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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li class="expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li class="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li class="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li class="expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li class="expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li class="expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li class="expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li class="expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li class="expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li class="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li class="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li class="expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li class="expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li class="expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li class="expanded "><a href="ch16-03-shared-state.html" class="active"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li class="expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li class="expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects That Allow for Values of Different Types</a></li><li class="expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li class="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li class="expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li class="expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning Our Single-Threaded Server into a Multithreaded Server</a></li><li class="expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li class="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<h2><a class="header" href="#shared-state-concurrency" id="shared-state-concurrency">Shared-State Concurrency</a></h2>
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<p>Message passing is a fine way of handling concurrency, but it’s not the only
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one. Consider this part of the slogan from the Go language documentation again:
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“do not communicate by sharing memory.”</p>
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<p>What would communicating by sharing memory look like? In addition, why would
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message-passing enthusiasts not use it and do the opposite instead?</p>
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<p>In a way, channels in any programming language are similar to single ownership,
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because once you transfer a value down a channel, you should no longer use that
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value. Shared memory concurrency is like multiple ownership: multiple threads
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||
can access the same memory location at the same time. As you saw in Chapter 15,
|
||
where smart pointers made multiple ownership possible, multiple ownership can
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||
add complexity because these different owners need managing. Rust’s type system
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||
and ownership rules greatly assist in getting this management correct. For an
|
||
example, let’s look at mutexes, one of the more common concurrency primitives
|
||
for shared memory.</p>
|
||
<h3><a class="header" href="#using-mutexes-to-allow-access-to-data-from-one-thread-at-a-time" id="using-mutexes-to-allow-access-to-data-from-one-thread-at-a-time">Using Mutexes to Allow Access to Data from One Thread at a Time</a></h3>
|
||
<p><em>Mutex</em> is an abbreviation for <em>mutual exclusion</em>, as in, a mutex allows only
|
||
one thread to access some data at any given time. To access the data in a
|
||
mutex, a thread must first signal that it wants access by asking to acquire the
|
||
mutex’s <em>lock</em>. The lock is a data structure that is part of the mutex that
|
||
keeps track of who currently has exclusive access to the data. Therefore, the
|
||
mutex is described as <em>guarding</em> the data it holds via the locking system.</p>
|
||
<p>Mutexes have a reputation for being difficult to use because you have to
|
||
remember two rules:</p>
|
||
<ul>
|
||
<li>You must attempt to acquire the lock before using the data.</li>
|
||
<li>When you’re done with the data that the mutex guards, you must unlock the
|
||
data so other threads can acquire the lock.</li>
|
||
</ul>
|
||
<p>For a real-world metaphor for a mutex, imagine a panel discussion at a
|
||
conference with only one microphone. Before a panelist can speak, they have to
|
||
ask or signal that they want to use the microphone. When they get the
|
||
microphone, they can talk for as long as they want to and then hand the
|
||
microphone to the next panelist who requests to speak. If a panelist forgets to
|
||
hand the microphone off when they’re finished with it, no one else is able to
|
||
speak. If management of the shared microphone goes wrong, the panel won’t work
|
||
as planned!</p>
|
||
<p>Management of mutexes can be incredibly tricky to get right, which is why so
|
||
many people are enthusiastic about channels. However, thanks to Rust’s type
|
||
system and ownership rules, you can’t get locking and unlocking wrong.</p>
|
||
<h4><a class="header" href="#the-api-of-mutext" id="the-api-of-mutext">The API of <code>Mutex<T></code></a></h4>
|
||
<p>As an example of how to use a mutex, let’s start by using a mutex in a
|
||
single-threaded context, as shown in Listing 16-12:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::sync::Mutex;
|
||
|
||
fn main() {
|
||
let m = Mutex::new(5);
|
||
|
||
{
|
||
let mut num = m.lock().unwrap();
|
||
*num = 6;
|
||
}
|
||
|
||
println!("m = {:?}", m);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 16-12: Exploring the API of <code>Mutex<T></code> in a
|
||
single-threaded context for simplicity</span></p>
|
||
<p>As with many types, we create a <code>Mutex<T></code> using the associated function <code>new</code>.
