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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="Registers an arbitrary action for the given signal."><title>register in signal_hook_registry - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.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"href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="signal_hook_registry" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../static.files/storage-e2aeef58.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-eab170b8.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]--><rustdoc-topbar><h2><a href="#">register</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../signal_hook_registry/index.html">signal_<wbr>hook_<wbr>registry</a><span class="version">1.4.8</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">register</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#panics" title="Panics">Panics</a></li><li><a href="#errors" title="Errors">Errors</a></li><li><a href="#safety" title="Safety">Safety</a></li><li><a href="#race-condition" title="Race condition">Race condition</a></li><li><a href="#performance" title="Performance">Performance</a></li><li><a href="#examples" title="Examples">Examples</a></li></ul></section><div id="rustdoc-modnav"><h2 class="in-crate"><a href="index.html">In crate signal_<wbr>hook_<wbr>registry</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="index.html">signal_hook_registry</a></div><h1>Function <span class="fn">register</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/signal_hook_registry/lib.rs.html#574-579">Source</a> </span></div><pre class="rust item-decl"><code>pub unsafe fn register<F>(signal: <a class="type" href="../libc/primitives/type.c_int.html" title="type libc::primitives::c_int">c_int</a>, action: F) -> <a class="enum" href="https://doc.rust-lang.org/1.93.1/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/1.93.1/std/io/error/struct.Error.html" title="struct std::io::error::Error">Error</a>><div class="where">where
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F: <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/ops/function/trait.Fn.html" title="trait core::ops::function::Fn">Fn</a>() + <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + 'static,</div></code></pre><details class="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>
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<p>This makes sure there’s a signal handler for the given signal. It then adds the action to the
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ones called each time the signal is delivered. If multiple actions are set for the same signal,
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all are called, in the order of registration.</p>
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<p>If there was a previous signal handler for the given signal, it is chained ‒ it will be called
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as part of this library’s signal handler, before any actions set through this function.</p>
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<p>On success, the function returns an ID that can be used to remove the action again with
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<a href="fn.unregister.html" title="fn signal_hook_registry::unregister"><code>unregister</code></a>.</p>
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<h2 id="panics"><a class="doc-anchor" href="#panics">§</a>Panics</h2>
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<p>If the signal is one of (see <a href="constant.FORBIDDEN.html" title="constant signal_hook_registry::FORBIDDEN"><code>FORBIDDEN</code></a>):</p>
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<ul>
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<li><code>SIGKILL</code></li>
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<li><code>SIGSTOP</code></li>
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<li><code>SIGILL</code></li>
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<li><code>SIGFPE</code></li>
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<li><code>SIGSEGV</code></li>
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</ul>
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<p>The first two are not possible to override (and the underlying C functions simply ignore all
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requests to do so, which smells of possible bugs, or return errors). The rest can be set, but
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generally needs very special handling to do so correctly (direct manipulation of the
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application’s address space, <code>longjmp</code> and similar). Unless you know very well what you’re
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doing, you’ll shoot yourself into the foot and this library won’t help you with that.</p>
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<h2 id="errors"><a class="doc-anchor" href="#errors">§</a>Errors</h2>
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<p>Since the library manipulates signals using the low-level C functions, all these can return
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errors. Generally, the errors mean something like the specified signal does not exist on the
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given platform ‒ after a program is debugged and tested on a given OS, it should never return
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an error.</p>
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<p>However, if an error <em>is</em> returned, there are no guarantees if the given action was registered
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or not.</p>
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<h2 id="safety"><a class="doc-anchor" href="#safety">§</a>Safety</h2>
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<p>This function is unsafe, because the <code>action</code> is run inside a signal handler. While Rust is
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somewhat vague about the consequences of such, it is reasonably to assume that similar
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restrictions as specified in C or C++ apply.</p>
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<p>In particular:</p>
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<ul>
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<li>Calling any OS functions that are not async-signal-safe as specified as POSIX is not allowed.</li>
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<li>Accessing globals or thread-locals without synchronization is not allowed (however, mutexes
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are not within the async-signal-safe functions, therefore the synchronization is limited to
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using atomics).</li>
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</ul>
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<p>The underlying reason is, signals are asynchronous (they can happen at arbitrary time) and are
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run in context of arbitrary thread (with some limited control of at which thread they can run).
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As a consequence, things like mutexes are prone to deadlocks, memory allocators can likely
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contain mutexes and the compiler doesn’t expect the interruption during optimizations.</p>
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<p>Things that generally are part of the async-signal-safe set (though check specifically) are
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routines to terminate the program, to further manipulate signals (by the low-level functions,
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not by this library) and to read and write file descriptors. The async-signal-safety is
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transitive - that is, a function composed only from computations (with local variables or with
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variables accessed with proper synchronizations) and other async-signal-safe functions is also
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safe.</p>
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<p>As panicking from within a signal handler would be a panic across FFI boundary (which is
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undefined behavior), the passed handler must not panic.</p>
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<p>Note that many innocently-looking functions do contain some of the forbidden routines (a lot of
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things lock or allocate).</p>
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<p>If you find these limitations hard to satisfy, choose from the helper functions in the
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<a href="https://docs.rs/signal-hook">signal-hook</a> crate ‒ these provide safe interface to use some
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common signal handling patters.</p>
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<h2 id="race-condition"><a class="doc-anchor" href="#race-condition">§</a>Race condition</h2>
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<p>Upon registering the first hook for a given signal into this library, there’s a short race
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condition under the following circumstances:</p>
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<ul>
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<li>The program already has a signal handler installed for this particular signal (through some
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other library, possibly).</li>
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<li>Concurrently, some other thread installs a different signal handler while it is being
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installed by this library.</li>
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<li>At the same time, the signal is delivered.</li>
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</ul>
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<p>Under such conditions signal-hook might wrongly “chain” to the older signal handler for a short
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while (until the registration is fully complete).</p>
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<p>Note that the exact conditions of the race condition might change in future versions of the
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library. The recommended way to avoid it is to register signals before starting any additional
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threads, or at least not to register signals concurrently.</p>
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<p>Alternatively, make sure all signals are handled through this library.</p>
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<h2 id="performance"><a class="doc-anchor" href="#performance">§</a>Performance</h2>
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<p>Even when it is possible to repeatedly install and remove actions during the lifetime of a
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program, the installation and removal is considered a slow operation and should not be done
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very often. Also, there’s limited (though huge) amount of distinct IDs (they are <code>u128</code>).</p>
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<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">extern crate </span>signal_hook_registry;
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<span class="kw">use </span>std::io::Error;
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<span class="kw">use </span>std::process;
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<span class="kw">fn </span>main() -> <span class="prelude-ty">Result</span><(), Error> {
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<span class="kw">let </span>signal = <span class="kw">unsafe </span>{
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signal_hook_registry::register(signal_hook::consts::SIGTERM, || process::abort())
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}<span class="question-mark">?</span>;
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<span class="comment">// Stuff here...
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</span>signal_hook_registry::unregister(signal); <span class="comment">// Not really necessary.
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</span><span class="prelude-val">Ok</span>(())
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}</code></pre></div></div></details></section></div></main></body></html> |