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311 lines
9.2 KiB
HTML
<html>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="keywords" content="threads, BTL, thread library, C++">
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<link rel="stylesheet" type="text/css" href="styles.css">
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<title>Boost.Threads, condition</title>
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</head>
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<body bgcolor="#FFFFFF" link="#0000FF" vlink="#800080" text="#000000">
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<table border="0" cellpadding="7" cellspacing="0" width="100%">
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<tr>
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<td valign="top" width="300">
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<h3><img src="../../../c++boost.gif" alt="C++ Boost" width="277" height="86"></h3>
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</td>
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<td valign="top">
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<h1 align="center">Boost.Threads</h1>
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<h2 align="center">condition</h2>
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</td>
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</tr>
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</table>
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<hr>
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<p><a href="#Introduction">Introduction</a><br>
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<a href="#Header">Header</a><br>
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<a href="#Synopsis">Synopsis</a><br>
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<a href="#Members">Members</a><br>
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<a href="#Example">Example</a></p>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>A <tt>condition</tt> is a synchronization primitive used to cause a thread to
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wait until a particular condition (or time) is met. A <tt>condition</tt> always works in
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conjunction with a <a href="mutex_concept.html">mutex model</a>. The
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<a href="mutex_concept.html">mutex model</a> must be locked prior to waiting on the <tt>condition</tt>,
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which is insured by passing a <a href="lock_concept.html">lock model</a> to the <code>condition</code> class <tt>wait</tt> functions. While the thread is waiting on the <tt>condition</tt> the
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<a href="mutex_concept.html">mutex model</a> is unlocked. When the thread returns from a call
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to one of the <code>condition</code> class <tt>wait</tt> functions, the <a href="mutex_concept.html">mutex model</a> is again
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locked.</p>
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<p>The <tt>condition</tt> type is often used to implement the
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<i>Monitor Object</i> pattern. See <a href="bibliography.html#Schmidt-00">[Schmidt
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00]</a> and <a href="bibliography.html#Hoare-74">[Hoare 74</a>]. Monitors
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are one of the most important patterns for creating reliable multithreaded
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programs.</p>
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<p>See <a href="thread_formal_definition.html"> Formal Definitions</a> for definitions of thread states <a href="thread_formal_definition.html#state">blocked</a>
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and <a href="thread_formal_definition.html#state">ready</a>. Note that
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"waiting" is a synonym for blocked.</p>
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<h2><a name="Header">Header</a></h2>
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<pre>
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#include <a href="../../../boost/thread/condition.hpp"><boost/thread/condition.hpp></a>
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</pre>
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<h2><a name="Synopsis">Synopsis</a></h2>
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<pre>namespace boost
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{
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class condition : private noncopyable
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{
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public:
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condition();
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~condition();
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void notify_one();
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void notify_all();
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template <typename L>
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void wait(L& lock);
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template <typename L, typename Pr>
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void wait(L& lock, Pr pred);
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template <typename L>
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bool timed_wait(L& lock, const xtime& xt);
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template <typename L, typename Pr>
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bool timed_wait(L& lock, const xtime& xt, Pr pred);
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};
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}</pre>
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<h2><a name="Members">Members</a></h2>
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<hr>
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<h3>Constructor</h3>
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<pre>
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condition();
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</pre>
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<p><b>Effects: </b>Constructs a <tt>condition</tt>.</p>
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<hr>
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<h3>Destructor</h3>
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<pre>
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~condition();
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</pre>
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<p><b>Effects: </b>Destroys <code>*this</code>.</p>
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<hr>
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<h3>notify_one</h3>
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<pre>
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void notify_one();
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</pre>
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<p><b>Effects: </b>If there is a thread waiting on <code>*this</code>, change
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that thread's state to ready.<b> </b> Otherwise there is no effect.</p>
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<p><b>Notes: </b>If more that one thread is waiting on the condition, it is
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unspecified which is made ready. </p>
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<hr>
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<h3>notify_all</h3>
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<pre>
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void notify_all();
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</pre>
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<p><b>Effects: </b>Change the state of all threads waiting on <code>*this</code>
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to ready. If there are no waiting threads, <code>notify()</code> has no effect.</p>
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<hr>
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<h3>wait</h3>
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<pre>
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template <typename L>
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void wait(L& lock);
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</pre>
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<p><b>Requires: </b>L meet the requirements of the <a href="lock_concept.html">Lock
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concept</a>.</p>
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<p><b>Effects: </b>Releases the lock on the <a href="mutex_concept.html">mutex model</a> associated with <tt>lock</tt>,
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blocks the current thread of execution until readied by a call to <code>*this->notify()</code>
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or <code>*this->notify_all()</code>, and then reacquires the lock. All
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effects occur in an atomic fashion.</p>
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<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code> if <code>!lock</code></p>
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<p><b>Danger: </b>This version should always be used within a loop checking that the state logically associated
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with the <tt>condition</tt> has become true. Without the loop, race conditions can ensue due to
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possible "spurious wake ups". The second version encapsulates this loop idiom internally and
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is generally the preferred method.</p>
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<pre>
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template <typename L, typename Pr>
