1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Requirements on asynchronous operations</title> 5<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../reference.html" title="Reference"> 9<link rel="prev" href="../reference.html" title="Reference"> 10<link rel="next" href="read_write_operations.html" title="Requirements on read and write operations"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 15<td align="center"><a href="../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="../reference.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="read_write_operations.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_asio.reference.asynchronous_operations"></a><a class="link" href="asynchronous_operations.html" title="Requirements on asynchronous operations">Requirements 28 on asynchronous operations</a> 29</h3></div></div></div> 30<p> 31 This section uses the names <code class="computeroutput">Alloc1</code>, <code class="computeroutput">Alloc2</code>, <code class="computeroutput">alloc1</code>, 32 <code class="computeroutput">alloc2</code>, <code class="computeroutput">Args</code>, <code class="computeroutput">CompletionHandler</code>, 33 <code class="computeroutput">completion_handler</code>, <code class="computeroutput">Executor1</code>, <code class="computeroutput">Executor2</code>, 34 <code class="computeroutput">ex1</code>, <code class="computeroutput">ex2</code>, <code class="computeroutput">f</code>, <code class="literal"><span class="emphasis"><em>i</em></span></code>, 35 <code class="literal"><span class="emphasis"><em>N</em></span></code>, <code class="computeroutput">Signature</code>, <code class="computeroutput">token</code>, 36 <code class="literal">T[sub <span class="emphasis"><em>i</em></span></code>], <code class="literal">t[sub <span class="emphasis"><em>i</em></span></code>], 37 <code class="computeroutput">work1</code>, and <code class="computeroutput">work2</code> as placeholders for specifying 38 the requirements below. 39 </p> 40<h5> 41<a name="boost_asio.reference.asynchronous_operations.h0"></a> 42 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.general_asynchronous_operation_concepts"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.general_asynchronous_operation_concepts">General 43 asynchronous operation concepts</a> 44 </h5> 45<p> 46 An <span class="emphasis"><em>initiating function</em></span> is a function which may be called 47 to start an asynchronous operation. A <span class="emphasis"><em>completion handler</em></span> 48 is a function object that will be invoked, at most once, with the result 49 of the asynchronous operation. 50 </p> 51<p> 52 The lifecycle of an asynchronous operation is comprised of the following 53 events and phases: 54 </p> 55<p> 56 — Event 1: The asynchronous operation is started by a call to the initiating 57 function. 58 </p> 59<p> 60 — Phase 1: The asynchronous operation is now <span class="emphasis"><em>outstanding</em></span>. 61 </p> 62<p> 63 — Event 2: The externally observable side effects of the asynchronous operation, 64 if any, are fully established. The completion handler is submitted to an 65 executor. 66 </p> 67<p> 68 — Phase 2: The asynchronous operation is now <span class="emphasis"><em>completed</em></span>. 69 </p> 70<p> 71 — Event 3: The completion handler is called with the result of the asynchronous 72 operation. 73 </p> 74<p> 75 In this library, all functions with the prefix <code class="computeroutput">async_</code> are initiating 76 functions. 77 </p> 78<h5> 79<a name="boost_asio.reference.asynchronous_operations.h1"></a> 80 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.completion_tokens_and_handlers"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.completion_tokens_and_handlers">Completion 81 tokens and handlers</a> 82 </h5> 83<p> 84 Initiating functions: 85 </p> 86<p> 87 — are function templates with template parameter <code class="computeroutput">CompletionToken</code>; 88 </p> 89<p> 90 — accept, as the final parameter, a <span class="emphasis"><em>completion token</em></span> object 91 <code class="computeroutput">token</code> of type <code class="computeroutput">CompletionToken</code>; 92 </p> 93<p> 94 — specify a <span class="emphasis"><em>completion signature</em></span>, which is a call signature 95 (C++Std [func.def]) <code class="computeroutput">Signature</code> that determines the arguments 96 to the completion handler. 97 </p> 98<p> 99 An initiating function determines the type <code class="computeroutput">CompletionHandler</code> 100 of its completion handler function object by performing <code class="computeroutput">typename async_result<decay_t<CompletionToken>, 101 Signature>::completion_handler_type</code>. The completion handler object 102 <code class="computeroutput">completion_handler</code> is initialized with <code class="computeroutput">forward<CompletionToken>(token)</code>. 