<!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 `extern` keyword in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, extern"><title>extern - Rust</title><link rel="stylesheet" type="text/css" href="../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../light1.35.0.css" id="themeStyle"><script src="../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow1.35.0.svg");}</style></head><body class="rustdoc keyword"><!--[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='../std/index.html'><img src='../rust-logo1.35.0.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='index.html'>std</a></p><script>window.sidebarCurrent = {name: 'extern', ty: 'keyword', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><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><a id="settings-menu" href="../settings.html"><img src="../wheel1.35.0.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></span><span class='in-band'>Keyword <a class="keyword" href=''>extern</a></span></h1><div class='docblock'><p>For external connections in Rust code.</p>
<p>The <code>extern</code> keyword is used in two places in Rust. One is in conjunction with the <a href="../std/index.html" title="`crate`"><code>crate</code></a>
keyword to make your Rust code aware of other Rust crates in your project, i.e., <code>extern crate lazy_static;</code>. The other use is in foreign function interfaces (FFI).</p>
<p><code>extern</code> is used in two different contexts within FFI. The first is in the form of external
blocks, for declaring function interfaces that Rust code can call foreign code by.</p>
<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore">
<span class="attribute">#[<span class="ident">link</span>(<span class="ident">name</span> <span class="op">=</span> <span class="string">"my_c_library"</span>)]</span>
<span class="kw">extern</span> <span class="string">"C"</span> {
<span class="kw">fn</span> <span class="ident">my_c_function</span>(<span class="ident">x</span>: <span class="ident">i32</span>) <span class="op">-></span> <span class="ident">bool</span>;
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Blink(name%20%3D%20%22my_c_library%22)%5D%0Aextern%20%22C%22%20%7B%0A%20%20%20%20fn%20my_c_function(x%3A%20i32)%20-%3E%20bool%3B%0A%7D%0A%7D">Run</a></pre></div>
<p>This code would attempt to link with <code>libmy_c_library.so</code> on unix-like systems and
<code>my_c_library.dll</code> on Windows at runtime, and panic if it can't find something to link to. Rust
code could then use <code>my_c_function</code> as if it were any other unsafe Rust function. Working with
non-Rust languages and FFI is inherently unsafe, so wrappers are usually built around C APIs.</p>
<p>The mirror use case of FFI is also done via the <code>extern</code> keyword:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="attribute">#[<span class="ident">no_mangle</span>]</span>
<span class="kw">pub</span> <span class="kw">extern</span> <span class="kw">fn</span> <span class="ident">callable_from_c</span>(<span class="ident">x</span>: <span class="ident">i32</span>) <span class="op">-></span> <span class="ident">bool</span> {
<span class="ident">x</span> <span class="op">%</span> <span class="number">3</span> <span class="op">==</span> <span class="number">0</span>
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Bno_mangle%5D%0Apub%20extern%20fn%20callable_from_c(x%3A%20i32)%20-%3E%20bool%20%7B%0A%20%20%20%20x%20%25%203%20%3D%3D%200%0A%7D%0A%7D">Run</a></pre></div>
<p>If compiled as a dylib, the resulting .so could then be linked to from a C library, and the
function could be used as if it was from any other library.</p>
<p>For more information on FFI, check the <a href="https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code">Rust book</a> or the <a href="https://doc.rust-lang.org/reference/items/external-blocks.html">Reference</a>.</p>
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