|
||
To access the data inside the mutex, we use the <code>lock</code> method to acquire the
|
||
lock. This call will block the current thread so it can’t do any work until
|
||
it’s our turn to have the lock.</p>
|
||
<p>The call to <code>lock</code> would fail if another thread holding the lock panicked. In
|
||
that case, no one would ever be able to get the lock, so we’ve chosen to
|
||
<code>unwrap</code> and have this thread panic if we’re in that situation.</p>
|
||
<p>After we’ve acquired the lock, we can treat the return value, named <code>num</code> in
|
||
this case, as a mutable reference to the data inside. The type system ensures
|
||
that we acquire a lock before using the value in <code>m</code>: <code>Mutex<i32></code> is not an
|
||
<code>i32</code>, so we <em>must</em> acquire the lock to be able to use the <code>i32</code> value. We
|
||
can’t forget; the type system won’t let us access the inner <code>i32</code> otherwise.</p>
|
||
<p>As you might suspect, <code>Mutex<T></code> is a smart pointer. More accurately, the call
|
||
to <code>lock</code> <em>returns</em> a smart pointer called <code>MutexGuard</code>, wrapped in a
|
||
<code>LockResult</code> that we handled with the call to <code>unwrap</code>. The <code>MutexGuard</code> smart
|
||
pointer implements <code>Deref</code> to point at our inner data; the smart pointer also
|
||
has a <code>Drop</code> implementation that releases the lock automatically when a
|
||
<code>MutexGuard</code> goes out of scope, which happens at the end of the inner scope in
|
||
Listing 16-12. As a result, we don’t risk forgetting to release the lock and
|
||
blocking the mutex from being used by other threads because the lock release
|
||
happens automatically.</p>
|
||
<p>After dropping the lock, we can print the mutex value and see that we were able
|
||
to change the inner <code>i32</code> to 6.</p>
|
||
<h4><a class="header" href="#sharing-a-mutext-between-multiple-threads" id="sharing-a-mutext-between-multiple-threads">Sharing a <code>Mutex<T></code> Between Multiple Threads</a></h4>
|
||
<p>Now, let’s try to share a value between multiple threads using <code>Mutex<T></code>.
|
||
We’ll spin up 10 threads and have them each increment a counter value by 1, so
|
||
the counter goes from 0 to 10. Note that the next few examples will have
|
||
compiler errors, and we’ll use those errors to learn more about using
|
||
<code>Mutex<T></code> and how Rust helps us use it correctly. Listing 16-13 has our
|
||
starting example:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::sync::Mutex;
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Mutex::new(0);
|
||
let mut handles = vec![];
|
||
|
||
for _ in 0..10 {
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
}
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 16-13: Ten threads each increment a counter
|
||
guarded by a <code>Mutex<T></code></span></p>
|
||
<p>We create a <code>counter</code> variable to hold an <code>i32</code> inside a <code>Mutex<T></code>, as we
|
||
did in Listing 16-12. Next, we create 10 threads by iterating over a range
|
||
of numbers. We use <code>thread::spawn</code> and give all the threads the same closure,
|
||
one that moves the counter into the thread, acquires a lock on the <code>Mutex<T></code>
|
||
by calling the <code>lock</code> method, and then adds 1 to the value in the mutex. When a
|
||
thread finishes running its closure, <code>num</code> will go out of scope and release the
|
||
lock so another thread can acquire it.</p>
|
||
<p>In the main thread, we collect all the join handles. Then, as we did in Listing
|
||
16-2, we call <code>join</code> on each handle to make sure all the threads finish. At
|
||
that point, the main thread will acquire the lock and print the result of this
|
||
program.</p>
|
||
<p>We hinted that this example wouldn’t compile. Now let’s find out why!</p>
|
||
<pre><code class="language-text">error[E0382]: capture of moved value: `counter`
|
||
--> src/main.rs:10:27
|
||
|
|
||
9 | let handle = thread::spawn(move || {
|
||
| ------- value moved (into closure) here
|
||
10 | let mut num = counter.lock().unwrap();
|
||
| ^^^^^^^ value captured here after move
|
||
|
|
||
= note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
|
||
which does not implement the `Copy` trait
|
||
|
||
error[E0382]: use of moved value: `counter`
|
||
--> src/main.rs:21:29
|
||
|
|
||
9 | let handle = thread::spawn(move || {
|
||
| ------- value moved (into closure) here
|
||
...