103 [<span class="emphasis"><em>Note:</em></span> No other requirements are placed on the type 104 <code class="computeroutput">CompletionToken</code>. —<span class="emphasis"><em>end note</em></span>] 105 </p> 106<p> 107 The type <code class="computeroutput">CompletionHandler</code> must satisfy the requirements of 108 <code class="computeroutput">Destructible</code> (C++Std [destructible]) and <code class="computeroutput">MoveConstructible</code> 109 (C++Std [moveconstructible]), and be callable with the specified call signature. 110 </p> 111<p> 112 In this library, all initiating functions specify a <span class="emphasis"><em>Completion 113 signature</em></span> element that defines the call signature <code class="computeroutput">Signature</code>. 114 The <span class="emphasis"><em>Completion signature</em></span> elements in this Technical 115 Specification have named parameters, and the results of an asynchronous operation 116 are specified in terms of these names. 117 </p> 118<h5> 119<a name="boost_asio.reference.asynchronous_operations.h2"></a> 120 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type">Automatic 121 deduction of initiating function return type</a> 122 </h5> 123<p> 124 The return type of an initiating function is <code class="computeroutput">typename async_result<decay_t<CompletionToken>, 125 Signature>::return_type</code>. 126 </p> 127<p> 128 For the sake of exposition, this library sometimes annotates functions with 129 a return type <span class="emphasis"><em><code class="literal">DEDUCED</code></em></span>. For every 130 function declaration that returns <span class="emphasis"><em><code class="literal">DEDUCED</code></em></span>, 131 the meaning is equivalent to specifying the return type as <code class="computeroutput">typename 132 async_result<decay_t<CompletionToken>, Signature>::return_type</code>. 133 </p> 134<h5> 135<a name="boost_asio.reference.asynchronous_operations.h3"></a> 136 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.production_of_initiating_function_return_value"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.production_of_initiating_function_return_value">Production 137 of initiating function return value</a> 138 </h5> 139<p> 140 An initiating function produces its return type as follows: 141 </p> 142<p> 143 — constructing an object <code class="computeroutput">result</code> of type <code class="computeroutput">async_result<decay_t<CompletionToken>, 144 Signature></code>, initialized as <code class="computeroutput">result(completion_handler)</code>; 145 and 146 </p> 147<p> 148 — using <code class="computeroutput">result.get()</code> as the operand of the return statement. 149 </p> 150<p> 151 [<span class="emphasis"><em>Example:</em></span> Given an asynchronous operation with <span class="emphasis"><em>Completion 152 signature</em></span> <code class="computeroutput">void(R1 r1, R2 r2)</code>, an initiating function 153 meeting these requirements may be implemented as follows: 154 </p> 155<pre class="programlisting">template<class CompletionToken> 156auto async_xyz(T1 t1, T2 t2, CompletionToken&& token) 157{ 158 typename async_result<decay_t<CompletionToken>, void(R1, R2)>::completion_handler_type 159 completion_handler(forward<CompletionToken>(token)); 160 161 async_result<decay_t<CompletionToken>, void(R1, R2)> result(completion_handler); 162 163 // initiate the operation and cause completion_handler to be invoked with 164 // the result 165 166 return result.get(); 167} 168</pre> 169<p> 170 For convenience, initiating functions may be implemented using the <code class="computeroutput">async_completion</code> 171 template: 172 </p> 173<pre class="programlisting">template<class CompletionToken> 174auto async_xyz(T1 t1, T2 t2, CompletionToken&& token) 175{ 176 async_completion<CompletionToken, void(R1, R2)> init(token); 177 178 // initiate the operation and cause init.completion_handler to be invoked 179 // with the result 180 181 return init.result.get(); 182} 183</pre> 184<p> 185 —<span class="emphasis"><em>end example</em></span>] 186 </p> 187<h5> 188<a name="boost_asio.reference.asynchronous_operations.h4"></a> 189 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.lifetime_of_initiating_function_arguments"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.lifetime_of_initiating_function_arguments">Lifetime 190 of initiating function arguments</a> 191 </h5> 192<p> 193 Unless otherwise specified, the lifetime of arguments to initiating functions 194 shall be treated as follows: 195 </p> 196<p> 197 — If