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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="Counting semaphore performing asynchronous permit acquisition."><title>Semaphore in tokio::sync - 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="tokio" 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 struct"><!--[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="#">Semaphore</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../tokio/index.html">tokio</a><span class="version">1.49.0</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Semaphore</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#examples" title="Examples">Examples</a><ul><li><a href="#limit-the-number-of-simultaneously-opened-files-in-your-program" title="Limit the number of simultaneously opened files in your program">Limit the number of simultaneously opened files in your program</a></li><li><a href="#limit-the-number-of-outgoing-requests-being-sent-at-the-same-time" title="Limit the number of outgoing requests being sent at the same time">Limit the number of outgoing requests being sent at the same time</a></li><li><a href="#limit-the-number-of-incoming-requests-being-handled-at-the-same-time" title="Limit the number of incoming requests being handled at the same time">Limit the number of incoming requests being handled at the same time</a></li><li><a href="#prevent-tests-from-running-in-parallel" title="Prevent tests from running in parallel">Prevent tests from running in parallel</a></li><li><a href="#rate-limiting-using-a-token-bucket" title="Rate limiting using a token bucket">Rate limiting using a token bucket</a></li></ul></li></ul><h3><a href="#implementations">Associated Constants</a></h3><ul class="block associatedconstant"><li><a href="#associatedconstant.MAX_PERMITS" title="MAX_PERMITS">MAX_PERMITS</a></li></ul><h3><a href="#implementations">Methods</a></h3><ul class="block method"><li><a href="#method.acquire" title="acquire">acquire</a></li><li><a href="#method.acquire_many" title="acquire_many">acquire_many</a></li><li><a href="#method.acquire_many_owned" title="acquire_many_owned">acquire_many_owned</a></li><li><a href="#method.acquire_owned" title="acquire_owned">acquire_owned</a></li><li><a href="#method.add_permits" title="add_permits">add_permits</a></li><li><a href="#method.available_permits" title="available_permits">available_permits</a></li><li><a href="#method.close" title="close">close</a></li><li><a href="#method.const_new" title="const_new">const_new</a></li><li><a href="#method.forget_permits" title="forget_permits">forget_permits</a></li><li><a href="#method.is_closed" title="is_closed">is_closed</a></li><li><a href="#method.new" title="new">new</a></li><li><a href="#method.try_acquire" title="try_acquire">try_acquire</a></li><li><a href="#method.try_acquire_many" title="try_acquire_many">try_acquire_many</a></li><li><a href="#method.try_acquire_many_owned" title="try_acquire_many_owned">try_acquire_many_owned</a></li><li><a href="#method.try_acquire_owned" title="try_acquire_owned">try_acquire_owned</a></li></ul><h3><a href="#trait-implementations">Trait Implementations</a></h3><ul class="block trait-implementation"><li><a href="#impl-Debug-for-Semaphore" title="Debug">Debug</a></li></ul><h3><a href="#synthetic-implementations">Auto Trait Implementations</a></h3><ul class="block synthetic-implementation"><li><a href="#impl-Freeze-for-Semaphore" title="!Freeze">!Freeze</a></li><li><a href="#impl-RefUnwindSafe-for-Semaphore" title="!RefUnwindSafe">!RefUnwindSafe</a></li><li><a href="#impl-UnwindSafe-for-Semaphore" title="!UnwindSafe">!UnwindSafe</a></li><li><a href="#impl-Send-for-Semaphore" title="Send">Send</a></li><li><a href="#impl-Sync-for-Semaphore" title="Sync">Sync</a></li><li><a href="#impl-Unpin-for-Semaphore" title="Unpin">Unpin</a></li></ul><h3><a href="#blanket-implementations">Blanket Implementations</a></h3><ul class="block blanket-implementation"><li><a href="#impl-Any-for-T" title="Any">Any</a></li><li><a href="#impl-Borrow%3CT%3E-for-T" title="Borrow&#60;T&#62;">Borrow&#60;T&#62;</a></li><li><a href="#impl-BorrowMut%3CT%3E-for-T" title="BorrowMut&#60;T&#62;">BorrowMut&#60;T&#62;</a></li><li><a href="#impl-From%3CT%3E-for-T" title="From&#60;T&#62;">From&#60;T&#62;</a></li><li><a href="#impl-Into%3CU%3E-for-T" title="Into&#60;U&#62;">Into&#60;U&#62;</a></li><li><a href="#impl-TryFrom%3CU%3E-for-T" title="TryFrom&#60;U&#62;">TryFrom&#60;U&#62;</a></li><li><a href="#impl-TryInto%3CU%3E-for-T" title="TryInto&#60;U&#62;">TryInto&#60;U&#62;</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="index.html">In tokio::<wbr>sync</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">tokio</a>::<wbr><a href="index.html">sync</a></div><h1>Struct <span class="struct">Semaphore</span>&nbsp;<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/sync/semaphore.rs.html#398-403">Source</a> </span></div><pre class="rust item-decl"><code>pub struct Semaphore { <span class="comment">/* private fields */</span> }</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Counting semaphore performing asynchronous permit acquisition.</p>
<p>A semaphore maintains a set of permits. Permits are used to synchronize
access to a shared resource. A semaphore differs from a mutex in that it
can allow more than one concurrent caller to access the shared resource at a
time.</p>
<p>When <code>acquire</code> is called and the semaphore has remaining permits, the
function immediately returns a permit. However, if no remaining permits are
available, <code>acquire</code> (asynchronously) waits until an outstanding permit is
dropped. At this point, the freed permit is assigned to the caller.</p>
<p>This <code>Semaphore</code> is fair, which means that permits are given out in the order
they were requested. This fairness is also applied when <code>acquire_many</code> gets
involved, so if a call to <code>acquire_many</code> at the front of the queue requests
more permits than currently available, this can prevent a call to <code>acquire</code>
from completing, even if the semaphore has enough permits complete the call
to <code>acquire</code>.</p>
<p>To use the <code>Semaphore</code> in a poll function, you can use the <a href="https://docs.rs/tokio-util/latest/tokio_util/sync/struct.PollSemaphore.html"><code>PollSemaphore</code></a>
utility.</p>
<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
<p>Basic usage:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::{Semaphore, TryAcquireError};
<span class="kw">let </span>semaphore = Semaphore::new(<span class="number">3</span>);