|
||
21 | println!("Result: {}", *counter.lock().unwrap());
|
||
| ^^^^^^^ value used here after move
|
||
|
|
||
= note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
|
||
which does not implement the `Copy` trait
|
||
|
||
error: aborting due to 2 previous errors
|
||
</code></pre>
|
||
<p>The error message states that the <code>counter</code> value is moved into the closure and
|
||
then captured when we call <code>lock</code>. That description sounds like what we wanted,
|
||
but it’s not allowed!</p>
|
||
<p>Let’s figure this out by simplifying the program. Instead of making 10 threads
|
||
in a <code>for</code> loop, let’s just make two threads without a loop and see what
|
||
happens. Replace the first <code>for</code> loop in Listing 16-13 with this code instead:</p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::sync::Mutex;
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Mutex::new(0);
|
||
let mut handles = vec![];
|
||
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
|
||
let handle2 = thread::spawn(move || {
|
||
let mut num2 = counter.lock().unwrap();
|
||
|
||
*num2 += 1;
|
||
});
|
||
handles.push(handle2);
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}
|
||
</code></pre>
|
||
<p>We make two threads and change the variable names used with the second thread
|
||
to <code>handle2</code> and <code>num2</code>. When we run the code this time, compiling gives us the
|
||
following:</p>
|
||
<pre><code class="language-text">error[E0382]: capture of moved value: `counter`
|
||
--> src/main.rs:16:24
|
||
|
|
||
8 | let handle = thread::spawn(move || {
|
||
| ------- value moved (into closure) here
|
||
...
|
||
16 | let mut num2 = counter.lock().unwrap();
|
||
| ^^^^^^^ value captured here after move
|
||
|
|
||
= note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
|
||
which does not implement the `Copy` trait
|
||
|
||
error[E0382]: use of moved value: `counter`
|
||
--> src/main.rs:26:29
|
||
|
|
||
8 | let handle = thread::spawn(move || {
|
||
| ------- value moved (into closure) here
|
||
...
|
||
26 | println!("Result: {}", *counter.lock().unwrap());
|
||
| ^^^^^^^ value used here after move
|
||
|
|
||
= note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
|
||
which does not implement the `Copy` trait
|
||
|
||
error: aborting due to 2 previous errors
|
||
</code></pre>
|
||
<p>Aha! The first error message indicates that <code>counter</code> is moved into the closure
|
||
for the thread associated with <code>handle</code>. That move is preventing us from
|
||
capturing <code>counter</code> when we try to call <code>lock</code> on it and store the result in
|
||
<code>num2</code> in the second thread! So Rust is telling us that we can’t move ownership
|
||
of <code>counter</code> into multiple threads. This was hard to see earlier because our
|
||
threads were in a loop, and Rust can’t point to different threads in different
|
||
iterations of the loop. Let’s fix the compiler error with a multiple-ownership
|
||
method we discussed in Chapter 15.</p>
|
||
<h4><a class="header" href="#multiple-ownership-with-multiple-threads" id="multiple-ownership-with-multiple-threads">Multiple Ownership with Multiple Threads</a></h4>
|
||
<p>In Chapter 15, we gave a value multiple owners by using the smart pointer
|
||
<code>Rc<T></code> to create a reference counted value. Let’s do the same here and see
|
||
what happens. We’ll wrap the <code>Mutex<T></code> in <code>Rc<T></code> in Listing 16-14 and clone
|
||
the <code>Rc<T></code> before moving ownership to the thread. Now that we’ve seen the
|
||
errors, we’ll also switch back to using the <code>for</code> loop, and we’ll keep the
|
||
<code>move</code> keyword with the closure.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::rc::Rc;
|
||
use std::sync::Mutex;
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Rc::new(Mutex::new(0));
|
||
let mut handles = vec![];
|
||
|
||
for _ in 0..10 {
|
||
let counter = Rc::clone(&counter);
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
}
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 16-14: Attempting to use <code>Rc<T></code> to allow
|
||
multiple threads to own the <code>Mutex<T></code></span></p>
|
||
<p>Once again, we compile and get... different errors! The compiler is teaching us
|
||
a lot.</p>
|
||
<pre><code class="language-text">error[E0277]: the trait bound `std::rc::Rc<std::sync::Mutex<i32>>:
|
||
std::marker::Send` is not satisfied in `[closure@src/main.rs:11:36:
|
||
15:10 counter:std::rc::Rc<std::sync::Mutex<i32>>]`
|
||