the parameter has a pointer type or has a type of lvalue reference to 198 non-const, the implementation may assume the validity of the pointee or referent, 199 respectively, until the completion handler is invoked. [<span class="emphasis"><em>Note:</em></span> 200 In other words, the program must guarantee the validity of the argument until 201 the completion handler is invoked. —<span class="emphasis"><em>end note</em></span>] 202 </p> 203<p> 204 — Otherwise, the implementation must not assume the validity of the argument 205 after the initiating function completes. [<span class="emphasis"><em>Note:</em></span> In other 206 words, the program is not required to guarantee the validity of the argument 207 after the initiating function completes. —<span class="emphasis"><em>end note</em></span>] The 208 implementation may make copies of the argument, and all copies shall be destroyed 209 no later than immediately after invocation of the completion handler. 210 </p> 211<h5> 212<a name="boost_asio.reference.asynchronous_operations.h5"></a> 213 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.non_blocking_requirements_on_initiating_functions"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.non_blocking_requirements_on_initiating_functions">Non-blocking 214 requirements on initiating functions</a> 215 </h5> 216<p> 217 An initiating function shall not block (C++Std [defns.block]) the calling 218 thread pending completion of the outstanding operation. 219 </p> 220<p> 221 [std_note Initiating functions may still block the calling thread for other 222 reasons. For example, an initiating function may lock a mutex in order to 223 synchronize access to shared data.] 224 </p> 225<h5> 226<a name="boost_asio.reference.asynchronous_operations.h6"></a> 227 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.associated_executor"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.associated_executor">Associated 228 executor</a> 229 </h5> 230<p> 231 Certain objects that participate in asynchronous operations have an <span class="emphasis"><em>associated 232 executor</em></span>. These are obtained as specified in the sections below. 233 </p> 234<h5> 235<a name="boost_asio.reference.asynchronous_operations.h7"></a> 236 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.associated_i_o_executor"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.associated_i_o_executor">Associated 237 I/O executor</a> 238 </h5> 239<p> 240 An asynchronous operation has an associated executor satisfying the <a class="link" href="Executor1.html" title="Executor requirements"><code class="computeroutput">Executor</code></a> requirements. 241 If not otherwise specified by the asynchronous operation, this associated 242 executor is an object of type <code class="computeroutput">system_executor</code>. 243 </p> 244<p> 245 All asynchronous operations in this library have an associated executor object 246 that is determined as follows: 247 </p> 248<p> 249 — If the initiating function is a member function, the associated executor 250 is that returned by the <code class="computeroutput">get_executor</code> member function on the 251 same object. 252 </p> 253<p> 254 — If the initiating function is not a member function, the associated executor 255 is that returned by the <code class="computeroutput">get_executor</code> member function of the 256 first argument to the initiating function. 257 </p> 258<p> 259 Let <code class="computeroutput">Executor1</code> be the type of the associated executor. Let <code class="computeroutput">ex1</code> 260 be a value of type <code class="computeroutput">Executor1</code>, representing the associated executor 261 object obtained as described above. 262 </p> 263<h5> 264<a name="boost_asio.reference.asynchronous_operations.h8"></a> 265 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.associated_completion_handler_executor"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.associated_completion_handler_executor">Associated 266 completion handler executor</a> 267 </h5> 268<p> 269 A completion handler object of type <code class="computeroutput">CompletionHandler</code> has an 270 associated executor of type <code class="computeroutput">Executor2</code> satisfying the <a class="link" href="Executor1.html" title="Executor requirements">Executor 271 requirements</a>. The type <code class="computeroutput">Executor2</code> is <code class="computeroutput">associated_executor_t<CompletionHandler, 272 Executor1></code>. Let <code class="computeroutput">ex2</code> be a value of type <code class="computeroutput">Executor2</code> 273 obtained by performing <code class="computeroutput">get_associated_executor(completion_handler, 274 ex1)</code>. 