update source to Ceph Pacific 16.2.2

[ceph.git] / ceph / src / boost / libs / asio / example / cpp03 / tutorial / timer_dox.txt

//
// Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

/**
\page tuttimer1 Timer.1 - Using a timer synchronously

This tutorial program introduces asio by showing how to perform a blocking
wait on a timer.

\dontinclude timer1/timer.cpp
\skip #include

We start by including the necessary header files.

All of the asio classes can be used by simply including the <tt>"asio.hpp"</tt>
header file.

\until asio.hpp

All programs that use asio need to have at least one boost::asio::io_context object.
This class provides access to I/O functionality. We declare an object of this
type first thing in the main function.

\until boost::asio::io_context

Next we declare an object of type boost::asio::steady_timer. The core asio classes
that provide I/O functionality (or as in this case timer functionality) always
take a reference to an io_context as their first constructor argument. The
second argument to the constructor sets the timer to expire 5 seconds from now.

\until boost::asio::steady_timer

In this simple example we perform a blocking wait on the timer.
That is, the call to boost::asio::steady_timer::wait() will not return until the
timer has expired, 5 seconds after it was created (i.e. <b>not</b> from when the
wait starts).

A timer is always in one of two states: "expired" or "not expired". If the
boost::asio::steady_timer::wait() function is called on an expired timer, it will
return immediately.

\until wait

Finally we print the obligatory <tt>"Hello, world!"</tt>
message to show when the timer has expired.

\until }

See the \ref tuttimer1src "full source listing" \n
Return to the \ref index "tutorial index" \n
Next: \ref tuttimer2

*/

/**
\page tuttimer1src Source listing for Timer.1
\include timer1/timer.cpp
Return to \ref tuttimer1
*/

/**
\page tuttimer2 Timer.2 - Using a timer asynchronously

This tutorial program demonstrates how to use asio's asynchronous callback
functionality by modifying the program from tutorial Timer.1 to perform an
asynchronous wait on the timer.

\dontinclude timer2/timer.cpp
\skip #include

\until asio.hpp

Using asio's asynchronous functionality means having a callback
function that will be called when an asynchronous operation completes. In this
program we define a function called <tt>print</tt> to be called when the
asynchronous wait finishes.

\until boost::asio::steady_timer 

Next, instead of doing a blocking wait as in tutorial Timer.1,
we call the boost::asio::steady_timer::async_wait() function to perform an
asynchronous wait. When calling this function we pass the <tt>print</tt>
callback handler that was defined above.

\skipline async_wait

Finally, we must call the boost::asio::io_context::run() member function
on the io_context object.

The asio library provides a guarantee that callback handlers will <b>only</b>
be called from threads that are currently calling boost::asio::io_context::run().
Therefore unless the boost::asio::io_context::run() function is called the callback for
the asynchronous wait completion will never be invoked.

The boost::asio::io_context::run() function will also continue to run while there is
still "work" to do. In this example, the work is the asynchronous wait on the
timer, so the call will not return until the timer has expired and the
callback has completed.

It is important to remember to give the io_context some work to do before
calling boost::asio::io_context::run(). For example, if we had omitted the above call
to boost::asio::steady_timer::async_wait(), the io_context would not have had any
work to do, and consequently boost::asio::io_context::run() would have returned
immediately.

\skip run
\until }

See the \ref tuttimer2src "full source listing" \n
Return to the \ref index "tutorial index" \n
Previous: \ref tuttimer1 \n
Next: \ref tuttimer3

*/

/**
\page tuttimer2src Source listing for Timer.2
\include timer2/timer.cpp
Return to \ref tuttimer2
*/

/**
\page tuttimer3 Timer.3 - Binding arguments to a handler

In this tutorial we will modify the program from tutorial Timer.2 so that the
timer fires once a second. This will show how to pass additional parameters to
your handler function.

\dontinclude timer3/timer.cpp
\skip #include

\until bind.hpp

To implement a repeating timer using asio you need to change
the timer's expiry time in your callback function, and to then start a new
asynchronous wait. Obviously this means that the callback function will need
to be able to access the timer object. To this end we add two new parameters
to the <tt>print</tt> function:

\li A pointer to a timer object.

\li A counter so that we can stop the program when the timer fires for the
sixth time.

\until {

As mentioned above, this tutorial program uses a counter to
stop running when the timer fires for the sixth time. However you will observe
that there is no explicit call to ask the io_context to stop. Recall that in
tutorial Timer.2 we learnt that the boost::asio::io_context::run() function completes
when there is no more "work" to do. By not starting a new asynchronous wait on
the timer when <tt>count</tt> reaches 5, the io_context will run out of work and
stop running.

\until ++

Next we move the expiry time for the timer along by one second
from the previous expiry time. By calculating the new expiry time relative to
the old, we can ensure that the timer does not drift away from the
whole-second mark due to any delays in processing the handler.

\until expires_at

Then we start a new asynchronous wait on the timer. As you can
see, the boost::bind() function is used to associate the extra parameters
with your callback handler. The boost::asio::steady_timer::async_wait() function
expects a handler function (or function object) with the signature
<tt>void(const boost::system::error_code&)</tt>. Binding the additional parameters
converts your <tt>print</tt> function into a function object that matches the
signature correctly.

See the <a href="http://www.boost.org/libs/bind/bind.html">Boost.Bind
documentation</a> for more information on how to use boost::bind().

In this example, the boost::asio::placeholders::error argument to boost::bind() is a
named placeholder for the error object passed to the handler. When initiating
the asynchronous operation, and if using boost::bind(), you must specify only
the arguments that match the handler's parameter list. In tutorial Timer.4 you
will see that this placeholder may be elided if the parameter is not needed by
the callback handler.

