2 // Copyright (c) 2003-2016 Christopher M. Kohlhoff (chris at kohlhoff dot com)
4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
9 \page tuttimer1 Timer.1 - Using a timer synchronously
11 This tutorial program introduces asio by showing how to perform a blocking
14 \dontinclude timer1/timer.cpp
17 We start by including the necessary header files.
19 All of the asio classes can be used by simply including the <tt>"asio.hpp"</tt>
24 Since this example uses timers, we need to include the appropriate
25 Boost.Date_Time header file for manipulating times.
29 All programs that use asio need to have at least one boost::asio::io_service object.
30 This class provides access to I/O functionality. We declare an object of this
31 type first thing in the main function.
33 \until boost::asio::io_service
35 Next we declare an object of type boost::asio::deadline_timer. The core asio classes
36 that provide I/O functionality (or as in this case timer functionality) always
37 take a reference to an io_service as their first constructor argument. The
38 second argument to the constructor sets the timer to expire 5 seconds from now.
40 \until boost::asio::deadline_timer
42 In this simple example we perform a blocking wait on the timer.
43 That is, the call to boost::asio::deadline_timer::wait() will not return until the
44 timer has expired, 5 seconds after it was created (i.e. <b>not</b> from when the
47 A deadline timer is always in one of two states: "expired" or "not expired". If
48 the boost::asio::deadline_timer::wait() function is called on an expired timer, it
49 will return immediately.
53 Finally we print the obligatory <tt>"Hello, world!"</tt>
54 message to show when the timer has expired.
58 See the \ref tuttimer1src "full source listing" \n
59 Return to the \ref index "tutorial index" \n
65 \page tuttimer1src Source listing for Timer.1
66 \include timer1/timer.cpp
67 Return to \ref tuttimer1
71 \page tuttimer2 Timer.2 - Using a timer asynchronously
73 This tutorial program demonstrates how to use asio's asynchronous callback
74 functionality by modifying the program from tutorial Timer.1 to perform an
75 asynchronous wait on the timer.
77 \dontinclude timer2/timer.cpp
82 Using asio's asynchronous functionality means having a callback
83 function that will be called when an asynchronous operation completes. In this
84 program we define a function called <tt>print</tt> to be called when the
85 asynchronous wait finishes.
87 \until boost::asio::deadline_timer
89 Next, instead of doing a blocking wait as in tutorial Timer.1,
90 we call the boost::asio::deadline_timer::async_wait() function to perform an
91 asynchronous wait. When calling this function we pass the <tt>print</tt>
92 callback handler that was defined above.
96 Finally, we must call the boost::asio::io_service::run() member function
97 on the io_service object.
99 The asio library provides a guarantee that callback handlers will <b>only</b>
100 be called from threads that are currently calling boost::asio::io_service::run().
101 Therefore unless the boost::asio::io_service::run() function is called the callback for
102 the asynchronous wait completion will never be invoked.
104 The boost::asio::io_service::run() function will also continue to run while there is
105 still "work" to do. In this example, the work is the asynchronous wait on the
106 timer, so the call will not return until the timer has expired and the
107 callback has completed.
109 It is important to remember to give the io_service some work to do before
110 calling boost::asio::io_service::run(). For example, if we had omitted the above call
111 to boost::asio::deadline_timer::async_wait(), the io_service would not have had any
112 work to do, and consequently boost::asio::io_service::run() would have returned
118 See the \ref tuttimer2src "full source listing" \n
119 Return to the \ref index "tutorial index" \n
120 Previous: \ref tuttimer1 \n
126 \page tuttimer2src Source listing for Timer.2
127 \include timer2/timer.cpp
128 Return to \ref tuttimer2
132 \page tuttimer3 Timer.3 - Binding arguments to a handler
134 In this tutorial we will modify the program from tutorial Timer.2 so that the
135 timer fires once a second. This will show how to pass additional parameters to
136 your handler function.