275 </p> 276<h5> 277<a name="boost_asio.reference.asynchronous_operations.h9"></a> 278 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.outstanding_work"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.outstanding_work">Outstanding 279 work</a> 280 </h5> 281<p> 282 Until the asynchronous operation has completed, the asynchronous operation 283 shall maintain: 284 </p> 285<p> 286 — an object <code class="computeroutput">work1</code> of type <code class="computeroutput">executor_work_guard<Executor1></code>, 287 initialized as <code class="computeroutput">work1(ex1)</code>, and where <code class="computeroutput">work1.owns_work() 288 == true</code>; and 289 </p> 290<p> 291 — an object <code class="computeroutput">work2</code> of type <code class="computeroutput">executor_work_guard<Executor2></code>, 292 initialized as <code class="computeroutput">work2(ex2)</code>, and where <code class="computeroutput">work2.owns_work() 293 == true</code>. 294 </p> 295<h5> 296<a name="boost_asio.reference.asynchronous_operations.h10"></a> 297 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.allocation_of_intermediate_storage"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.allocation_of_intermediate_storage">Allocation 298 of intermediate storage</a> 299 </h5> 300<p> 301 Asynchronous operations may allocate memory. [<span class="emphasis"><em>Note:</em></span> 302 Such as a data structure to store copies of the <code class="computeroutput">completion_handler</code> 303 object and the initiating function's arguments. —<span class="emphasis"><em>end note</em></span>] 304 </p> 305<p> 306 Let <code class="computeroutput">Alloc1</code> be a type, satisfying the <a class="link" href="ProtoAllocator.html" title="Proto-allocator requirements"><code class="computeroutput">ProtoAllocator</code></a> 307 requirements, that represents the asynchronous operation's default allocation 308 strategy. [<span class="emphasis"><em>Note:</em></span> Typically <code class="computeroutput">std::allocator<void></code>. 309 —<span class="emphasis"><em>end note</em></span>] Let <code class="computeroutput">alloc1</code> be a value of type 310 <code class="computeroutput">Alloc1</code>. 311 </p> 312<p> 313 A completion handler object of type <code class="computeroutput">CompletionHandler</code> has an 314 associated allocator object <code class="computeroutput">alloc2</code> of type <code class="computeroutput">Alloc2</code> 315 satisfying the <a class="link" href="ProtoAllocator.html" title="Proto-allocator requirements"><code class="computeroutput">ProtoAllocator</code></a> 316 requirements. The type <code class="computeroutput">Alloc2</code> is <code class="computeroutput">associated_allocator_t<CompletionHandler, 317 Alloc1></code>. Let <code class="computeroutput">alloc2</code> be a value of type <code class="computeroutput">Alloc2</code> 318 obtained by performing <code class="computeroutput">get_associated_allocator(completion_handler, 319 alloc1)</code>. 320 </p> 321<p> 322 The asynchronous operations defined in this library: 323 </p> 324<p> 325 — If required, allocate memory using only the completion handler's associated 326 allocator. 327 </p> 328<p> 329 — Prior to completion handler execution, deallocate any memory allocated using 330 the completion handler's associated allocator. 331 </p> 332<p> 333 [std_note The implementation may perform operating system or underlying API 334 calls that perform memory allocations not using the associated allocator. 335 Invocations of the allocator functions may not introduce data races (See 336 C++Std [res.on.data.races]).] 337 </p> 338<h5> 339<a name="boost_asio.reference.asynchronous_operations.h11"></a> 340 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.execution_of_completion_handler_on_completion_of_asynchronous_operation"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.execution_of_completion_handler_on_completion_of_asynchronous_operation">Execution 341 of completion handler on completion of asynchronous operation</a> 342 </h5> 343<p> 344 Let <code class="computeroutput">Args...</code> be the argument types of the completion signature 345 <code class="computeroutput">Signature</code> and let <code class="literal"><span class="emphasis"><em>N</em></span></code> 346 be <code class="computeroutput">sizeof...(Args)</code>. Let <code class="literal"><span class="emphasis"><em>i</em></span></code> 347 be in the range [<code class="computeroutput">0</code>,<code class="literal"><span class="emphasis"><em>N</em></span></code>). 348 Let <code class="literal">T[sub <span class="emphasis"><em>i</em></span></code>] be the <code class="literal"><span class="emphasis"><em>i</em></span></code>th 349 type in <code class="computeroutput">Args...