140 lines
6.2 KiB
HTML
140 lines
6.2 KiB
HTML
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Asynchronous Operations</title>
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<link rel="stylesheet" href="../../../boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
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<link rel="home" href="../../../index.html" title="Asio">
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<link rel="up" href="../model.html" title="Asynchronous Model">
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<link rel="prev" href="../model.html" title="Asynchronous Model">
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<link rel="next" href="async_agents.html" title="Asynchronous Agents">
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<meta name="viewport" content="width=device-width, initial-scale=1">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../../asio.png"></td></tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="../model.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../model.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="async_agents.html"><img src="../../../next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="asio.overview.model.async_ops"></a><a class="link" href="async_ops.html" title="Asynchronous Operations">Asynchronous Operations</a>
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</h4></div></div></div>
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<p>
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<span class="inlinemediaobject"><img src="../../../async_op_model.png" width="421"></span>
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</p>
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<p>
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An <span class="emphasis"><em>asynchronous operation</em></span> is the basic unit of composition
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in the Asio asynchronous model. Asynchronous operations represent work
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that is launched and performed in the background, while the user's code
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that initiated the work can continue with other things.
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</p>
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<p>
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Conceptually, the lifecycle of an asynchronous operation can be described
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in terms of the following events and phases:
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../../async_op_phases.png" width="861"></span>
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</p>
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<p>
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An <span class="emphasis"><em>initiating function</em></span> is a function which may be
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called by the user to start an asynchronous operation.
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</p>
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<p>
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A <span class="emphasis"><em>completion handler</em></span> is a user-provided, move-only
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function object that will be invoked, at most once, with the result of
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the asynchronous operation. The invocation of the completion handler tells
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the user about something that has already happened: the operation completed,
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and the side effects of the operation were established.
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</p>
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<p>
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The initiating function and completion handler are incorporated into the
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user's code as follows:
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../../async_op_init_complete.png" width="496"></span>
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</p>
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<p>
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Synchronous operations, being embodied as single functions, have several
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inherent semantic properties as a consequence. Asynchronous operations
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adopt some of these semantic properties from their synchronous counterparts,
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in order to facilitate flexible and efficient composition.
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</p>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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Property of synchronous operations
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</p>
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</th>
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<th>
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<p>
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Equivalent property of asynchronous operations
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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When a synchronous operation is generic (i.e. a template) the
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return type is deterministically derived from the function and
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its arguments.
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</p>
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</td>
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<td>
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<p>
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When an asynchronous operation is generic, the completion handler's
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arguments' types and order are deterministically derived from
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the initiating function and its arguments.
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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If a synchronous operation requires a temporary resource (such
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as memory, a file descriptor, or a thread), this resource is
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released before returning from the function.
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</p>
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</td>
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<td>
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<p>
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If an asynchronous operation requires a temporary resource (such
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as memory, a file descriptor, or a thread), this resource is
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released before calling the completion handler.
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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<p>
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The latter is an important property of asynchronous operations, in that
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it allows a completion handler to initiate further asynchronous operations
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without overlapping resource usage. Consider the trivial (and relatively
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common) case of the same operation being repeated over and over in a chain:
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</p>
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<p>
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<span class="inlinemediaobject"><img src="../../../async_op_trivial_chain.png" width="375"></span>
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</p>
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<p>
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By ensuring that resources are released before the completion handler runs,
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we avoid doubling the peak resource usage of the chain of operations.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="../model.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../model.html"><img src="../../../up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../home.png" alt="Home"></a><a accesskey="n" href="async_agents.html"><img src="../../../next.png" alt="Next"></a>
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</div>
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</body>
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</html>
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