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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `AsyncRead` trait in crate `tokio`."><meta name="keywords" content="rust, rustlang, rust-lang, AsyncRead"><title>tokio::io::AsyncRead - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc trait"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../tokio/index.html'><div class='logo-container rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><p class="location">Trait AsyncRead</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.poll_read">poll_read</a></div><a class="sidebar-title" href="#foreign-impls">Implementations on Foreign Types</a><div class="sidebar-links"><a href="#impl-AsyncRead-for-%26%27_%20%5Bu8%5D">&'_ [u8]</a><a href="#impl-AsyncRead-for-%26%27_%20mut%20T">&'_ mut T</a><a href="#impl-AsyncRead-for-Box%3CT%3E">Box<T></a><a href="#impl-AsyncRead-for-Cursor%3CT%3E">Cursor<T></a><a href="#impl-AsyncRead-for-Pin%3CP%3E">Pin<P></a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class="location"><a href="../index.html">tokio</a>::<wbr><a href="index.html">io</a></p><script>window.sidebarCurrent = {name: "AsyncRead", ty: "trait", relpath: ""};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><span class="help-button">?</span>
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<a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../../src/tokio/io/async_read.rs.html#43-58" title="goto source code">[src]</a></span><span class="in-band">Trait <a href="../index.html">tokio</a>::<wbr><a href="index.html">io</a>::<wbr><a class="trait" href="">AsyncRead</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class="rust trait">pub trait AsyncRead {
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fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/nightly/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="../../tokio/io/struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br> ) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/nightly/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.unit.html">()</a>>>;
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}</pre></div><div class="docblock"><p>Reads bytes from a source.</p>
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<p>This trait is analogous to the <a href="https://doc.rust-lang.org/nightly/std/io/trait.Read.html"><code>std::io::Read</code></a> trait, but integrates with
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the asynchronous task system. In particular, the <a href="../../tokio/io/trait.AsyncRead.html#tymethod.poll_read"><code>poll_read</code></a> method,
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unlike <a href="https://doc.rust-lang.org/nightly/std/io/trait.Read.html#tymethod.read"><code>Read::read</code></a>, will automatically queue the current task for wakeup
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and return if data is not yet available, rather than blocking the calling
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thread.</p>
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<p>Specifically, this means that the <code>poll_read</code> function will return one of
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the following:</p>
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<ul>
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<li>
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<p><code>Poll::Ready(Ok(()))</code> means that data was immediately read and placed into
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the output buffer. The amount of data read can be determined by the
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increase in the length of the slice returned by <code>ReadBuf::filled</code>. If the
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difference is 0, EOF has been reached.</p>
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</li>
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<li>
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<p><code>Poll::Pending</code> means that no data was read into the buffer
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provided. The I/O object is not currently readable but may become readable
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in the future. Most importantly, <strong>the current future's task is scheduled
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to get unparked when the object is readable</strong>. This means that like
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<code>Future::poll</code> you'll receive a notification when the I/O object is
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readable again.</p>
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</li>
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<li>
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<p><code>Poll::Ready(Err(e))</code> for other errors are standard I/O errors coming from the
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underlying object.</p>
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</li>
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</ul>
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<p>This trait importantly means that the <code>read</code> method only works in the
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context of a future's task. The object may panic if used outside of a task.</p>
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<p>Utilities for working with <code>AsyncRead</code> values are provided by
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<a href="crate::io::AsyncReadExt"><code>AsyncReadExt</code></a>.</p>
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</div><h2 id="required-methods" class="small-section-header">Required methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><h3 id="tymethod.poll_read" class="method"><code>fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/nightly/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="../../tokio/io/struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/nightly/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.unit.html">()</a>>></code></h3><div class="docblock"><p>Attempts to read from the <code>AsyncRead</code> into <code>buf</code>.</p>
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<p>On success, returns <code>Poll::Ready(Ok(()))</code> and fills <code>buf</code> with data
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read. If no data was read (<code>buf.filled().is_empty()</code>) it implies that
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EOF has been reached.</p>
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<p>If no data is available for reading, the method returns <code>Poll::Pending</code>
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and arranges for the current task (via <code>cx.waker()</code>) to receive a
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notification when the object becomes readable or is closed.</p>
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</div></div><span class="loading-content">Loading content...</span><h2 id="foreign-impls" class="small-section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor"></a></h2><h3 id="impl-AsyncRead-for-Box%3CT%3E" class="impl"><code class="in-band">impl<T: ?<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="../../tokio/io/trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>> <a class="trait" href="../../tokio/io/trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="struct" href="https://doc.rust-lang.org/nightly/alloc/boxed/struct.Box.html" title="struct alloc::boxed::Box">Box</a><T></code><a href="#impl-AsyncRead-for-Box%3CT%3E" class="anchor"></a><a class="srclink" href="../../src/tokio/io/async_read.rs.html#72-74" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.poll_read" class="method hidden"><code>fn <a href="#method.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/nightly/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="../../tokio/io/struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/nightly/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.unit.html">()</a>>></code><a class="srclink" href="../../src/tokio/io/async_read.rs.html#73" title="goto source code">[src]</a></h4></div><h3 id="impl-AsyncRead-for-%26%27_%20mut%20T" class="impl"><code class="in-band">impl<T: ?<a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="../../tokio/io/trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>, '_> <a class="trait" href="../../tokio/io/trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&'_ mut </a>T</code><a href="#impl-AsyncRead-for-%26%27_%20mut%20T" class="anchor"></a><a class="srclink" href="../../src/tokio/io/async_read.rs.html#76-78" title="goto source code">[src]</a></h3><div class="impl-items"><h4 id="method.poll_read-1" class="method hidden"><code>fn <a href="#method.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/nightly/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/nightly/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="../../tokio/io/struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/tas
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