\until boost::asio::io_context

A new <tt>count</tt> variable is added so that we can stop the
program when the timer fires for the sixth time.

\until boost::asio::steady_timer

As in Step 4, when making the call to
boost::asio::steady_timer::async_wait() from <tt>main</tt> we bind the additional
parameters needed for the <tt>print</tt> function.

\until run

Finally, just to prove that the <tt>count</tt> variable was
being used in the <tt>print</tt> handler function, we will print out its new
value.

\until }

See the \ref tuttimer3src "full source listing" \n
Return to the \ref index "tutorial index" \n
Previous: \ref tuttimer2 \n
Next: \ref tuttimer4

*/

/**
\page tuttimer3src Source listing for Timer.3
\include timer3/timer.cpp
Return to \ref tuttimer3
*/

/**
\page tuttimer4 Timer.4 - Using a member function as a handler

In this tutorial we will see how to use a class member function as a callback
handler. The program should execute identically to the tutorial program from
tutorial Timer.3.

\dontinclude timer4/timer.cpp
\skip #include

\until bind.hpp

Instead of defining a free function <tt>print</tt> as the
callback handler, as we did in the earlier tutorial programs, we now define a
class called <tt>printer</tt>.

\until public

The constructor of this class will take a reference to the
io_context object and use it when initialising the <tt>timer_</tt> member. The
counter used to shut down the program is now also a member of the class.

\until {

The boost::bind() function works just as well with class
member functions as with free functions. Since all non-static class member
functions have an implicit <tt>this</tt> parameter, we need to bind
<tt>this</tt> to the function. As in tutorial Timer.3, boost::bind()
converts our callback handler (now a member function) into a function object
that can be invoked as though it has the signature <tt>void(const
boost::system::error_code&)</tt>.

You will note that the boost::asio::placeholders::error placeholder is not specified
here, as the <tt>print</tt> member function does not accept an error object as
a parameter.

\until }

In the class destructor we will print out the final value of
the counter.

\until }

The <tt>print</tt> member function is very similar to the
<tt>print</tt> function from tutorial Timer.3, except that it now operates on
the class data members instead of having the timer and counter passed in as
parameters.

\until };

The <tt>main</tt> function is much simpler than before, as it
now declares a local <tt>printer</tt> object before running the io_context as
normal.

\until }

See the \ref tuttimer4src "full source listing" \n
Return to the \ref index "tutorial index" \n
Previous: \ref tuttimer3 \n
Next: \ref tuttimer5 \n

*/

/**
\page tuttimer4src Source listing for Timer.4
\include timer4/timer.cpp
Return to \ref tuttimer4
*/

/**
\page tuttimer5 Timer.5 - Synchronising handlers in multithreaded programs

This tutorial demonstrates the use of the boost::asio::strand class template to
synchronise callback handlers in a multithreaded program.

The previous four tutorials avoided the issue of handler synchronisation by
calling the boost::asio::io_context::run() function from one thread only. As you
already know, the asio library provides a guarantee that callback handlers will
<b>only</b> be called from threads that are currently calling
boost::asio::io_context::run(). Consequently, calling boost::asio::io_context::run() from
only one thread ensures that callback handlers cannot run concurrently.

The single threaded approach is usually the best place to start when
developing applications using asio. The downside is the limitations it places
on programs, particularly servers, including:

<ul>
<li>Poor responsiveness when handlers can take a long time to complete.</li>
<li>An inability to scale on multiprocessor systems.</li>
</ul>

If you find yourself running into these limitations, an alternative approach
is to have a pool of threads calling boost::asio::io_context::run(). However, as this
allows handlers to execute concurrently, we need a method of synchronisation
when handlers might be accessing a shared, thread-unsafe resource.

\dontinclude timer5/timer.cpp
\skip #include

\until bind.hpp

We start by defining a class called <tt>printer</tt>, similar
to the class in the previous tutorial. This class will extend the previous
tutorial by running two timers in parallel.

\until public

In addition to initialising a pair of boost::asio::steady_timer members, the
constructor initialises the <tt>strand_</tt> member, an object of type
boost::asio::strand<boost::asio::io_context::executor_type>.

The boost::asio::strand class template is an executor adapter that guarantees
that, for those handlers that are dispatched through it, an executing handler
will be allowed to complete before the next one is started. This is guaranteed
irrespective of the number of threads that are calling
boost::asio::io_context::run(). Of course, the handlers may still execute
concurrently with other handlers that were <b>not</b> dispatched through an
boost::asio::strand, or were dispatched through a different boost::asio::strand
object.

\until {

When initiating the asynchronous operations, each callback handler is "bound"
to an boost::asio::strand<boost::asio::io_context::executor_type> object. The
boost::asio::bind_executor() function returns a new handler that automatically
dispatches its contained handler through the boost::asio::strand object. By
binding the handlers to the same boost::asio::strand, we are ensuring that they
cannot execute concurrently.

\until }
\until }

In a multithreaded program, the handlers for asynchronous
operations should be synchronised if they access shared resources. In this
tutorial, the shared resources used by the handlers (<tt>print1</tt> and
<tt>print2</tt>) are <tt>std::cout</tt> and the <tt>count_</tt> data member.

\until };

The <tt>main</tt> function now causes boost::asio::io_context::run() to
be called from two threads: the main thread and one additional thread. This is
accomplished using an boost::thread object.

Just as it would with a call from a single thread, concurrent calls to
boost::asio::io_context::run() will continue to execute while there is "work" left to
do. The background thread will not exit until all asynchronous operations have
completed.

\until }

See the \ref tuttimer5src "full source listing" \n
Return to the \ref index "tutorial index" \n
Previous: \ref tuttimer4 \n

*/

/**
\page tuttimer5src Source listing for Timer.5
\include timer5/timer.cpp
Return to \ref tuttimer5
*/