link></entry>
							<entry>
								<para><classname>boost::try_mutex</classname></para>
								<para><classname>boost::recursive_try_mutex</classname></para>
							</entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.TimedMutex">TimedMutex</link></entry>
							<entry><link linkend="threads.concepts.TryMutex">TryMutex</link></entry>
							<entry>
								<para><classname>boost::timed_mutex</classname></para>
								<para><classname>boost::recursive_timed_mutex</classname></para>
							</entry>
						</row>
					</tbody>
				</tgroup>
			</table>
		</section>
		
		<section id="threads.concepts.lock-concepts">
			<title>Lock Concepts</title>

			<para>A lock object provides a safe means for locking and unlocking a mutex
			object (an object whose type is a model of <link
			linkend="threads.concepts.Mutex">Mutex</link> or one of its refinements). In
			other words they are an implementation of the <emphasis>Scoped
			Locking</emphasis> &cite.SchmidtStalRohnertBuschmann; pattern. The <link
			linkend="threads.concepts.ScopedLock">ScopedLock</link>,
			<link linkend="threads.concepts.ScopedTryLock">ScopedTryLock</link>, and 
			<link linkend="threads.concepts.ScopedTimedLock">ScopedTimedLock</link>
			concepts formalize the requirements.</para>
			<para>Lock objects are constructed with a reference to a mutex object and
			typically acquire ownership of the mutex object by setting its state to
			locked. They also ensure ownership is relinquished in the destructor. Lock
			objects also expose functions to query the lock status and to manually lock
			and unlock the mutex object.</para>
			<para>Lock objects are meant to be short lived, expected to be used at block
			scope only. The lock objects are not <link
			linkend="threads.glossary.thread-safe">thread-safe</link>. Lock objects must
			maintain state to indicate whether or not they've been locked and this state
			is not protected by any synchronization concepts. For this reason a lock
			object should never be shared between multiple threads.</para>

			<section id="threads.concepts.Lock">
				<title>Lock Concept</title>
				
				<para>For a Lock type <code>L</code> 
				and an object <code>lk</code> 
				and const object <code>clk</code> of that type, 
				the following expressions must be well-formed
				and have the indicated effects.</para>
				
				<table>
					<title>Lock Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry><code>(&amp;lk)-&gt;~L();</code></entry>
								<entry><code>if (locked()) unlock();</code></entry>
							</row>
							<row>
								<entry><code>(&amp;clk)-&gt;operator const void*()</code></entry>
								<entry>Returns type void*, non-zero if the associated mutex
								object has been locked by <code>clk</code>, otherwise 0.</entry>
							</row>
							<row>
								<entry><code>clk.locked()</code></entry>
								<entry>Returns a <code>bool</code>, <code>(&amp;clk)-&gt;operator
								const void*() != 0</code></entry>
							</row>
							<row>
								<entry><code>lk.lock()</code></entry>
								<entry>
									<para>Throws <classname>boost::lock_error</classname>
									if <code>locked()</code>.</para>
									
									<para>If the associated mutex object is
									already locked by some other thread, places the current thread in the 
									<link linkend="threads.glossary.thread-state">Blocked</link> state until
									the associated mutex is unlocked, after which the current thread
									is placed in the <link
									linkend="threads.glossary.thread-state">Ready</link> state,
									eventually to be returned to the <link
									linkend="threads.glossary.thread-state">Running</link> state. If
									the associated mutex object is already locked by the same thread
									the behavior is dependent on the <link
									linkend="threads.concepts.locking-strategies">locking
									strategy</link> of the associated mutex object.</para>
									
									<para>Postcondition: <code>locked() == true</code></para>
								</entry>
							</row>
							<row>
								<entry><code>lk.unlock()</code></entry>
								<entry>
									<para>Throws <classname>boost::lock_error</classname>
									if <code>!locked()</code>.</para>
									
									<para>Unlocks the associated mutex.</para>
									
									<para>Postcondition: <code>!locked()</code></para></entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.ScopedLock">
				<title>ScopedLock Concept</title>
				