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void wait(L& lock, Pr pred);
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</pre>
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<p><b>Requires: </b>L meet the requirements of the <a href="lock_concept.html">Lock
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concept</a>. Return from <code>pred()</code> convertible to bool.</p>
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<p><b>Effects: </b>As if: <code>while !(pred()) wait(lock)</code></p>
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<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code> if <code>!lock</code></p>
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<hr>
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<h3>timed_wait</h3>
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<pre>
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template <typename L>
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bool timed_wait(L& lock, const xtime& xt);
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</pre>
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<p><b>Requires: </b>L meet the requirements of the <a href="lock_concept.html">Lock
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concept</a>.</p>
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<p><b>Effects: </b>Releases the lock on the <a href="mutex_concept.html">mutex model</a> associated with the <tt>lock</tt>,
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blocks the current thread of execution until readied by a call to <code>*this->notify()</code>
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or <code>*this->notify_all()</code>, or until <tt>xt,</tt> and then reacquires the lock. All
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effects occur in an atomic fashion.</p>
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<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code> if <code>!lock</code></p>
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<p><b>Danger:</b> This version should
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always be used within a loop checking that the state logically associated with the <tt>condition</tt> has
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become true. Without the loop, race conditions can ensue due to "spurious wake ups". The second version
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encapsulates this loop idiom internally and is generally the preferred method.</p>
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<p><b>Returns:</b> <tt>false</tt> if <code>xt </code>expires, otherwise <code>true</code>. </p>
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<pre>
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template <typename L, typename Pr>
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bool timed_wait(L& lock, const xtime& xt, Pr pred);
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</pre>
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<p><b>Requires: </b>L meet the requirements of the <a href="lock_concept.html">Lock
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concept</a>. Return from <code>pred()</code> convertible to bool.</p>
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<p><b>Effects: </b>As if: <b> </b></p>
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<pre> while (!pred())
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{
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if (!timed_wait(lock, xt))
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return false;
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}
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</pre>
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<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code> if <code>!lock</code></p>
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<p><b>Returns:</b> <tt>false</tt> if <code>xt </code>expires, otherwise <code>true</code>.</p>
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<hr>
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<h2><a name="Example">Example</a> Usage</h2>
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<pre>
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#include <iostream>
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#include <vector>
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#include <a href="../../../boost/utility.hpp"><boost/utility.hpp></a>
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#include <a href="../../../boost/thread/condition.hpp"><boost/thread/condition.hpp></a>
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#include <a href="../../../boost/thread/thread.hpp"><boost/thread/thread.hpp></a>
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class bounded_buffer : private boost::noncopyable
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{
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private:
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int begin, end, buffered;
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std::vector<int> circular_buf;
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boost::condition buffer_not_full, buffer_not_empty;
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boost::mutex monitor;
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typedef boost::mutex::lock lock;
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public:
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buffer(int n) : begin(0), end(0), buffered(0), circular_buf(n) { }
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void send (int m) {
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lock lk(monitor);
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while (buffered == circular_buf.size())
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buffer_not_full.wait(lk);
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circular_buf[end] = m;
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end = (end+1) % circular_buf.size();
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++buffered;
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buffer_not_empty.notify_one();
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}
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int receive() {
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lock lk(monitor);
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while (buffered == 0 && !finished)
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buffer_not_empty.wait(lk);
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int i = circular_buf[begin];
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begin = (begin+1) % circular_buf.size();
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--buffered;
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buffer_not_full.notify_one();
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return i;
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}
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};
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bounded_buffer buf(2);
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void sender(void*) {
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int n = 0;
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while (n < 100) {
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buf.send(n);
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std::cout << "sent: " << n << std::endl;
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++n;
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}
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buf.send(-1);
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}
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void receiver(void*) {
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int n;
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do {
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n = buf.receive();
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std::cout << "received: " << n << std::endl;
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} while (n != -1); // -1 indicates end of buffer
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}
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int main(int, char*[])
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{
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boost::thread::create(&sender, 0);
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boost::thread::create(&receiver, 0);
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boost::thread::join_all();
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return 0;
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}
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</pre>
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<p>Typical output (dependent on scheduling policies) is:</p>
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<pre>
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sent: 0
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sent: 1
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received: 0
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received: 1
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sent: 2
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sent: 3
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received: 2
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received: 3
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sent: 4
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received: 4
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</pre>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->17 July, 2001<!--webbot bot="Timestamp" endspan i-checksum="21075" -->
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</p>
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<p><i>Copyright <a href="mailto:williamkempf@hotmail.com">William E. Kempf</a>
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2001 all rights reserved.</i></p>
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</body>
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</html>
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