| <!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="Registers an arbitrary action for the given signal."><meta name="keywords" content="rust, rustlang, rust-lang, register"><title>register in signal_hook_registry - 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" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.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">☰</button><a class="sidebar-logo" href="../signal_hook_registry/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../signal_hook_registry/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In signal_hook_registry</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Function <a href="index.html">signal_hook_registry</a>::<wbr><a class="fn" href="#">register</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></h1><span class="out-of-band"><a class="srclink" href="../src/signal_hook_registry/lib.rs.html#500-505">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust fn"><code>pub unsafe fn register<F>(signal: <a class="type" href="../libc/unix/type.c_int.html" title="type libc::unix::c_int">c_int</a>, action: F) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><<a class="struct" href="struct.SigId.html" title="struct signal_hook_registry::SigId">SigId</a>, <a class="struct" href="https://doc.rust-lang.org/nightly/std/io/error/struct.Error.html" title="struct std::io::error::Error">Error</a>><span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.Fn.html" title="trait core::ops::function::Fn">Fn</a>() + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static,</span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Registers an arbitrary action for the given signal.</p> |
| <p>This makes sure there’s a signal handler for the given signal. It then adds the action to the |
| ones called each time the signal is delivered. If multiple actions are set for the same signal, |
| all are called, in the order of registration.</p> |
| <p>If there was a previous signal handler for the given signal, it is chained ‒ it will be called |
| as part of this library’s signal handler, before any actions set through this function.</p> |
| <p>On success, the function returns an ID that can be used to remove the action again with |
| <a href="fn.unregister.html" title="unregister"><code>unregister</code></a>.</p> |
| <h2 id="panics"><a href="#panics">Panics</a></h2> |
| <p>If the signal is one of (see <a href="constant.FORBIDDEN.html" title="FORBIDDEN"><code>FORBIDDEN</code></a>):</p> |
| <ul> |
| <li><code>SIGKILL</code></li> |
| <li><code>SIGSTOP</code></li> |
| <li><code>SIGILL</code></li> |
| <li><code>SIGFPE</code></li> |
| <li><code>SIGSEGV</code></li> |
| </ul> |
| <p>The first two are not possible to override (and the underlying C functions simply ignore all |
| requests to do so, which smells of possible bugs, or return errors). The rest can be set, but |
| generally needs very special handling to do so correctly (direct manipulation of the |
| application’s address space, <code>longjmp</code> and similar). Unless you know very well what you’re |
| doing, you’ll shoot yourself into the foot and this library won’t help you with that.</p> |
| <h2 id="errors"><a href="#errors">Errors</a></h2> |
| <p>Since the library manipulates signals using the low-level C functions, all these can return |
| errors. Generally, the errors mean something like the specified signal does not exist on the |
| given platform ‒ after a program is debugged and tested on a given OS, it should never return |
| an error.</p> |
| <p>However, if an error <em>is</em> returned, there are no guarantees if the given action was registered |
| or not.</p> |
| <h2 id="safety"><a href="#safety">Safety</a></h2> |
| <p>This function is unsafe, because the <code>action</code> is run inside a signal handler. The set of |
| functions allowed to be called from within is very limited (they are called async-signal-safe |
| functions by POSIX). These specifically do <em>not</em> contain mutexes and memory |
| allocation/deallocation. They <em>do</em> contain routines to terminate the program, to further |
| manipulate signals (by the low-level functions, not by this library) and to read and write file |
| descriptors. Calling program’s own functions consisting only of these is OK, as is manipulating |
| program’s variables ‒ however, as the action can be called on any thread that does not have the |
| given signal masked (by default no signal is masked on any thread), and mutexes are a no-go, |
| this is harder than it looks like at first.</p> |
| <p>As panicking from within a signal handler would be a panic across FFI boundary (which is |
| undefined behavior), the passed handler must not panic.</p> |
| <p>If you find these limitations hard to satisfy, choose from the helper functions in the |
| <a href="https://docs.rs/signal-hook">signal-hook</a> crate ‒ these provide safe interface to use some |
| common signal handling patters.</p> |
| <h2 id="race-condition"><a href="#race-condition">Race condition</a></h2> |
| <p>Upon registering the first hook for a given signal into this library, there’s a short race |
| condition under the following circumstances:</p> |
| <ul> |
| <li>The program already has a signal handler installed for this particular signal (through some |
| other library, possibly).</li> |
| <li>Concurrently, some other thread installs a different signal handler while it is being |
| installed by this library.</li> |
| <li>At the same time, the signal is delivered.</li> |
| </ul> |
| <p>Under such conditions signal-hook might wrongly “chain” to the older signal handler for a short |
| while (until the registration is fully complete).</p> |
| <p>Note that the exact conditions of the race condition might change in future versions of the |
| library. The recommended way to avoid it is to register signals before starting any additional |
| threads, or at least not to register signals concurrently.</p> |
| <p>Alternatively, make sure all signals are handled through this library.</p> |
| <h2 id="performance"><a href="#performance">Performance</a></h2> |
| <p>Even when it is possible to repeatedly install and remove actions during the lifetime of a |
| program, the installation and removal is considered a slow operation and should not be done |
| very often. Also, there’s limited (though huge) amount of distinct IDs (they are <code>u128</code>).</p> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">extern crate </span>signal_hook_registry; |
| |
| <span class="kw">use </span>std::io::Error; |
| <span class="kw">use </span>std::process; |
| |
| <span class="kw">fn </span>main() -> <span class="prelude-ty">Result</span><(), Error> { |
| <span class="kw">let </span>signal = <span class="kw">unsafe </span>{ |
| signal_hook_registry::register(signal_hook::consts::SIGTERM, || process::abort()) |
| }<span class="question-mark">?</span>; |
| <span class="comment">// Stuff here... |
| </span>signal_hook_registry::unregister(signal); <span class="comment">// Not really necessary. |
| </span><span class="prelude-val">Ok</span>(()) |
| }</code></pre></div> |
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