				<para>A ScopedLock is a refinement of <link
				linkend="threads.concepts.Lock">Lock</link>. 
				For a ScopedLock type <code>L</code> 
				and an object <code>lk</code> of that type, 
				and an object <code>m</code> of a type meeting the 
				<link linkend="threads.concepts.Mutex">Mutex</link> requirements, 
				and an object <code>b</code> of type <code>bool</code>, 
				the following expressions must be well-formed 
				and have the indicated effects.</para>
				
				<table>
					<title>ScopedLock Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry><code>L lk(m);</code></entry>
								<entry>Constructs an object <code>lk</code>, and associates mutex
								object <code>m</code> with it, then calls
								<code>lock()</code></entry>
							</row>
							<row>
								<entry><code>L lk(m,b);</code></entry>
								<entry>Constructs an object <code>lk</code>, and associates mutex
								object <code>m</code> with it, then if <code>b</code>, calls
								<code>lock()</code></entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.TryLock">
				<title>TryLock Concept</title>
				
				<para>A TryLock is a refinement of <link
				linkend="threads.concepts.Lock">Lock</link>. 
				For a TryLock type <code>L</code> 
				and an object <code>lk</code> of that type, 
				the following expressions must be well-formed 
				and have the indicated effects.</para>
				
				<table>
					<title>TryLock Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry><code>lk.try_lock()</code></entry>
								<entry>
									<para>Throws  <classname>boost::lock_error</classname>
									if locked().</para>
									
									<para>Makes a
									non-blocking attempt to lock the associated mutex object,
									returning <code>true</code> if the lock attempt is successful,
									otherwise <code>false</code>. If the associated mutex object is
									already locked by the same thread the behavior is dependent on the
									<link linkend="threads.concepts.locking-strategies">locking
									strategy</link> of the associated mutex object.</para>
								</entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.ScopedTryLock">
				<title>ScopedTryLock Concept</title>
				
				<para>A ScopedTryLock is a refinement of <link
				linkend="threads.concepts.TryLock">TryLock</link>. 
				For a ScopedTryLock type <code>L</code> 
				and an object <code>lk</code> of that type, 
				and an object <code>m</code> of a type meeting the 
				<link linkend="threads.concepts.TryMutex">TryMutex</link> requirements, 
				and an object <code>b</code> of type <code>bool</code>, 
				the following expressions must be well-formed 
				and have the indicated effects.</para>
				
				<table>
					<title>ScopedTryLock Expressions</title>
					
					<tgroup cols="2">
						<thead>
							<row>
								<entry>Expression</entry>
								<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry><code>L lk(m);</code></entry>
								<entry>Constructs an object <code>lk</code>, and associates mutex
								object <code>m</code> with it, then calls
								<code>try_lock()</code></entry>
							</row>
							<row>
								<entry><code>L lk(m,b);</code></entry>
								<entry>Constructs an object <code>lk</code>, and associates mutex
								object <code>m</code> with it, then if <code>b</code>, calls
								<code>lock()</code></entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.TimedLock">
				<title>TimedLock Concept</title>
				
				<para>A TimedLock is a refinement of <link
				linkend="threads.concepts.TryLock">TryLock</link>. 
				For a TimedLock type <code>L</code> 
				and an object <code>lk</code> of that type, 
				and an object <code>t</code> of type <classname>boost::xtime</classname>, 
				the following expressions must be well-formed 
				and have the indicated effects.</para>
				
				<table>
					<title>TimedLock Expressions</title>
					<tgroup cols="2">
						<thead>
							<row>
							<entry>Expression</entry>
							<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry><code>lk.timed_lock(t)</code></entry>
								<entry>
									<para>Throws <classname>boost::lock_error</classname>
									if locked().</para>
									
									<para>Makes a blocking attempt
									to lock the associated mutex object, and returns <code>true</code>
									if successful within the specified time <code>t</code>, otherwise
									<code>false</code>. If the associated mutex object is already
									locked by the same thread the behavior is dependent on the <link
									linkend="threads.concepts.locking-strategies">locking
									strategy</link> of the associated mutex object.</para>
								</entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		
			<section id="threads.concepts.ScopedTimedLock">
				<title>ScopedTimedLock Concept</title>
				