<span class="kw">let </span>a_permit = semaphore.acquire().<span class="kw">await</span>.unwrap();
<span class="kw">let </span>two_permits = semaphore.acquire_many(<span class="number">2</span>).<span class="kw">await</span>.unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">0</span>);
<span class="kw">let </span>permit_attempt = semaphore.try_acquire();
<span class="macro">assert_eq!</span>(permit_attempt.err(), <span class="prelude-val">Some</span>(TryAcquireError::NoPermits));</code></pre></div><h3 id="limit-the-number-of-simultaneously-opened-files-in-your-program"><a class="doc-anchor" href="#limit-the-number-of-simultaneously-opened-files-in-your-program">§</a>Limit the number of simultaneously opened files in your program</h3>
<p>Most operating systems have limits on the number of open file
handles. Even in systems without explicit limits, resource constraints
implicitly set an upper bound on the number of open files. If your
program attempts to open a large number of files and exceeds this
limit, it will result in an error.</p>
<p>This example uses a Semaphore with 100 permits. By acquiring a permit from
the Semaphore before accessing a file, you ensure that your program opens
no more than 100 files at a time. When trying to open the 101st
file, the program will wait until a permit becomes available before
proceeding to open another file.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::io::Result;
<span class="kw">use </span>tokio::fs::File;
<span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">use </span>tokio::io::AsyncWriteExt;
<span class="kw">static </span>PERMITS: Semaphore = Semaphore::const_new(<span class="number">100</span>);
<span class="kw">async fn </span>write_to_file(message: <span class="kw-2">&amp;</span>[u8]) -&gt; <span class="prelude-ty">Result</span>&lt;()&gt; {
<span class="kw">let </span>_permit = PERMITS.acquire().<span class="kw">await</span>.unwrap();
<span class="kw">let </span><span class="kw-2">mut </span>buffer = File::create(<span class="string">"example.txt"</span>).<span class="kw">await</span><span class="question-mark">?</span>;
buffer.write_all(message).<span class="kw">await</span><span class="question-mark">?</span>;
<span class="prelude-val">Ok</span>(()) <span class="comment">// Permit goes out of scope here, and is available again for acquisition
</span>}</code></pre></div><h3 id="limit-the-number-of-outgoing-requests-being-sent-at-the-same-time"><a class="doc-anchor" href="#limit-the-number-of-outgoing-requests-being-sent-at-the-same-time">§</a>Limit the number of outgoing requests being sent at the same time</h3>
<p>In some scenarios, it might be required to limit the number of outgoing
requests being sent in parallel. This could be due to limits of a consumed
API or the network resources of the system the application is running on.</p>
<p>This example uses an <code>Arc&lt;Semaphore&gt;</code> with 10 permits. Each task spawned is
given a reference to the semaphore by cloning the <code>Arc&lt;Semaphore&gt;</code>. Before
a task sends a request, it must acquire a permit from the semaphore by
calling <a href="struct.Semaphore.html#method.acquire" title="method tokio::sync::Semaphore::acquire"><code>Semaphore::acquire</code></a>. This ensures that at most 10 requests are
sent in parallel at any given time. After a task has sent a request, it
drops the permit to allow other tasks to send requests.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::Semaphore;
<span class="comment">// Define maximum number of parallel requests.
</span><span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">5</span>));
<span class="comment">// Spawn many tasks that will send requests.
</span><span class="kw">let </span><span class="kw-2">mut </span>jhs = Vec::new();
<span class="kw">for </span>task_id <span class="kw">in </span><span class="number">0</span>..<span class="number">50 </span>{
<span class="kw">let </span>semaphore = semaphore.clone();
<span class="kw">let </span>jh = tokio::spawn(<span class="kw">async move </span>{
<span class="comment">// Acquire permit before sending request.
</span><span class="kw">let </span>_permit = semaphore.acquire().<span class="kw">await</span>.unwrap();
<span class="comment">// Send the request.
</span><span class="kw">let </span>response = send_request(task_id).<span class="kw">await</span>;
<span class="comment">// Drop the permit after the request has been sent.
</span>drop(_permit);
<span class="comment">// Handle response.
// ...
</span>response
});
jhs.push(jh);
}
<span class="comment">// Collect responses from tasks.
</span><span class="kw">let </span><span class="kw-2">mut </span>responses = Vec::new();
<span class="kw">for </span>jh <span class="kw">in </span>jhs {
<span class="kw">let </span>response = jh.<span class="kw">await</span>.unwrap();
responses.push(response);
}
<span class="comment">// Process responses.
// ...</span></code></pre></div><h3 id="limit-the-number-of-incoming-requests-being-handled-at-the-same-time"><a class="doc-anchor" href="#limit-the-number-of-incoming-requests-being-handled-at-the-same-time">§</a>Limit the number of incoming requests being handled at the same time</h3>
<p>Similar to limiting the number of simultaneously opened files, network handles
are a limited resource. Allowing an unbounded amount of requests to be processed
could result in a denial-of-service, among many other issues.</p>
<p>This example uses an <code>Arc&lt;Semaphore&gt;</code> instead of a global variable.
To limit the number of requests that can be processed at the time,
we acquire a permit for each task before spawning it. Once acquired,
a new task is spawned; and once finished, the permit is dropped inside
of the task to allow others to spawn. Permits must be acquired via
<a href="struct.Semaphore.html#method.acquire_owned" title="method tokio::sync::Semaphore::acquire_owned"><code>Semaphore::acquire_owned</code></a> to be movable across the task boundary.
(Since our semaphore is not a global variable — if it was, then <code>acquire</code> would be enough.)</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">use </span>tokio::net::TcpListener;
<span class="attr">#[tokio::main]
</span><span class="kw">async fn </span>main() -&gt; std::io::Result&lt;()&gt; {
<span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">3</span>));
<span class="kw">let </span>listener = TcpListener::bind(<span class="string">"127.0.0.1:8080"</span>).<span class="kw">await</span><span class="question-mark">?</span>;
<span class="kw">loop </span>{
<span class="comment">// Acquire permit before accepting the next socket.