138 \dontinclude timer3/timer.cpp
141 \until posix_time.hpp
143 To implement a repeating timer using asio you need to change
144 the timer's expiry time in your callback function, and to then start a new
145 asynchronous wait. Obviously this means that the callback function will need
146 to be able to access the timer object. To this end we add two new parameters
147 to the <tt>print</tt> function:
149 \li A pointer to a timer object.
151 \li A counter so that we can stop the program when the timer fires for the
156 As mentioned above, this tutorial program uses a counter to
157 stop running when the timer fires for the sixth time. However you will observe
158 that there is no explicit call to ask the io_service to stop. Recall that in
159 tutorial Timer.2 we learnt that the boost::asio::io_service::run() function completes
160 when there is no more "work" to do. By not starting a new asynchronous wait on
161 the timer when <tt>count</tt> reaches 5, the io_service will run out of work and
166 Next we move the expiry time for the timer along by one second
167 from the previous expiry time. By calculating the new expiry time relative to
168 the old, we can ensure that the timer does not drift away from the
169 whole-second mark due to any delays in processing the handler.
173 Then we start a new asynchronous wait on the timer. As you can
174 see, the boost::bind() function is used to associate the extra parameters
175 with your callback handler. The boost::asio::deadline_timer::async_wait() function
176 expects a handler function (or function object) with the signature
177 <tt>void(const boost::system::error_code&)</tt>. Binding the additional parameters
178 converts your <tt>print</tt> function into a function object that matches the
181 See the <a href="http://www.boost.org/libs/bind/bind.html">Boost.Bind
182 documentation</a> for more information on how to use boost::bind().
184 In this example, the boost::asio::placeholders::error argument to boost::bind() is a
185 named placeholder for the error object passed to the handler. When initiating
186 the asynchronous operation, and if using boost::bind(), you must specify only
187 the arguments that match the handler's parameter list. In tutorial Timer.4 you
188 will see that this placeholder may be elided if the parameter is not needed by
189 the callback handler.
191 \until boost::asio::io_service
193 A new <tt>count</tt> variable is added so that we can stop the
194 program when the timer fires for the sixth time.
196 \until boost::asio::deadline_timer
198 As in Step 4, when making the call to
199 boost::asio::deadline_timer::async_wait() from <tt>main</tt> we bind the additional
200 parameters needed for the <tt>print</tt> function.
204 Finally, just to prove that the <tt>count</tt> variable was
205 being used in the <tt>print</tt> handler function, we will print out its new
210 See the \ref tuttimer3src "full source listing" \n
211 Return to the \ref index "tutorial index" \n
212 Previous: \ref tuttimer2 \n
218 \page tuttimer3src Source listing for Timer.3
219 \include timer3/timer.cpp
220 Return to \ref tuttimer3
224 \page tuttimer4 Timer.4 - Using a member function as a handler
226 In this tutorial we will see how to use a class member function as a callback
227 handler. The program should execute identically to the tutorial program from
230 \dontinclude timer4/timer.cpp
233 \until posix_time.hpp
235 Instead of defining a free function <tt>print</tt> as the
236 callback handler, as we did in the earlier tutorial programs, we now define a
237 class called <tt>printer</tt>.
241 The constructor of this class will take a reference to the
242 io_service object and use it when initialising the <tt>timer_</tt> member. The
243 counter used to shut down the program is now also a member of the class.
247 The boost::bind() function works just as well with class
248 member functions as with free functions. Since all non-static class member
249 functions have an implicit <tt>this</tt> parameter, we need to bind
250 <tt>this</tt> to the function. As in tutorial Timer.3, boost::bind()
251 converts our callback handler (now a member function) into a function object
252 that can be invoked as though it has the signature <tt>void(const
253 boost::system::error_code&)</tt>.