</code> and let <code class="literal">t[sub <span class="emphasis"><em>i</em></span></code>] 350 be the <code class="literal"><span class="emphasis"><em>i</em></span></code>th completion handler argument 351 associated with <code class="literal">T[sub <span class="emphasis"><em>i</em></span></code>]. 352 </p> 353<p> 354 Let <code class="computeroutput">f</code> be a function object, callable as <code class="computeroutput">f()</code>, that 355 invokes <code class="computeroutput">completion_handler</code> as if by <code class="literal">completion_handler(forward<T[sub 356 <span class="emphasis"><em>0</em></span></code>>(t[sub <span class="emphasis"><em>0</em></span>]), ..., 357 forward<T[sub <span class="emphasis"><em>N-1</em></span>]>(t[sub <span class="emphasis"><em>N-1</em></span>]))]. 358 </p> 359<p> 360 If an asynchonous operation completes immediately (that is, within the thread 361 of execution calling the initiating function, and before the initiating function 362 returns), the completion handler shall be submitted for execution as if by 363 performing <code class="computeroutput">ex2.post(std::move(f), alloc2)</code>. Otherwise, the completion 364 handler shall be submitted for execution as if by performing <code class="computeroutput">ex2.dispatch(std::move(f), 365 alloc2)</code>. 366 </p> 367<h5> 368<a name="boost_asio.reference.asynchronous_operations.h12"></a> 369 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.completion_handlers_and_exceptions"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.completion_handlers_and_exceptions">Completion 370 handlers and exceptions</a> 371 </h5> 372<p> 373 Completion handlers are permitted to throw exceptions. The effect of any 374 exception propagated from the execution of a completion handler is determined 375 by the executor which is executing the completion handler. 376 </p> 377<h5> 378<a name="boost_asio.reference.asynchronous_operations.h13"></a> 379 <span class="phrase"><a name="boost_asio.reference.asynchronous_operations.default_completion_tokens"></a></span><a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens">Default 380 completion tokens</a> 381 </h5> 382<p> 383 Every I/O executor type has an associated default completion token type. 384 This is specified via the <code class="computeroutput">default_completion_token</code> trait. This 385 trait may be used in asynchronous operation declarations as follows: 386 </p> 387<pre class="programlisting">template < 388 typename IoObject, 389 typename CompletionToken = 390 typename default_completion_token< 391 typename IoObject::executor_type 392 >::type 393 > 394auto async_xyz( 395 IoObject& io_object, 396 CompletionToken&& token = 397 typename default_completion_token< 398 typename IoObject::executor_type 399 >::type{} 400 ); 401</pre> 402<p> 403 If not specialised, this trait type is <code class="computeroutput">void</code>, meaning no default 404 completion token type is available for the given I/O executor. 405 </p> 406<p> 407 [<span class="emphasis"><em>Example:</em></span> The <code class="computeroutput">default_completion_token</code> 408 trait is specialised for the <code class="computeroutput">use_awaitable</code> completion token 409 so that it may be used as shown in the following example: 410 </p> 411<pre class="programlisting">auto socket = use_awaitable.as_default_on(tcp::socket(my_context)); 412// ... 413co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable. 414</pre> 415<p> 416 In this example, the type of the <code class="computeroutput">socket</code> object is transformed 417 from <code class="computeroutput">tcp::socket</code> to have an I/O executor with the default completion 418 token set to <code class="computeroutput">use_awaitable</code>. 419 </p> 420<p> 421 Alternatively, the socket type may be computed directly: 422 </p> 423<pre class="programlisting">using tcp_socket = use_awaitable_t<>::as_default_on_t<tcp::socket>; 424tcp_socket socket(my_context); 425// ... 426co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable. 427</pre> 428<p> 429 —<span class="emphasis"><em>end example</em></span>] 430 </p> 431</div> 432<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 433<td align="left"></td> 434<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 435 Kohlhoff<p> 436 Distributed under the Boost Software License, Version 1.0. (See accompanying 437 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 438 </p> 439</div></td> 440</tr></table> 441<hr> 442<div class="spirit-nav"> 443<a accesskey="p" href="../reference.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="read_write_operations.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 444</div> 445</body> 446</html> 447