//
// We use `acquire_owned` so that we can move `permit` into
// other tasks.
</span><span class="kw">let </span>permit = semaphore.clone().acquire_owned().<span class="kw">await</span>.unwrap();
<span class="kw">let </span>(<span class="kw-2">mut </span>socket, <span class="kw">_</span>) = listener.accept().<span class="kw">await</span><span class="question-mark">?</span>;
tokio::spawn(<span class="kw">async move </span>{
<span class="comment">// Do work using the socket.
</span>handle_connection(<span class="kw-2">&amp;mut </span>socket).<span class="kw">await</span>;
<span class="comment">// Drop socket while the permit is still live.
</span>drop(socket);
<span class="comment">// Drop the permit, so more tasks can be created.
</span>drop(permit);
});
}
}</code></pre></div><h3 id="prevent-tests-from-running-in-parallel"><a class="doc-anchor" href="#prevent-tests-from-running-in-parallel">§</a>Prevent tests from running in parallel</h3>
<p>By default, Rust runs tests in the same file in parallel. However, in some
cases, running two tests in parallel may lead to problems. For example, this
can happen when tests use the same database.</p>
<p>Consider the following scenario:</p>
<ol>
<li><code>test_insert</code>: Inserts a key-value pair into the database, then retrieves
the value using the same key to verify the insertion.</li>
<li><code>test_update</code>: Inserts a key, then updates the key to a new value and
verifies that the value has been accurately updated.</li>
<li><code>test_others</code>: A third test that doesn’t modify the database state. It
can run in parallel with the other tests.</li>
</ol>
<p>In this example, <code>test_insert</code> and <code>test_update</code> need to run in sequence to
work, but it doesn’t matter which test runs first. We can leverage a
semaphore with a single permit to address this challenge.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::Semaphore;
<span class="comment">// Initialize a static semaphore with only one permit, which is used to
// prevent test_insert and test_update from running in parallel.
</span><span class="kw">static </span>PERMIT: Semaphore = Semaphore::const_new(<span class="number">1</span>);
<span class="comment">// Initialize the database that will be used by the subsequent tests.
</span><span class="kw">static </span>DB: Database = Database::setup();
<span class="attr">#[tokio::test]
</span><span class="kw">async fn </span>test_insert() {
<span class="comment">// Acquire permit before proceeding. Since the semaphore has only one permit,
// the test will wait if the permit is already acquired by other tests.
</span><span class="kw">let </span>permit = PERMIT.acquire().<span class="kw">await</span>.unwrap();
<span class="comment">// Do the actual test stuff with database
// Insert a key-value pair to database
</span><span class="kw">let </span>(key, value) = (<span class="string">"name"</span>, <span class="number">0</span>);
DB.insert(key, value).<span class="kw">await</span>;
<span class="comment">// Verify that the value has been inserted correctly.
</span><span class="macro">assert_eq!</span>(DB.get(key).<span class="kw">await</span>, value);
<span class="comment">// Undo the insertion, so the database is empty at the end of the test.
</span>DB.delete(key).<span class="kw">await</span>;
<span class="comment">// Drop permit. This allows the other test to start running.
</span>drop(permit);
}
<span class="attr">#[tokio::test]
</span><span class="kw">async fn </span>test_update() {
<span class="comment">// Acquire permit before proceeding. Since the semaphore has only one permit,
// the test will wait if the permit is already acquired by other tests.
</span><span class="kw">let </span>permit = PERMIT.acquire().<span class="kw">await</span>.unwrap();
<span class="comment">// Do the same insert.
</span><span class="kw">let </span>(key, value) = (<span class="string">"name"</span>, <span class="number">0</span>);
DB.insert(key, value).<span class="kw">await</span>;
<span class="comment">// Update the existing value with a new one.
</span><span class="kw">let </span>new_value = <span class="number">1</span>;
DB.update(key, new_value).<span class="kw">await</span>;
<span class="comment">// Verify that the value has been updated correctly.
</span><span class="macro">assert_eq!</span>(DB.get(key).<span class="kw">await</span>, new_value);
<span class="comment">// Undo any modificattion.
</span>DB.delete(key).<span class="kw">await</span>;
<span class="comment">// Drop permit. This allows the other test to start running.
</span>drop(permit);
}
<span class="attr">#[tokio::test]
</span><span class="kw">async fn </span>test_others() {
<span class="comment">// This test can run in parallel with test_insert and test_update,
// so it does not use PERMIT.
</span>}</code></pre></div><h3 id="rate-limiting-using-a-token-bucket"><a class="doc-anchor" href="#rate-limiting-using-a-token-bucket">§</a>Rate limiting using a token bucket</h3>
<p>This example showcases the <a href="struct.Semaphore.html#method.add_permits" title="method tokio::sync::Semaphore::add_permits"><code>add_permits</code></a> and <a href="struct.SemaphorePermit.html#method.forget" title="method tokio::sync::SemaphorePermit::forget"><code>SemaphorePermit::forget</code></a> methods.</p>
<p>Many applications and systems have constraints on the rate at which certain
operations should occur. Exceeding this rate can result in suboptimal
performance or even errors.</p>
<p>This example implements rate limiting using a <a href="https://en.wikipedia.org/wiki/Token_bucket">token bucket</a>. A token bucket is a form of rate
limiting that doesn’t kick in immediately, to allow for short bursts of incoming requests that
arrive at the same time.</p>
<p>With a token bucket, each incoming request consumes a token, and the tokens are refilled at a
certain rate that defines the rate limit. When a burst of requests arrives, tokens are
immediately given out until the bucket is empty. Once the bucket is empty, requests will have to
wait for new tokens to be added.</p>
<p>Unlike the example that limits how many requests can be handled at the same time, we do not add
tokens back when we finish handling a request. Instead, tokens are added only by a timer task.</p>
<p>Note that this implementation is suboptimal when the duration is small, because it consumes a
lot of cpu constantly looping and sleeping.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">use </span>tokio::time::{interval, Duration};
<span class="kw">struct </span>TokenBucket {
sem: Arc&lt;Semaphore&gt;,
jh: tokio::task::JoinHandle&lt;()&gt;,
}
<span class="kw">impl </span>TokenBucket {
<span class="kw">fn </span>new(duration: Duration, capacity: usize) -&gt; <span class="self">Self </span>{
<span class="kw">let </span>sem = Arc::new(Semaphore::new(capacity));
<span class="comment">// refills the tokens at the end of each interval
</span><span class="kw">let </span>jh = tokio::spawn({
<span class="kw">let </span>sem = sem.clone();
<span class="kw">let </span><span class="kw-2">mut </span>interval = interval(duration);
interval.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
<span class="kw">async move </span>{
<span class="kw">loop </span>{
interval.tick().<span class="kw">await</span>;
<span class="kw">if </span>sem.available_permits() &lt; capacity {
sem.add_permits(<span class="number">1</span>);
}
}
}
});
<span class="self">Self </span>{ jh, sem }
}
<span class="kw">async fn </span>acquire(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="comment">// This can return an error if the semaphore is closed, but we
// never close it, so this error can never happen.
