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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="Reads bytes from a source."><meta name="keywords" content="rust, rustlang, rust-lang, AsyncRead"><title>AsyncRead in tokio::io - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><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="../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../dark.css" disabled><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script src="../../crates.js"></script><script defer src="../../main.js"></script>
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</a><h2 class="location">Trait AsyncRead</h2><div class="sidebar-elems"><div class="block items"><h3 class="sidebar-title"><a href="#required-methods">Required Methods</a></h3><div class="sidebar-links"><a href="#tymethod.poll_read">poll_read</a></div><h3 class="sidebar-title"><a href="#foreign-impls">Implementations on Foreign Types</a></h3><div class="sidebar-links"><a href="#impl-AsyncRead-for-%26%5Bu8%5D">&[u8]</a><a href="#impl-AsyncRead-for-%26mut%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><h3 class="sidebar-title"><a href="#implementors">Implementors</a></h3></div><h2 class="location">Other items in<br><a href="../index.html">tokio</a>::<wbr><a href="index.html">io</a></h2><div id="sidebar-vars" data-name="AsyncRead" data-ty="trait" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../tokio/index.html"><img class="rust-logo" src="../../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../../brush.svg"></button><div id="theme-choices" role="menu"></div></div><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" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><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><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span><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></h1><div class="docblock item-decl"><pre class="rust trait"><code>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/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br> ) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>>;
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}</code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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/1.59.0/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="trait.AsyncRead.html#tymethod.poll_read"><code>poll_read</code></a> method,
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unlike <a href="https://doc.rust-lang.org/1.59.0/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></details><h2 id="required-methods" class="small-section-header">Required methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><details class="rustdoc-toggle" open><summary><div id="tymethod.poll_read" class="method has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#53-57" title="goto source code">[src]</a></div><h4 class="code-header">fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>></h4></div></summary><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 places data in the
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unfilled portion of <code>buf</code>. If no data was read (<code>buf.filled().len()</code> is
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unchanged), it implies that 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></details></div><h2 id="foreign-impls" class="small-section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor"></a></h2><details class="rustdoc-toggle implementors-toggle"><summary><div id="impl-AsyncRead-for-Box%3CT%3E" class="impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#72-74" title="goto source code">[src]</a></div><a href="#impl-AsyncRead-for-Box%3CT%3E" class="anchor"></a><h3 class="code-header in-band">impl<T: ?<a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> + <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>> <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="struct" href="https://doc.rust-lang.org/1.59.0/alloc/boxed/struct.Box.html" title="struct alloc::boxed::Box">Box</a><T></h3></div></summary><div class="impl-items"><div id="method.poll_read" class="method trait-impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#73" title="goto source code">[src]</a></div><a href="#method.poll_read" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>></h4></div></div></details><details class="rustdoc-toggle implementors-toggle"><summary><div id="impl-AsyncRead-for-%26mut%20T" class="impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#76-78" title="goto source code">[src]</a></div><a href="#impl-AsyncRead-for-%26mut%20T" class="anchor"></a><h3 class="code-header in-band">impl<T: ?<a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> + <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>> <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>T</h3></div></summary><div class="impl-items"><div id="method.poll_read-1" class="method trait-impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#77" title="goto source code">[src]</a></div><a href="#method.poll_read-1" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>></h4></div></div></details><details class="rustdoc-toggle implementors-toggle"><summary><div id="impl-AsyncRead-for-Pin%3CP%3E" class="impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#80-92" title="goto source code">[src]</a></div><a href="#impl-AsyncRead-for-Pin%3CP%3E" class="anchor"></a><h3 class="code-header in-band">impl<P> <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><P> <span class="where fmt-newline">where<br> P: <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/ops/deref/trait.DerefMut.html" title="trait core::ops::deref::DerefMut">DerefMut</a> + <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,<br> P::<a class="associatedtype" href="https://doc.rust-lang.org/1.59.0/core/ops/deref/trait.Deref.html#associatedtype.Target" title="type core::ops::deref::Deref::Target">Target</a>: <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a>, </span></h3></div></summary><div class="impl-items"><div id="method.poll_read-2" class="method trait-impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#85-91" title="goto source code">[src]</a></div><a href="#method.poll_read-2" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>></h4></div></div></details><details class="rustdoc-toggle implementors-toggle"><summary><div id="impl-AsyncRead-for-%26%5Bu8%5D" class="impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#94-106" title="goto source code">[src]</a></div><a href="#impl-AsyncRead-for-%26%5Bu8%5D" class="anchor"></a><h3 class="code-header in-band">impl <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.slice.html">&[</a><a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.u8.html">u8</a><a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.slice.html">]</a></h3></div></summary><div class="impl-items"><div id="method.poll_read-3" class="method trait-impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#95-105" title="goto source code">[src]</a></div><a href="#method.poll_read-3" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> _cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>></h4></div></div></details><details class="rustdoc-toggle implementors-toggle"><summary><div id="impl-AsyncRead-for-Cursor%3CT%3E" class="impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#108-131" title="goto source code">[src]</a></div><a href="#impl-AsyncRead-for-Cursor%3CT%3E" class="anchor"></a><h3 class="code-header in-band">impl<T: <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/convert/trait.AsRef.html" title="trait core::convert::AsRef">AsRef</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.slice.html">[</a><a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.u8.html">u8</a><a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.slice.html">]</a>> + <a class="trait" href="https://doc.rust-lang.org/1.59.0/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>> <a class="trait" href="trait.AsyncRead.html" title="trait tokio::io::AsyncRead">AsyncRead</a> for <a class="struct" href="https://doc.rust-lang.org/1.59.0/std/io/cursor/struct.Cursor.html" title="struct std::io::cursor::Cursor">Cursor</a><T></h3></div></summary><div class="impl-items"><div id="method.poll_read-4" class="method trait-impl has-srclink"><div class="rightside"><a class="srclink" href="../../src/tokio/io/async_read.rs.html#109-130" title="goto source code">[src]</a></div><a href="#method.poll_read-4" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.poll_read" class="fnname">poll_read</a>(<br> self: <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/pin/struct.Pin.html" title="struct core::pin::Pin">Pin</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.reference.html">&mut </a>Self>, <br> _cx: &mut <a class="struct" href="https://doc.rust-lang.org/1.59.0/core/task/wake/struct.Context.html" title="struct core::task::wake::Context">Context</a><'_>, <br> buf: &mut <a class="struct" href="struct.ReadBuf.html" title="struct tokio::io::ReadBuf">ReadBuf</a><'_><br>) -> <a class="enum" href="https://doc.rust-lang.org/1.59.0/core/task/poll/enum.Poll.html" title="enum core::task::poll::Poll">Poll</a><<a class="type" href="https://doc.rust-lang.org/1.59.0/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a><<a class="primitive" href="https://doc.rust-lang.org/1.59.0/std/primitive.unit.html">()</a>>></h4></div></div></details><h2 id="implementors" class="small-section-header">Implementors<a href="#implementors" class="anchor"></a></h2><div class="item-list" id="implementors-list"></div><script type="text/javascript" src="../../implementors/tokio/io/trait.AsyncRead.js" async></script></section><section id="search" class="content hidden"></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.59.0 (9d1b2106e 2022-02-23)" ></div>
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