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<html>
<head>
<meta http-equiv="Content-Type" content=
"text/html; charset=iso-8859-1">
<meta name="keywords" content="threads, BTL, thread library, C++">
<link rel="stylesheet" type="text/css" href="styles.css">
<title>Boost.Threads, condition</title>
</head>
<body bgcolor="#ffffff" link="#0000ff" vlink="#800080" text="#000000">
<table summary="header" border="0" cellpadding="7" cellspacing="0"
width="100%">
<tr>
<td valign="top" width="300">
<h3><img height="86" alt="C++ Boost" src=
"../../../c++boost.gif" width="277"></h3>
</td>
<td valign="top">
<h1 align="center">Boost.Threads</h1>
<h2 align="center">condition</h2>
</td>
</tr>
</table>
<hr>
<p><a href="#Introduction">Introduction</a><br>
<a href="#Header">Header</a><br>
<a href="#Synopsis">Synopsis</a><br>
<a href="#Members">Members</a><br>
<a href="#Example">Example</a></p>
<h2><a name="Introduction">Introduction</a></h2>
<p>An object of class <code>condition</code> is a synchronization
primitive used to cause a thread to wait until a particular shared-data
condition (or time) is met. A <code>condition</code> object is always
used in conjunction with a mutex object modeling a <a href=
"mutex_concept.html">Mutex Concept</a>. The mutex must be locked prior
to waiting on the <code>condition</code>, which is ensured by passing a
lock object modeling a <a href="lock_concept.html">Lock Concept</a> to
the <code>condition</code> object's <code>wait</code> functions.
While the thread is waiting on the <code>condition</code> object, the
mutex associated with the lock is unlocked. When the thread returns
from a call to one of the <code>condition</code> object's <code>
wait</code> functions, the mutex is again locked. The tricky
lock/unlock/lock sequence is performed automatically by the <code>
condition</code> object's <code>wait</code> functions.</p>
<p>The <code>condition</code> type is often used to implement the <i>
Monitor Object</i> and other important patterns. See <a href=
"bibliography.html#Schmidt-00">[Schmidt-00]</a> and <a href=
"bibliography.html#Hoare-74">[Hoare 74]</a>. Monitors are one of the
most important patterns for creating reliable multithreaded
programs.</p>
<p>See <a href="definitions.html">Formal Definitions</a> for
definitions of thread states <a href="definitions.html#state">
blocked</a> and <a href="definitions.html#state">ready</a>. Note that
"waiting" is a synonym for blocked.</p>
<h2><a name="Header">Header</a></h2>
<pre>
#include <a href=
"../../../boost/thread/condition.hpp"><boost/thread/condition.hpp></a>
</pre>
<h2><a name="Synopsis">Synopsis</a></h2>
<pre>
namespace boost {
class condition : private <a href=
"../../utility/utility.htm#Class noncopyable">boost::noncopyable</a> // Exposition only.
// Class condition meets the <a href=
"overview.html#NonCopyable">NonCopyable</a> requirement.
{
public:
condition();
~condition();
void notify_one();
void notify_all();
template <typename <a href="scoped_lock.html">ScopedLock</a>>
void wait(<a href="scoped_lock.html">ScopedLock</a>& lock);
template <typename <a href="scoped_lock.html">ScopedLock</a>, typename <a
href="http://www.sgi.com/tech/stl/Predicate.html">Predicate</a>>
void wait(<a href="scoped_lock.html">ScopedLock</a>& lock, <a href=
"http://www.sgi.com/tech/stl/Predicate.html">Predicate</a> pred);
template <typename <a href="scoped_lock.html">ScopedLock</a>>
bool timed_wait(<a href=
"scoped_lock.html">ScopedLock</a>& lock, const xtime& xt);
template <typename <a href="scoped_lock.html">ScopedLock</a>, typename <a
href="http://www.sgi.com/tech/stl/Predicate.html">Predicate</a>>
bool timed_wait(<a href=
"scoped_lock.html">ScopedLock</a>& lock, const xtime& xt, <a href=
"http://www.sgi.com/tech/stl/Predicate.html">Predicate</a> pred);
};
} // namespace boost
</pre>
<h2><a name="Members">Members</a></h2>
<hr>
<h3>Constructor</h3>
<pre>
condition();
</pre>
<p><b>Effects:</b> Constructs a <code>condition</code>.</p>
<hr>
<h3>Destructor</h3>
<pre>
~condition();
</pre>
<p><b>Effects:</b> Destroys <code>*this</code>.</p>
<hr>
<h3>notify_one</h3>
<pre>
void notify_one();
</pre>
<p><b>Effects:</b> If there is a thread waiting on <code>*this</code>,
change that thread's state to ready. Otherwise there is no
effect.</p>
<p><b>Notes:</b> If more that one thread is waiting on the condition,
it is unspecified which is made ready.</p>
<hr>
<h3>notify_all</h3>
<pre>
void notify_all();
</pre>
<p><b>Effects:</b> Change the state of all threads waiting on <code>
*this</code> to ready. If there are no waiting threads, <code>
notify_all()</code> has no effect.</p>
<hr>
<h3>wait</h3>
<pre>
template <typename ScopedLock>
void wait(ScopedLock& lock);
</pre>
<p><b>Requires:</b> ScopedLock meets the <a href=
"lock_concept.html#ScopedLock">ScopedLock</a> requirements.</p>
<p><b>Effects:</b> Releases the lock on the <a href=