				<para>A ScopedTimedLock is a refinement of <link
				linkend="threads.concepts.TimedLock">TimedLock</link>. 
				For a ScopedTimedLock type <code>L</code> 
				and an object <code>lk</code> of that type, 
				and an object <code>m</code> of a type meeting the 
				<link linkend="threads.concepts.TimedMutex">TimedMutex</link> requirements, 
				and an object <code>b</code> of type <code>bool</code>, 
				and an object <code>t</code> of type <classname>boost::xtime</classname>, 
				the following expressions must be well-formed 
				and have the indicated effects.</para>
				
				<table>
					<title>ScopedTimedLock Expressions</title>
					<tgroup cols="2">
						<thead>
							<row>
							<entry>Expression</entry>
							<entry>Effects</entry>
							</row>
						</thead>
						
						<tbody>
							<row>
								<entry><code>L lk(m,t);</code></entry>
								<entry>Constructs an object <code>lk</code>, and associates mutex
								object <code>m</code> with it, then calls
								<code>timed_lock(t)</code></entry>
							</row>
							<row>
								<entry><code>L lk(m,b);</code></entry>
								<entry>Constructs an object <code>lk</code>, and associates mutex
								object <code>m</code> with it, then if <code>b</code>, calls
								<code>lock()</code></entry>
							</row>
						</tbody>
					</tgroup>
				</table>
			</section>
		</section>

		<section id="threads.concepts.lock-models">
			<title>Lock Models</title>
			
			<para>&Boost.Threads; currently supplies twelve models of
			<link linkend="threads.concepts.Lock">Lock</link>
			and its refinements.</para>
			
			<table>
				<title>Lock Models</title>
				
				<tgroup cols="3">
					<thead>
						<row>
							<entry>Concept</entry>
							<entry>Refines</entry>
							<entry>Models</entry>
						</row>
					</thead>
					
					<tbody>
						<row>
							<entry><link linkend="threads.concepts.Lock">Lock</link></entry>
							<entry></entry>
							<entry></entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.ScopedLock">ScopedLock</link></entry>
							<entry><link linkend="threads.concepts.Lock">Lock</link></entry>
							<entry>
								<para><classname>boost::mutex::scoped_lock</classname></para>
								<para><classname>boost::recursive_mutex::scoped_lock</classname></para>
								
								<para><classname>boost::try_mutex::scoped_lock</classname></para>
								<para><classname>boost::recursive_try_mutex::scoped_lock</classname></para>
								
								<para><classname>boost::timed_mutex::scoped_lock</classname></para>
								<para><classname>boost::recursive_timed_mutex::scoped_lock</classname></para>
							</entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.TryLock">TryLock</link></entry>
							<entry><link linkend="threads.concepts.Lock">Lock</link></entry>
							<entry></entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.ScopedTryLock">ScopedTryLock</link></entry>
							<entry><link linkend="threads.concepts.TryLock">TryLock</link></entry>
							<entry>
								<para><classname>boost::try_mutex::scoped_try_lock</classname></para>
								<para><classname>boost::recursive_try_mutex::scoped_try_lock</classname></para>
								
								<para><classname>boost::timed_mutex::scoped_try_lock</classname></para>
								<para><classname>boost::recursive_timed_mutex::scoped_try_lock</classname></para>
							</entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.TimedLock">TimedLock</link></entry>
							<entry><link linkend="threads.concepts.TryLock">TryLock</link></entry>
							<entry></entry>
						</row>
						<row>
							<entry><link linkend="threads.concepts.ScopedTimedLock">ScopedTimedLock</link></entry>
							<entry><link linkend="threads.concepts.TimedLock">TimedLock</link></entry>
							<entry>
								<para><classname>boost::timed_mutex::scoped_timed_lock</classname></para>
								<para><classname>boost::recursive_timed_mutex::scoped_timed_lock</classname></para>
							</entry>
						</row>
					</tbody>
				</tgroup>
			</table>
		</section>
	</section>

</section>
