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<p class="hpath"><a href="index.html">Top</a>: <a href="async.html">Multithreading</a>:
thread </p>
<blockquote>
<pre class="lang">#include <pasync.h>
class thread {
thread(bool autofree);
~thread();
void start();
void waitfor();
void signal();
pthread_id_t get_id();
bool get_running();
bool get_finished();
bool get_signaled();
virtual void execute() = 0;
virtual void cleanup() = 0;
bool relax(int milliseconds);
}</pre>
</blockquote>
<p>Create a descendant of <span class="lang">thread</span> to represent an execution
thread in a multithreaded application. All threads are running within the framework
of the parent process and share the same global variables. Each thread, however,
has its own stack. The execution of a thread can be started either from the main
process or from another thread. Further, the newly launched thread executes its
code concurrently.</p>
<p>Multithreading can significantly improve your application when a process consumes
time by waiting for data from some communication device, a slow storage device
or user input. When such pieces of code run concurrently the CPU is loaded more
efficiently. Also, many networking services use threading to serve multiple users
simultaneously.</p>
<p>When threads concurrently access and modify shared data structures precautions
should be taken to preserve logical integrity of these data structures. Synchronization
between threads is performed using either of: <a href="async.semaphore.html">semaphore</a>,
<a href="async.mutex.html">mutex</a>, <a href="async.rwlock.html">rwlock</a>,
<a href="async.trigger.html">trigger</a> or <a href="async.msgqueue.html">msgqueue</a>.</p>
<p><span class="def">thread::thread(bool autofree)</span> creates a thread object,
but does not run it. If <span class="lang">autofree</span> is <span class="lang">true</span>
the thread will destroy itself upon termination. Note, that you can use <span class="lang">autofree</span>
only for dynamically allocated thread objects. A variable that holds a pointer
to a dynamic <span class="lang">autofree</span> thread object can be considered
invalid after a call to <span class="lang">start()</span>.</p>
<p><span class="def">thread::~thread()</span> destroys the thread object. Usually
both the constructor and the destructor for non-autofree threads are called from
a different context. In contrary, autofree threads call their destructors from
their own context.</p>
<p><span class="def">void thread::start()</span> runs the thread. The overridden
virtual method <span class="lang">execute()</span> is called asynchronously. <span class="lang">Start()</span>
itself returns immediately. Each thread object can be run only once.</p>
<p><span class="def">void thread::waitfor()</span> waits for the thread to terminate.
This function is called when a thread needs to synchronize its execution with
the completion of the target non-autofree thread. For non-autofree threads this
method must be called <b>at least once</b> and within the scope of a thread that
called <span class="lang">start()</span>. For convenience, you can place a call
to <span class="lang">waitfor()</span> in the overridden destructor. <span class="lang">waitfor()</span>,
however, can not be called for autofree threads.</p>
<p><span class="def">void thread::signal()</span> sets signaled attribute to true
and possibly wakes up the thread if in a `relaxed' state (see <span class="lang">relax()</span>
below). <span class="lang">signal()</span> is usually called from a different
thread to let the given thread know that the execution should be terminated as
soon as possible. If the given thread performs an iteration one of the conditions
of leaving the loop should be <span class="lang">(!get_signaled())</span>. <span class="lang">Signal()</span>
can be called only once for a thread object.</p>
<p><span class="def">pthread_id_t thread::get_id()</span> returns the thread
ID assigned by the operating system. This ID can then be used in call to <span class="lang">pthrequal()</span>.</p>
<p><span class="def">bool thread::get_running()</span> returns <span class="lang">true</span>
if the given thread object is running. This property is never set to <span class="lang">false</span>
once the thread started. To check whether the thread finished its job, i.e. returned
from <span class="lang">execute()</span>, use <span class="lang">get_finished()</span>.</p>
<p><span class="def">bool thread::get_finished()</span> returns <span class="lang">true</span>
if thread has already terminated its execution, or, in other words, has left <span class="lang">execute()</span>.</p>
<p><span class="def">bool thread::get_signaled()</span> returns <span class="lang">true</span>
if the thread object is in signaled state, i.e. <span class="lang">signal()</span>
has been called from a concurrent thread. See also <span class="lang">signal()</span>.</p>
<p><span class="def">virtual void thread::execute()</span> -- this pure virtual
method should be overridden in the descendant class to implement the functionality
of your thread. <span class="lang">execute()</span> can be viewed as <span class="lang">main()</span>
for your mini-process. Typically, you create a class descendant from <span class="lang">thread</span>
and override at least two virtual methods: <span class="lang">execute()</span>
and <span class="lang">cleanup()</span>.</p>
<p><span class="def">virtual void thread::cleanup()</span> -- this pure virtual
method should be overridden in the descendant class. <span class="lang">cleanup()</span>
is called either when the thread terminates normally or when an exception is raised
within the thread's context. When implementing this method you might want to clean
up any memory and other resources allocated by your thread object. <span class="lang">cleanup()</span>
is guaranteed to be called once upon termination of the thread. To properly shut
down the thread, avoid using operations that can cause exceptions in <span class="lang">cleanup()</span>.</p>
<p><span class="def">bool thread::relax(int milliseconds)</span> is a protected
member function which can be called only from within the overridden <span class="lang">execute()</span>
method. This function suspends the execution of the thread until either of: the
specified amount of time is elapsed, in which case the function returns false,
or <span class="lang">signal()</span> is called from a concurrent thread, in which
case <span class="lang">relax()</span> returns true. If parameter <span class="lang">milliseconds</span>
is -1 the function will wait infinitely. The relax/signal mechanism is useful
for threads doing some job that requires taking periodic actions in the background.</p>
<p class="seealso">See also: <a href="async.semaphore.html">semaphore</a>, <a href="async.examples.html">mutex</a>,
<a href="async.rwlock.html">rwlock</a>, <a href="async.trigger.html">trigger</a>,
<a href="async.utils.html">Utilities</a>, <a href="async.examples.html">Examples</a></p>
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