</span><span class="kw">let </span>permit = <span class="self">self</span>.sem.acquire().<span class="kw">await</span>.unwrap();
<span class="comment">// To avoid releasing the permit back to the semaphore, we use
// the `SemaphorePermit::forget` method.
</span>permit.forget();
}
}
<span class="kw">impl </span>Drop <span class="kw">for </span>TokenBucket {
<span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
<span class="comment">// Kill the background task so it stops taking up resources when we
// don't need it anymore.
</span><span class="self">self</span>.jh.abort();
}
}
<span class="kw">let </span>capacity = <span class="number">5</span>;
<span class="kw">let </span>update_interval = Duration::from_secs_f32(<span class="number">1.0 </span>/ capacity <span class="kw">as </span>f32);
<span class="kw">let </span>bucket = TokenBucket::new(update_interval, capacity);
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">5 </span>{
bucket.acquire().<span class="kw">await</span>;
<span class="comment">// do the operation
</span>}</code></pre></div></div></details><h2 id="implementations" class="section-header">Implementations<a href="#implementations" class="anchor">§</a></h2><div id="implementations-list"><details class="toggle implementors-toggle" open><summary><section id="impl-Semaphore" class="impl"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#446-971">Source</a><a href="#impl-Semaphore" class="anchor">§</a><h3 class="code-header">impl <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section></summary><div class="impl-items"><details class="toggle" open><summary><section id="associatedconstant.MAX_PERMITS" class="associatedconstant"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#450">Source</a><h4 class="code-header">pub const <a href="#associatedconstant.MAX_PERMITS" class="constant">MAX_PERMITS</a>: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a> = super::batch_semaphore::Semaphore::MAX_PERMITS</h4></section></summary><div class="docblock"><p>The maximum number of permits which a semaphore can hold. It is <code>usize::MAX &gt;&gt; 3</code>.</p>
<p>Exceeding this limit typically results in a panic.</p>
</div></details><details class="toggle method-toggle" open><summary><section id="method.new" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#456-484">Source</a><h4 class="code-header">pub fn <a href="#method.new" class="fn">new</a>(permits: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>) -&gt; Self</h4></section></summary><div class="docblock"><p>Creates a new semaphore with the initial number of permits.</p>
<p>Panics if <code>permits</code> exceeds <a href="struct.Semaphore.html#associatedconstant.MAX_PERMITS" title="associated constant tokio::sync::Semaphore::MAX_PERMITS"><code>Semaphore::MAX_PERMITS</code></a>.</p>
</div></details><details class="toggle method-toggle" open><summary><section id="method.const_new" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#504-510">Source</a><h4 class="code-header">pub const fn <a href="#method.const_new" class="fn">const_new</a>(permits: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>) -&gt; Self</h4></section></summary><div class="docblock"><p>Creates a new semaphore with the initial number of permits.</p>
<p>When using the <code>tracing</code> <a href="../index.html#unstable-features" title="mod tokio">unstable feature</a>, a <code>Semaphore</code> created with
<code>const_new</code> will not be instrumented. As such, it will not be visible
in <a href="https://github.com/tokio-rs/console"><code>tokio-console</code></a>. Instead, <a href="struct.Semaphore.html#method.new" title="associated function tokio::sync::Semaphore::new"><code>Semaphore::new</code></a> should be used to
create an instrumented object if that is needed.</p>
<h5 id="examples-1"><a class="doc-anchor" href="#examples-1">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">static </span>SEM: Semaphore = Semaphore::const_new(<span class="number">10</span>);</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.available_permits" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#532-534">Source</a><h4 class="code-header">pub fn <a href="#method.available_permits" class="fn">available_permits</a>(&amp;self) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a></h4></section></summary><div class="docblock"><p>Returns the current number of available permits.</p>
</div></details><details class="toggle method-toggle" open><summary><section id="method.add_permits" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#539-541">Source</a><h4 class="code-header">pub fn <a href="#method.add_permits" class="fn">add_permits</a>(&amp;self, n: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>)</h4></section></summary><div class="docblock"><p>Adds <code>n</code> new permits to the semaphore.</p>
<p>The maximum number of permits is <a href="struct.Semaphore.html#associatedconstant.MAX_PERMITS" title="associated constant tokio::sync::Semaphore::MAX_PERMITS"><code>Semaphore::MAX_PERMITS</code></a>, and this function will panic if the limit is exceeded.</p>
</div></details><details class="toggle method-toggle" open><summary><section id="method.forget_permits" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#547-549">Source</a><h4 class="code-header">pub fn <a href="#method.forget_permits" class="fn">forget_permits</a>(&amp;self, n: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a>) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.usize.html">usize</a></h4></section></summary><div class="docblock"><p>Decrease a semaphore’s permits by a maximum of <code>n</code>.</p>
<p>If there are insufficient permits and it’s not possible to reduce by <code>n</code>,
return the number of permits that were actually reduced.</p>
</div></details><details class="toggle method-toggle" open><summary><section id="method.acquire" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#585-602">Source</a><h4 class="code-header">pub async fn <a href="#method.acquire" class="fn">acquire</a>(&amp;self) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit">SemaphorePermit</a>&lt;'_&gt;, <a class="struct" href="struct.AcquireError.html" title="struct tokio::sync::AcquireError">AcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Acquires a permit from the semaphore.</p>
<p>If the semaphore has been closed, this returns an <a href="struct.AcquireError.html" title="struct tokio::sync::AcquireError"><code>AcquireError</code></a>.
