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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="Asynchronously copies the entire contents of a reader into a writer."><title>copy_buf in tokio::io - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../static.files/rustdoc-42caa33d.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="tokio" data-themes="" data-resource-suffix="" data-rustdoc-version="1.84.1 (e71f9a9a9 2025-01-27)" data-channel="1.84.1" data-search-js="search-92e6798f.js" data-settings-js="settings-0f613d39.js" ><script src="../../static.files/storage-59e33391.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-044be391.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../tokio/index.html">tokio</a><span class="version">1.42.0</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">copy_<wbr>buf</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#errors" title="Errors">Errors</a></li><li><a href="#examples" title="Examples">Examples</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="index.html">In tokio::<wbr>io</a></h2></div></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><span class="rustdoc-breadcrumbs"><a href="../index.html">tokio</a>::<wbr><a href="index.html">io</a></span><h1>Function <span class="fn">copy_buf</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../src/tokio/io/util/copy_buf.rs.html#60-70">Source</a> </span></div><pre class="rust item-decl"><code>pub async fn copy_buf&lt;'a, R, W&gt;(
reader: <a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.reference.html">&amp;'a mut R</a>,
writer: <a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.reference.html">&amp;'a mut W</a>,
) -&gt; <a class="type" href="https://doc.rust-lang.org/1.84.1/std/io/error/type.Result.html" title="type std::io::error::Result">Result</a>&lt;<a class="primitive" href="https://doc.rust-lang.org/1.84.1/std/primitive.u64.html">u64</a>&gt;<div class="where">where
R: <a class="trait" href="trait.AsyncBufRead.html" title="trait tokio::io::AsyncBufRead">AsyncBufRead</a> + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,
W: <a class="trait" href="trait.AsyncWrite.html" title="trait tokio::io::AsyncWrite">AsyncWrite</a> + <a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a> + ?<a class="trait" href="https://doc.rust-lang.org/1.84.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Asynchronously copies the entire contents of a reader into a writer.</p>
<p>This function returns a future that will continuously read data from
<code>reader</code> and then write it into <code>writer</code> in a streaming fashion until
<code>reader</code> returns EOF or fails.</p>
<p>On success, the total number of bytes that were copied from <code>reader</code> to
<code>writer</code> is returned.</p>
<p>This is a <a href="fn.copy.html" title="fn tokio::io::copy"><code>tokio::io::copy</code></a> alternative for <a href="trait.AsyncBufRead.html" title="trait tokio::io::AsyncBufRead"><code>AsyncBufRead</code></a> readers
with no extra buffer allocation, since <a href="trait.AsyncBufRead.html" title="trait tokio::io::AsyncBufRead"><code>AsyncBufRead</code></a> allow access
to the readers inner buffer.</p>
<h2 id="errors"><a class="doc-anchor" href="#errors">§</a>Errors</h2>
<p>The returned future will finish with an error will return an error
immediately if any call to <code>poll_fill_buf</code> or <code>poll_write</code> returns an
error.</p>
<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::io;
<span class="kw">let </span><span class="kw-2">mut </span>reader: <span class="kw-2">&amp;</span>[u8] = <span class="string">b"hello"</span>;
<span class="kw">let </span><span class="kw-2">mut </span>writer: Vec&lt;u8&gt; = <span class="macro">vec!</span>[];
io::copy_buf(<span class="kw-2">&amp;mut </span>reader, <span class="kw-2">&amp;mut </span>writer).<span class="kw">await</span><span class="question-mark">?</span>;
<span class="macro">assert_eq!</span>(<span class="string">b"hello"</span>, <span class="kw-2">&amp;</span>writer[..]);</code></pre></div>
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