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<h1 class="topictitle1">ReaderWriterMutex Concept</h1>
<div>
<div class="section">
<p>The ReaderWriterMutex concept extends the Mutex
Concept to include the notion of reader-writer locks. It introduces a boolean
parameter
<samp class="codeph">write</samp> that specifies whether a writer lock
(<samp class="codeph">write</samp> =true) or reader lock (<samp class="codeph">write</samp> =false)
is being requested. Multiple reader locks can be held simultaneously on a
ReaderWriterMutex if it does not have a writer lock on it. A writer lock on a
ReaderWriterMutex excludes all other threads from holding a lock on the mutex
at the same time.
</p>
<p> The following table shows the requirements for a
ReaderWriterMutex
<samp class="codeph">RW</samp>. They form a superset of the Mutex Concept.
</p>
<div class="tablenoborder"><a name="tbl28"><!-- --></a><table cellpadding="4" summary="" id="tbl28" width="100%" frame="hsides" border="1" rules="all"><caption><span class="tablecap">ReaderWriterMutex Concept</span></caption>
<thead align="left">
<tr>
<th class="cellrowborder" valign="top" width="NaN%" id="d101181e52">
<p>Pseudo-Signature
</p>
</th>
<th class="row-nocellborder" valign="top" width="NaN%" id="d101181e58">
<p>Semantics
</p>
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">RW()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Construct unlocked mutex.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">~RW()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Destroy unlocked mutex.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">typename RW::scoped_lock</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Corresponding scoped-lock type.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">RW::scoped_lock()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Construct lock without acquiring mutex.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">RW::scoped_lock(RW&, bool
write=true)</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Construct lock and acquire lock on mutex.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">RW::~scoped_lock()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Release lock (if acquired).
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">RW::scoped_lock::acquire(RW&,bool
write=true)</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Acquire lock on mutex.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">bool
RW::scoped_lock::try_acquire(RW&, bool write=true)</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Try to acquire lock on mutex. Return
<samp class="codeph">true</samp> if lock acquired,
<samp class="codeph">false</samp> otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">RW::scoped_lock::release()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Release lock.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">bool
RW::scoped_lock::upgrade_to_writer()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Change reader lock to writer lock.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">bool
RW::scoped_lock::downgrade_to_reader()</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>Change writer lock to reader lock.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">static const bool RW::is_rw_mutex =
true</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>True.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">static const bool
RW::is_recursive_mutex</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>True if mutex is recursive; false
otherwise. For all current reader-writer mutexes, false.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d101181e52 ">
<p><samp class="codeph">static const bool
RW::is_fair_mutex</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d101181e58 ">
<p>True if mutex is fair; false otherwise.
</p>
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="tablenoborder"><table cellpadding="4" summary="" frame="border" border="1" cellspacing="0" rules="all"><span class="tabledesc">The following table provides explain the semantics of the
ReaderWriterMutex concept in detail.
</span><thead align="left">
<tr>
<th class="cellrowborder" valign="top" width="33.89830508474576%" id="d101181e329">Member
</th>
<th class="cellrowborder" valign="top" width="66.10169491525423%" id="d101181e332">Description
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">Model Types</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Classes
<samp class="codeph"><samp class="codeph">spin_rw_mutex</samp></samp> and
<samp class="codeph"><samp class="codeph">queuing_rw_mutex</samp></samp> model the
ReaderWriterMutex concept.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">ReaderWriterMutex()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Constructs unlocked
<samp class="codeph">ReaderWriterMutex</samp>.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">~ReaderWriterMutex()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Destroys unlocked
<samp class="codeph">ReaderWriterMutex</samp>. The effect of destroying a
locked
<samp class="codeph">ReaderWriterMutex</samp> is undefined.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">ReaderWriterMutex::scoped_lock()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Constructs a
<samp class="codeph">scoped_lock</samp> object that does not hold a lock
on any mutex.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword"> ReaderWriterMutex::scoped_lock(
ReaderWriterMutex& rw, bool write =true)</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Constructs a
<samp class="codeph">scoped_lock</samp> object that acquires a lock on
mutex
<em>rw</em>. The lock is a writer lock if
<samp class="codeph"><em>write</em></samp> is true; a reader lock otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">ReaderWriterMutex::~scoped_lock()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>If the object holds a lock on a
<samp class="codeph">ReaderWriterMutex</samp>, releases the lock.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">void ReaderWriterMutex::
scoped_lock:: acquire( ReaderWriterMutex& rw, bool write=true )</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Acquires a lock on mutex
<em>rw</em>. The lock is a writer lock if
<samp class="codeph"><em>write</em></samp> is true; a reader lock otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword"> bool ReaderWriterMutex::
scoped_lock::try_acquire( ReaderWriterMutex& rw, bool write=true
)</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Attempts to acquire a lock on mutex
<em>rw</em>. The lock is a writer lock if
<samp class="codeph"><em>write</em></samp> is true; a reader lock otherwise.
</p>
<p><strong>Returns</strong>:
<samp class="codeph">true</samp> if the lock is acquired,
<samp class="codeph">false</samp> otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword"> void ReaderWriterMutex::
scoped_lock::release()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Releases lock. The effect is undefined if
no lock is held.
</p>
<p><strong>Returns</strong>:
<samp class="codeph">false</samp> if lock was released in favor of another
upgrade request and then reacquired;
<samp class="codeph">true</samp> otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">bool ReaderWriterMutex::
scoped_lock::upgrade_to_writer()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Changes reader lock to a writer lock. The
effect is undefined if the object does not already hold a reader lock.
</p>
<p><strong>Returns</strong>:
<samp class="codeph">false</samp> if lock was released and reacquired;
<samp class="codeph">true</samp> otherwise.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d101181e329 "><span class="keyword">bool ReaderWriterMutex::
scoped_lock::downgrade_to_reader()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d101181e332 ">
<p>Changes writer lock to a reader lock. The
effect is undefined if the object does not already hold a writer lock.
</p>
<p><strong>Returns</strong>:
<samp class="codeph">false</samp> if lock was released and reacquired;
<samp class="codeph">true</samp> otherwise.
</p>
<p>Intel's current implementations for
<samp class="codeph">spin_rw_mutex</samp> and
<samp class="codeph">queuing_rw_mutex</samp> always return
<samp class="codeph">true</samp>. Different implementations might
sometimes return
<samp class="codeph">false</samp>.
</p>
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../../../reference/synchronization/mutexes.htm">Mutexes</a></div>
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