Otherwise, this returns a <a href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit"><code>SemaphorePermit</code></a> representing the
acquired permit.</p>
<h5 id="cancel-safety"><a class="doc-anchor" href="#cancel-safety">§</a>Cancel safety</h5>
<p>This method uses a queue to fairly distribute permits in the order they
were requested. Cancelling a call to <code>acquire</code> makes you lose your place
in the queue.</p>
<h5 id="examples-2"><a class="doc-anchor" href="#examples-2">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">let </span>semaphore = Semaphore::new(<span class="number">2</span>);
<span class="kw">let </span>permit_1 = semaphore.acquire().<span class="kw">await</span>.unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">1</span>);
<span class="kw">let </span>permit_2 = semaphore.acquire().<span class="kw">await</span>.unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">0</span>);
drop(permit_1);
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">1</span>);</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.acquire_many" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#632-650">Source</a><h4 class="code-header">pub async fn <a href="#method.acquire_many" class="fn">acquire_many</a>(
&amp;self,
n: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.u32.html">u32</a>,
) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit">SemaphorePermit</a>&lt;'_&gt;, <a class="struct" href="struct.AcquireError.html" title="struct tokio::sync::AcquireError">AcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Acquires <code>n</code> permits from the semaphore.</p>
<p>If the semaphore has been closed, this returns an <a href="struct.AcquireError.html" title="struct tokio::sync::AcquireError"><code>AcquireError</code></a>.
Otherwise, this returns a <a href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit"><code>SemaphorePermit</code></a> representing the
acquired permits.</p>
<h5 id="cancel-safety-1"><a class="doc-anchor" href="#cancel-safety-1">§</a>Cancel safety</h5>
<p>This method uses a queue to fairly distribute permits in the order they
were requested. Cancelling a call to <code>acquire_many</code> makes you lose your
place in the queue.</p>
<h5 id="examples-3"><a class="doc-anchor" href="#examples-3">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">let </span>semaphore = Semaphore::new(<span class="number">5</span>);
<span class="kw">let </span>permit = semaphore.acquire_many(<span class="number">3</span>).<span class="kw">await</span>.unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">2</span>);</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.try_acquire" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#680-688">Source</a><h4 class="code-header">pub fn <a href="#method.try_acquire" class="fn">try_acquire</a>(&amp;self) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit">SemaphorePermit</a>&lt;'_&gt;, <a class="enum" href="enum.TryAcquireError.html" title="enum tokio::sync::TryAcquireError">TryAcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Tries to acquire a permit from the semaphore.</p>
<p>If the semaphore has been closed, this returns a <a href="enum.TryAcquireError.html#variant.Closed" title="variant tokio::sync::TryAcquireError::Closed"><code>TryAcquireError::Closed</code></a>
and a <a href="enum.TryAcquireError.html#variant.NoPermits" title="variant tokio::sync::TryAcquireError::NoPermits"><code>TryAcquireError::NoPermits</code></a> if there are no permits left. Otherwise,
this returns a <a href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit"><code>SemaphorePermit</code></a> representing the acquired permits.</p>
<h5 id="examples-4"><a class="doc-anchor" href="#examples-4">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::{Semaphore, TryAcquireError};
<span class="kw">let </span>semaphore = Semaphore::new(<span class="number">2</span>);
<span class="kw">let </span>permit_1 = semaphore.try_acquire().unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">1</span>);
<span class="kw">let </span>permit_2 = semaphore.try_acquire().unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">0</span>);
<span class="kw">let </span>permit_3 = semaphore.try_acquire();
<span class="macro">assert_eq!</span>(permit_3.err(), <span class="prelude-val">Some</span>(TryAcquireError::NoPermits));</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.try_acquire_many" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#715-723">Source</a><h4 class="code-header">pub fn <a href="#method.try_acquire_many" class="fn">try_acquire_many</a>(
&amp;self,
n: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.u32.html">u32</a>,
) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit">SemaphorePermit</a>&lt;'_&gt;, <a class="enum" href="enum.TryAcquireError.html" title="enum tokio::sync::TryAcquireError">TryAcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Tries to acquire <code>n</code> permits from the semaphore.</p>
<p>If the semaphore has been closed, this returns a <a href="enum.TryAcquireError.html#variant.Closed" title="variant tokio::sync::TryAcquireError::Closed"><code>TryAcquireError::Closed</code></a>
and a <a href="enum.TryAcquireError.html#variant.NoPermits" title="variant tokio::sync::TryAcquireError::NoPermits"><code>TryAcquireError::NoPermits</code></a> if there are not enough permits left.
Otherwise, this returns a <a href="struct.SemaphorePermit.html" title="struct tokio::sync::SemaphorePermit"><code>SemaphorePermit</code></a> representing the acquired permits.</p>
<h5 id="examples-5"><a class="doc-anchor" href="#examples-5">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::{Semaphore, TryAcquireError};
<span class="kw">let </span>semaphore = Semaphore::new(<span class="number">4</span>);
<span class="kw">let </span>permit_1 = semaphore.try_acquire_many(<span class="number">3</span>).unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">1</span>);
<span class="kw">let </span>permit_2 = semaphore.try_acquire_many(<span class="number">2</span>);
<span class="macro">assert_eq!</span>(permit_2.err(), <span class="prelude-val">Some</span>(TryAcquireError::NoPermits));</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.acquire_owned" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#767-784">Source</a><h4 class="code-header">pub async fn <a href="#method.acquire_owned" class="fn">acquire_owned</a>(
self: <a class="struct" href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc">Arc</a>&lt;Self&gt;,
) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit">OwnedSemaphorePermit</a>, <a class="struct" href="struct.AcquireError.html" title="struct tokio::sync::AcquireError">AcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Acquires a permit from the semaphore.</p>
<p>The semaphore must be wrapped in an <a href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc"><code>Arc</code></a> to call this method.