"mutex_concept.html">mutex model</a> associated with <code>lock</code>,
blocks the current thread of execution until readied by a call to
<code>this->notify_one()</code> or <code>
this->notify_all()</code>, and then reacquires the lock. All effects
occur in an atomic fashion.</p>
<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code>
if <code>!lock.locked()</code></p>
<p><b>Danger:</b> This version should always be used within a loop
checking that the state logically associated with the <code>
condition</code> has become true. Without the loop, race conditions can
ensue due to possible "spurious wake ups". The second version
encapsulates this loop idiom internally and is generally the preferred
method.</p>
<pre>
template <typename ScopedLock, typename Pr>
void wait(ScopedLock& lock, Pr pred);
</pre>
<p><b>Requires:</b> ScopedLock meets the <a href=
"lock_concept.html#ScopedLock">ScopedLock</a> requirements, return from
<code>pred()</code> convertible to bool.</p>
<p><b>Effects:</b> As if:</p>
<code> while (!pred()) wait(lock)</code>
<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code>
if <code>!lock.locked()</code></p>
<hr>
<h3>timed_wait</h3>
<pre>
template <typename ScopedLock>
bool timed_wait(ScopedLock& lock, const <a href=
"xtime.html">xtime</a>& xt);
</pre>
<p><b>Requires:</b> ScopedLock meets the <a href=
"lock_concept.html#ScopedLock">ScopedLock</a>
requirements.</p>
<p><b>Effects:</b> Releases the lock on the <a href=
"mutex_concept.html">mutex model</a> associated with the <code>
lock</code>, blocks the current thread of execution until readied by a
call to <code>this->notify_one()</code> or <code>
this->notify_all()</code>, or until <code>xt</code>, and then
reacquires the lock. All effects occur in an atomic fashion.</p>
<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code>
if <code>!lock.locked()</code></p>
<p><b>Danger:</b> This version should always be used within a loop
checking that the state logically associated with the <code>
condition</code> has become true. Without the loop, race conditions can
ensue due to "spurious wake ups". The second version
encapsulates this loop idiom internally and is generally the preferred
method.</p>
<p><b>Returns:</b> <code>false</code> if <code>xt</code> is reached,
otherwise <code>true</code>.</p>
<pre>
template <typename ScopedLock, typename Pr>
bool timed_wait(ScopedLock& lock, const <a href=
"xtime.html">xtime</a>& xt, Pr pred);
</pre>
<p><b>Requires:</b> ScopedLock meets the <a href=
"lock_concept.html#ScopedLock">ScopedLock</a> requirements,
return from <code>pred()</code> convertible to bool.</p>
<p><b>Effects:</b> As if:</p>
<code> while (!pred())<br>
{<br>
if (!timed_wait(lock, xt))<br>
return
false;<br>
}</code>
<p><b>Throws:</b> <code><a href="lock_error.html">lock_error</a></code>
if <code>!lock.locked()</code></p>
<p><b>Returns:</b> <code>false</code> if <code>xt</code> is reached,
otherwise <code>true</code>.</p>
<hr>
<h2><a name="Example">Example Usage</a></h2>
<pre>
#include <iostream>
#include <vector>
#include <a href="../../../boost/utility.hpp"><boost/utility.hpp></a>
#include <a href=
"../../../boost/thread/condition.hpp"><boost/thread/condition.hpp></a>
#include <a href=
"../../../boost/thread/thread.hpp"><boost/thread/thread.hpp></a>
class bounded_buffer : private boost::noncopyable
{
public:
typedef boost::mutex::scoped_lock lock;
bounded_buffer(int n) : begin(0), end(0), buffered(0), circular_buf(n) { }
void send (int m) {
lock lk(monitor);
while (buffered == circular_buf.size())
buffer_not_full.wait(lk);
circular_buf[end] = m;
end = (end+1) % circular_buf.size();
++buffered;
buffer_not_empty.notify_one();
}
int receive() {
lock lk(monitor);
while (buffered == 0)
buffer_not_empty.wait(lk);
int i = circular_buf[begin];
begin = (begin+1) % circular_buf.size();
--buffered;
buffer_not_full.notify_one();
return i;
}
private:
int begin, end, buffered;
std::vector<int> circular_buf;
boost::condition buffer_not_full, buffer_not_empty;
boost::mutex monitor;
};
bounded_buffer buf(2);
void sender() {
int n = 0;
while (n < 100) {
buf.send(n);
std::cout << "sent: " << n << std::endl;
++n;
}
buf.send(-1);
}
void receiver() {
int n;
do {
n = buf.receive();
std::cout << "received: " << n << std::endl;
} while (n != -1); // -1 indicates end of buffer
}
int main(int, char*[])
{
boost::thread thrd1(&sender);
boost::thread thrd2(&receiver);
thrd1.join();
thrd2.join();
return 0;
}
</pre>
<p>Typical output (dependent on scheduling policies) is:</p>
<pre>
sent: 0
sent: 1
received: 0
received: 1
sent: 2
sent: 3
received: 2
received: 3
sent: 4
received: 4
</pre>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->05 November, 2001<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
<p><i>© Copyright <a href="mailto:williamkempf@hotmail.com">
William E. Kempf</a> 2001 all rights reserved.</i></p>
</body>
</html>
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