255 You will note that the boost::asio::placeholders::error placeholder is not specified
256 here, as the <tt>print</tt> member function does not accept an error object as
261 In the class destructor we will print out the final value of
266 The <tt>print</tt> member function is very similar to the
267 <tt>print</tt> function from tutorial Timer.3, except that it now operates on
268 the class data members instead of having the timer and counter passed in as
273 The <tt>main</tt> function is much simpler than before, as it
274 now declares a local <tt>printer</tt> object before running the io_service as
279 See the \ref tuttimer4src "full source listing" \n
280 Return to the \ref index "tutorial index" \n
281 Previous: \ref tuttimer3 \n
282 Next: \ref tuttimer5 \n
287 \page tuttimer4src Source listing for Timer.4
288 \include timer4/timer.cpp
289 Return to \ref tuttimer4
293 \page tuttimer5 Timer.5 - Synchronising handlers in multithreaded programs
295 This tutorial demonstrates the use of the boost::asio::strand class to synchronise
296 callback handlers in a multithreaded program.
298 The previous four tutorials avoided the issue of handler synchronisation by
299 calling the boost::asio::io_service::run() function from one thread only. As you
300 already know, the asio library provides a guarantee that callback handlers will
301 <b>only</b> be called from threads that are currently calling
302 boost::asio::io_service::run(). Consequently, calling boost::asio::io_service::run() from
303 only one thread ensures that callback handlers cannot run concurrently.
305 The single threaded approach is usually the best place to start when
306 developing applications using asio. The downside is the limitations it places
307 on programs, particularly servers, including:
310 <li>Poor responsiveness when handlers can take a long time to complete.</li>
311 <li>An inability to scale on multiprocessor systems.</li>
314 If you find yourself running into these limitations, an alternative approach
315 is to have a pool of threads calling boost::asio::io_service::run(). However, as this
316 allows handlers to execute concurrently, we need a method of synchronisation
317 when handlers might be accessing a shared, thread-unsafe resource.
319 \dontinclude timer5/timer.cpp
322 \until posix_time.hpp
324 We start by defining a class called <tt>printer</tt>, similar
325 to the class in the previous tutorial. This class will extend the previous
326 tutorial by running two timers in parallel.
330 In addition to initialising a pair of boost::asio::deadline_timer members, the
331 constructor initialises the <tt>strand_</tt> member, an object of type
334 An boost::asio::strand guarantees that, for those handlers that are dispatched through
335 it, an executing handler will be allowed to complete before the next one is
336 started. This is guaranteed irrespective of the number of threads that are
337 calling boost::asio::io_service::run(). Of course, the handlers may still execute
338 concurrently with other handlers that were <b>not</b> dispatched through an
339 boost::asio::strand, or were dispatched through a different boost::asio::strand object.
343 When initiating the asynchronous operations, each callback handler is "wrapped"
344 using the boost::asio::strand object. The boost::asio::strand::wrap() function returns a new
345 handler that automatically dispatches its contained handler through the
346 boost::asio::strand object. By wrapping the handlers using the same boost::asio::strand, we
347 are ensuring that they cannot execute concurrently.
352 In a multithreaded program, the handlers for asynchronous
353 operations should be synchronised if they access shared resources. In this
354 tutorial, the shared resources used by the handlers (<tt>print1</tt> and
355 <tt>print2</tt>) are <tt>std::cout</tt> and the <tt>count_</tt> data member.
359 The <tt>main</tt> function now causes boost::asio::io_service::run() to
360 be called from two threads: the main thread and one additional thread. This is
361 accomplished using an boost::thread object.
363 Just as it would with a call from a single thread, concurrent calls to
364 boost::asio::io_service::run() will continue to execute while there is "work" left to
365 do. The background thread will not exit until all asynchronous operations have
370 See the \ref tuttimer5src "full source listing" \n
371 Return to the \ref index "tutorial index" \n
372 Previous: \ref tuttimer4 \n
377 \page tuttimer5src Source listing for Timer.5
378 \include timer5/timer.cpp
379 Return to \ref tuttimer5