If the semaphore has been closed, this returns an <a href="struct.AcquireError.html" title="struct tokio::sync::AcquireError"><code>AcquireError</code></a>.
Otherwise, this returns a <a href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit"><code>OwnedSemaphorePermit</code></a> representing the
acquired permit.</p>
<h5 id="cancel-safety-2"><a class="doc-anchor" href="#cancel-safety-2">§</a>Cancel safety</h5>
<p>This method uses a queue to fairly distribute permits in the order they
were requested. Cancelling a call to <code>acquire_owned</code> makes you lose your
place in the queue.</p>
<h5 id="examples-6"><a class="doc-anchor" href="#examples-6">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">3</span>));
<span class="kw">let </span><span class="kw-2">mut </span>join_handles = Vec::new();
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">5 </span>{
<span class="kw">let </span>permit = semaphore.clone().acquire_owned().<span class="kw">await</span>.unwrap();
join_handles.push(tokio::spawn(<span class="kw">async move </span>{
<span class="comment">// perform task...
// explicitly own `permit` in the task
</span>drop(permit);
}));
}
<span class="kw">for </span>handle <span class="kw">in </span>join_handles {
handle.<span class="kw">await</span>.unwrap();
}</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.acquire_many_owned" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#828-848">Source</a><h4 class="code-header">pub async fn <a href="#method.acquire_many_owned" class="fn">acquire_many_owned</a>(
self: <a class="struct" href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc">Arc</a>&lt;Self&gt;,
n: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.u32.html">u32</a>,
) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit">OwnedSemaphorePermit</a>, <a class="struct" href="struct.AcquireError.html" title="struct tokio::sync::AcquireError">AcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Acquires <code>n</code> permits from the semaphore.</p>
<p>The semaphore must be wrapped in an <a href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc"><code>Arc</code></a> to call this method.
If the semaphore has been closed, this returns an <a href="struct.AcquireError.html" title="struct tokio::sync::AcquireError"><code>AcquireError</code></a>.
Otherwise, this returns a <a href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit"><code>OwnedSemaphorePermit</code></a> representing the
acquired permit.</p>
<h5 id="cancel-safety-3"><a class="doc-anchor" href="#cancel-safety-3">§</a>Cancel safety</h5>
<p>This method uses a queue to fairly distribute permits in the order they
were requested. Cancelling a call to <code>acquire_many_owned</code> makes you lose
your place in the queue.</p>
<h5 id="examples-7"><a class="doc-anchor" href="#examples-7">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">10</span>));
<span class="kw">let </span><span class="kw-2">mut </span>join_handles = Vec::new();
<span class="kw">for _ in </span><span class="number">0</span>..<span class="number">5 </span>{
<span class="kw">let </span>permit = semaphore.clone().acquire_many_owned(<span class="number">2</span>).<span class="kw">await</span>.unwrap();
join_handles.push(tokio::spawn(<span class="kw">async move </span>{
<span class="comment">// perform task...
// explicitly own `permit` in the task
</span>drop(permit);
}));
}
<span class="kw">for </span>handle <span class="kw">in </span>join_handles {
handle.<span class="kw">await</span>.unwrap();
}</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.try_acquire_owned" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#882-890">Source</a><h4 class="code-header">pub fn <a href="#method.try_acquire_owned" class="fn">try_acquire_owned</a>(
self: <a class="struct" href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc">Arc</a>&lt;Self&gt;,
) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit">OwnedSemaphorePermit</a>, <a class="enum" href="enum.TryAcquireError.html" title="enum tokio::sync::TryAcquireError">TryAcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Tries to acquire a permit from the semaphore.</p>
<p>The semaphore must be wrapped in an <a href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc"><code>Arc</code></a> to call this method. If
the semaphore has been closed, this returns a <a href="enum.TryAcquireError.html#variant.Closed" title="variant tokio::sync::TryAcquireError::Closed"><code>TryAcquireError::Closed</code></a>
and a <a href="enum.TryAcquireError.html#variant.NoPermits" title="variant tokio::sync::TryAcquireError::NoPermits"><code>TryAcquireError::NoPermits</code></a> if there are no permits left.
Otherwise, this returns a <a href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit"><code>OwnedSemaphorePermit</code></a> representing the
acquired permit.</p>
<h5 id="examples-8"><a class="doc-anchor" href="#examples-8">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::{Semaphore, TryAcquireError};
<span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">2</span>));
<span class="kw">let </span>permit_1 = Arc::clone(<span class="kw-2">&amp;</span>semaphore).try_acquire_owned().unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">1</span>);
<span class="kw">let </span>permit_2 = Arc::clone(<span class="kw-2">&amp;</span>semaphore).try_acquire_owned().unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">0</span>);
<span class="kw">let </span>permit_3 = semaphore.try_acquire_owned();
<span class="macro">assert_eq!</span>(permit_3.err(), <span class="prelude-val">Some</span>(TryAcquireError::NoPermits));</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.try_acquire_many_owned" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#921-932">Source</a><h4 class="code-header">pub fn <a href="#method.try_acquire_many_owned" class="fn">try_acquire_many_owned</a>(
self: <a class="struct" href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc">Arc</a>&lt;Self&gt;,
n: <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.u32.html">u32</a>,
) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit">OwnedSemaphorePermit</a>, <a class="enum" href="enum.TryAcquireError.html" title="enum tokio::sync::TryAcquireError">TryAcquireError</a>&gt;</h4></section></summary><div class="docblock"><p>Tries to acquire <code>n</code> permits from the semaphore.</p>
<p>The semaphore must be wrapped in an <a href="https://doc.rust-lang.org/1.93.1/alloc/sync/struct.Arc.html" title="struct alloc::sync::Arc"><code>Arc</code></a> to call this method. If
the semaphore has been closed, this returns a <a href="enum.TryAcquireError.html#variant.Closed" title="variant tokio::sync::TryAcquireError::Closed"><code>TryAcquireError::Closed</code></a>
and a <a href="enum.TryAcquireError.html#variant.NoPermits" title="variant tokio::sync::TryAcquireError::NoPermits"><code>TryAcquireError::NoPermits</code></a> if there are no permits left.
Otherwise, this returns a <a href="struct.OwnedSemaphorePermit.html" title="struct tokio::sync::OwnedSemaphorePermit"><code>OwnedSemaphorePermit</code></a> representing the
acquired permit.</p>
<h5 id="examples-9"><a class="doc-anchor" href="#examples-9">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::{Semaphore, TryAcquireError};
<span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">4</span>));
<span class="kw">let </span>permit_1 = Arc::clone(<span class="kw-2">&amp;</span>semaphore).try_acquire_many_owned(<span class="number">3</span>).unwrap();
<span class="macro">assert_eq!</span>(semaphore.available_permits(), <span class="number">1</span>);
<span class="kw">let </span>permit_2 = semaphore.try_acquire_many_owned(<span class="number">2</span>);
<span class="macro">assert_eq!</span>(permit_2.err(), <span class="prelude-val">Some</span>(TryAcquireError::NoPermits));</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.close" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#963-965">Source</a><h4 class="code-header">pub fn <a href="#method.close" class="fn">close</a>(&amp;self)</h4></section></summary><div class="docblock"><p>Closes the semaphore.</p>
<p>This prevents the semaphore from issuing new permits and notifies all pending waiters.</p>
<h5 id="examples-10"><a class="doc-anchor" href="#examples-10">§</a>Examples</h5>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>tokio::sync::Semaphore;
<span class="kw">use </span>std::sync::Arc;
<span class="kw">use </span>tokio::sync::TryAcquireError;
<span class="kw">let </span>semaphore = Arc::new(Semaphore::new(<span class="number">1</span>));
<span class="kw">let </span>semaphore2 = semaphore.clone();
tokio::spawn(<span class="kw">async move </span>{
<span class="kw">let </span>permit = semaphore.acquire_many(<span class="number">2</span>).<span class="kw">await</span>;
<span class="macro">assert!</span>(permit.is_err());
<span class="macro">println!</span>(<span class="string">"waiter received error"</span>);
});
<span class="macro">println!</span>(<span class="string">"closing semaphore"</span>);
semaphore2.close();
<span class="comment">// Cannot obtain more permits
</span><span class="macro">assert_eq!</span>(semaphore2.try_acquire().err(), <span class="prelude-val">Some</span>(TryAcquireError::Closed))</code></pre></div></div></details><details class="toggle method-toggle" open><summary><section id="method.is_closed" class="method"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#968-970">Source</a><h4 class="code-header">pub fn <a href="#method.is_closed" class="fn">is_closed</a>(&amp;self) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.bool.html">bool</a></h4></section></summary><div class="docblock"><p>Returns true if the semaphore is closed</p>
</div></details></div></details></div><h2 id="trait-implementations" class="section-header">Trait Implementations<a href="#trait-implementations" class="anchor">§</a></h2><div id="trait-implementations-list"><details class="toggle implementors-toggle" open><summary><section id="impl-Debug-for-Semaphore" class="impl"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#397">Source</a><a href="#impl-Debug-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.fmt" class="method trait-impl"><a class="src rightside" href="../../src/tokio/sync/semaphore.rs.html#397">Source</a><a href="#method.fmt" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/fmt/trait.Debug.html#tymethod.fmt" class="fn">fmt</a>(&amp;self, f: &amp;mut <a class="struct" href="https://doc.rust-lang.org/1.93.1/core/fmt/struct.Formatter.html" title="struct core::fmt::Formatter">Formatter</a>&lt;'_&gt;) -&gt; <a class="type" href="https://doc.rust-lang.org/1.93.1/core/fmt/type.Result.html" title="type core::fmt::Result">Result</a></h4></section></summary><div class='docblock'>Formats the value using the given formatter. <a href="https://doc.rust-lang.org/1.93.1/core/fmt/trait.Debug.html#tymethod.fmt">Read more</a></div></details></div></details></div><h2 id="synthetic-implementations" class="section-header">Auto Trait Implementations<a href="#synthetic-implementations" class="anchor">§</a></h2><div id="synthetic-implementations-list"><section id="impl-Freeze-for-Semaphore" class="impl"><a href="#impl-Freeze-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl !<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Freeze.html" title="trait core::marker::Freeze">Freeze</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section><section id="impl-RefUnwindSafe-for-Semaphore" class="impl"><a href="#impl-RefUnwindSafe-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl !<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/panic/unwind_safe/trait.RefUnwindSafe.html" title="trait core::panic::unwind_safe::RefUnwindSafe">RefUnwindSafe</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section><section id="impl-Send-for-Semaphore" class="impl"><a href="#impl-Send-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section><section id="impl-Sync-for-Semaphore" class="impl"><a href="#impl-Sync-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section><section id="impl-Unpin-for-Semaphore" class="impl"><a href="#impl-Unpin-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section><section id="impl-UnwindSafe-for-Semaphore" class="impl"><a href="#impl-UnwindSafe-for-Semaphore" class="anchor">§</a><h3 class="code-header">impl !<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/panic/unwind_safe/trait.UnwindSafe.html" title="trait core::panic::unwind_safe::UnwindSafe">UnwindSafe</a> for <a class="struct" href="struct.Semaphore.html" title="struct tokio::sync::Semaphore">Semaphore</a></h3></section></div><h2 id="blanket-implementations" class="section-header">Blanket Implementations<a href="#blanket-implementations" class="anchor">§</a></h2><div id="blanket-implementations-list"><details class="toggle implementors-toggle"><summary><section id="impl-Any-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/any.rs.html#138">Source</a><a href="#impl-Any-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/any/trait.Any.html" title="trait core::any::Any">Any</a> for T<div class="where">where
T: 'static + ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.type_id" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/any.rs.html#139">Source</a><a href="#method.type_id" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/any/trait.Any.html#tymethod.type_id" class="fn">type_id</a>(&amp;self) -&gt; <a class="struct" href="https://doc.rust-lang.org/1.93.1/core/any/struct.TypeId.html" title="struct core::any::TypeId">TypeId</a></h4></section></summary><div class='docblock'>Gets the <code>TypeId</code> of <code>self</code>. <a href="https://doc.rust-lang.org/1.93.1/core/any/trait.Any.html#tymethod.type_id">Read more</a></div></details></div></details><details class="toggle implementors-toggle"><summary><section id="impl-Borrow%3CT%3E-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/borrow.rs.html#212">Source</a><a href="#impl-Borrow%3CT%3E-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/borrow/trait.Borrow.html" title="trait core::borrow::Borrow">Borrow</a>&lt;T&gt; for T<div class="where">where
T: ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.borrow" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/borrow.rs.html#214">Source</a><a href="#method.borrow" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/borrow/trait.Borrow.html#tymethod.borrow" class="fn">borrow</a>(&amp;self) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&amp;T</a></h4></section></summary><div class='docblock'>Immutably borrows from an owned value. <a href="https://doc.rust-lang.org/1.93.1/core/borrow/trait.Borrow.html#tymethod.borrow">Read more</a></div></details></div></details><details class="toggle implementors-toggle"><summary><section id="impl-BorrowMut%3CT%3E-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/borrow.rs.html#221">Source</a><a href="#impl-BorrowMut%3CT%3E-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/borrow/trait.BorrowMut.html" title="trait core::borrow::BorrowMut">BorrowMut</a>&lt;T&gt; for T<div class="where">where
T: ?<a class="trait" href="https://doc.rust-lang.org/1.93.1/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>,</div></h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.borrow_mut" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/borrow.rs.html#222">Source</a><a href="#method.borrow_mut" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut" class="fn">borrow_mut</a>(&amp;mut self) -&gt; <a class="primitive" href="https://doc.rust-lang.org/1.93.1/std/primitive.reference.html">&amp;mut T</a></h4></section></summary><div class='docblock'>Mutably borrows from an owned value. <a href="https://doc.rust-lang.org/1.93.1/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut">Read more</a></div></details></div></details><details class="toggle implementors-toggle"><summary><section id="impl-From%3CT%3E-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#785">Source</a><a href="#impl-From%3CT%3E-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.From.html" title="trait core::convert::From">From</a>&lt;T&gt; for T</h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.from" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#788">Source</a><a href="#method.from" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.From.html#tymethod.from" class="fn">from</a>(t: T) -&gt; T</h4></section></summary><div class="docblock"><p>Returns the argument unchanged.</p>
</div></details></div></details><details class="toggle implementors-toggle"><summary><section id="impl-Into%3CU%3E-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#767-769">Source</a><a href="#impl-Into%3CU%3E-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T, U&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.Into.html" title="trait core::convert::Into">Into</a>&lt;U&gt; for T<div class="where">where
U: <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.From.html" title="trait core::convert::From">From</a>&lt;T&gt;,</div></h3></section></summary><div class="impl-items"><details class="toggle method-toggle" open><summary><section id="method.into" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#777">Source</a><a href="#method.into" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.Into.html#tymethod.into" class="fn">into</a>(self) -&gt; U</h4></section></summary><div class="docblock"><p>Calls <code>U::from(self)</code>.</p>
<p>That is, this conversion is whatever the implementation of
<code><a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.From.html" title="trait core::convert::From">From</a>&lt;T&gt; for U</code> chooses to do.</p>
</div></details></div></details><details class="toggle implementors-toggle"><summary><section id="impl-TryFrom%3CU%3E-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#827-829">Source</a><a href="#impl-TryFrom%3CU%3E-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T, U&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a>&lt;U&gt; for T<div class="where">where
U: <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.Into.html" title="trait core::convert::Into">Into</a>&lt;T&gt;,</div></h3></section></summary><div class="impl-items"><details class="toggle" open><summary><section id="associatedtype.Error-1" class="associatedtype trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#831">Source</a><a href="#associatedtype.Error-1" class="anchor">§</a><h4 class="code-header">type <a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html#associatedtype.Error" class="associatedtype">Error</a> = <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/convert/enum.Infallible.html" title="enum core::convert::Infallible">Infallible</a></h4></section></summary><div class='docblock'>The type returned in the event of a conversion error.</div></details><details class="toggle method-toggle" open><summary><section id="method.try_from" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#834">Source</a><a href="#method.try_from" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html#tymethod.try_from" class="fn">try_from</a>(value: U) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;T, &lt;T as <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a>&lt;U&gt;&gt;::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html#associatedtype.Error" title="type core::convert::TryFrom::Error">Error</a>&gt;</h4></section></summary><div class='docblock'>Performs the conversion.</div></details></div></details><details class="toggle implementors-toggle"><summary><section id="impl-TryInto%3CU%3E-for-T" class="impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#811-813">Source</a><a href="#impl-TryInto%3CU%3E-for-T" class="anchor">§</a><h3 class="code-header">impl&lt;T, U&gt; <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryInto.html" title="trait core::convert::TryInto">TryInto</a>&lt;U&gt; for T<div class="where">where
U: <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a>&lt;T&gt;,</div></h3></section></summary><div class="impl-items"><details class="toggle" open><summary><section id="associatedtype.Error" class="associatedtype trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#815">Source</a><a href="#associatedtype.Error" class="anchor">§</a><h4 class="code-header">type <a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryInto.html#associatedtype.Error" class="associatedtype">Error</a> = &lt;U as <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a>&lt;T&gt;&gt;::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html#associatedtype.Error" title="type core::convert::TryFrom::Error">Error</a></h4></section></summary><div class='docblock'>The type returned in the event of a conversion error.</div></details><details class="toggle method-toggle" open><summary><section id="method.try_into" class="method trait-impl"><a class="src rightside" href="https://doc.rust-lang.org/1.93.1/src/core/convert/mod.rs.html#818">Source</a><a href="#method.try_into" class="anchor">§</a><h4 class="code-header">fn <a href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryInto.html#tymethod.try_into" class="fn">try_into</a>(self) -&gt; <a class="enum" href="https://doc.rust-lang.org/1.93.1/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;U, &lt;U as <a class="trait" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html" title="trait core::convert::TryFrom">TryFrom</a>&lt;T&gt;&gt;::<a class="associatedtype" href="https://doc.rust-lang.org/1.93.1/core/convert/trait.TryFrom.html#associatedtype.Error" title="type core::convert::TryFrom::Error">Error</a>&gt;</h4></section></summary><div class='docblock'>Performs the conversion.</div></details></div></details></div></section></div></main></body></html>
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