--> src/main.rs:11:22
|
||
|
|
||
11 | let handle = thread::spawn(move || {
|
||
| ^^^^^^^^^^^^^ `std::rc::Rc<std::sync::Mutex<i32>>`
|
||
cannot be sent between threads safely
|
||
|
|
||
= help: within `[closure@src/main.rs:11:36: 15:10
|
||
counter:std::rc::Rc<std::sync::Mutex<i32>>]`, the trait `std::marker::Send` is
|
||
not implemented for `std::rc::Rc<std::sync::Mutex<i32>>`
|
||
= note: required because it appears within the type
|
||
`[closure@src/main.rs:11:36: 15:10 counter:std::rc::Rc<std::sync::Mutex<i32>>]`
|
||
= note: required by `std::thread::spawn`
|
||
</code></pre>
|
||
<p>Wow, that error message is very wordy! Here are some important parts to focus
|
||
on: the first inline error says <code>`std::rc::Rc<std::sync::Mutex<i32>>` cannot be sent between threads safely</code>. The reason for this is in the next important
|
||
part to focus on, the error message. The distilled error message says <code>the trait bound `Send` is not satisfied</code>. We’ll talk about <code>Send</code> in the next
|
||
section: it’s one of the traits that ensures the types we use with threads are
|
||
meant for use in concurrent situations.</p>
|
||
<p>Unfortunately, <code>Rc<T></code> is not safe to share across threads. When <code>Rc<T></code>
|
||
manages the reference count, it adds to the count for each call to <code>clone</code> and
|
||
subtracts from the count when each clone is dropped. But it doesn’t use any
|
||
concurrency primitives to make sure that changes to the count can’t be
|
||
interrupted by another thread. This could lead to wrong counts—subtle bugs that
|
||
could in turn lead to memory leaks or a value being dropped before we’re done
|
||
with it. What we need is a type exactly like <code>Rc<T></code> but one that makes changes
|
||
to the reference count in a thread-safe way.</p>
|
||
<h4><a class="header" href="#atomic-reference-counting-with-arct" id="atomic-reference-counting-with-arct">Atomic Reference Counting with <code>Arc<T></code></a></h4>
|
||
<p>Fortunately, <code>Arc<T></code> <em>is</em> a type like <code>Rc<T></code> that is safe to use in
|
||
concurrent situations. The <em>a</em> stands for <em>atomic</em>, meaning it’s an <em>atomically
|
||
reference counted</em> type. Atomics are an additional kind of concurrency
|
||
primitive that we won’t cover in detail here: see the standard library
|
||
documentation for <code>std::sync::atomic</code> for more details. At this point, you just
|
||
need to know that atomics work like primitive types but are safe to share
|
||
across threads.</p>
|
||
<p>You might then wonder why all primitive types aren’t atomic and why standard
|
||
library types aren’t implemented to use <code>Arc<T></code> by default. The reason is that
|
||
thread safety comes with a performance penalty that you only want to pay when
|
||
you really need to. If you’re just performing operations on values within a
|
||
single thread, your code can run faster if it doesn’t have to enforce the
|
||
guarantees atomics provide.</p>
|
||
<p>Let’s return to our example: <code>Arc<T></code> and <code>Rc<T></code> have the same API, so we fix
|
||
our program by changing the <code>use</code> line, the call to <code>new</code>, and the call to
|
||
<code>clone</code>. The code in Listing 16-15 will finally compile and run:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::sync::{Mutex, Arc};
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let counter = Arc::new(Mutex::new(0));
|
||
let mut handles = vec![];
|
||
|
||
for _ in 0..10 {
|
||
let counter = Arc::clone(&counter);
|
||
let handle = thread::spawn(move || {
|
||
let mut num = counter.lock().unwrap();
|
||
|
||
*num += 1;
|
||
});
|
||
handles.push(handle);
|
||
}
|
||
|
||
for handle in handles {
|
||
handle.join().unwrap();
|
||
}
|
||
|
||
println!("Result: {}", *counter.lock().unwrap());
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 16-15: Using an <code>Arc<T></code> to wrap the <code>Mutex<T></code>
|
||
to be able to share ownership across multiple threads</span></p>
|
||
<p>This code will print the following:</p>
|
||
<pre><code class="language-text">Result: 10
|
||
</code></pre>
|
||
<p>We did it! We counted from 0 to 10, which may not seem very impressive, but it
|
||
did teach us a lot about <code>Mutex<T></code> and thread safety. You could also use this
|
||
program’s structure to do more complicated operations than just incrementing a
|
||
counter. Using this strategy, you can divide a calculation into independent
|
||
parts, split those parts across threads, and then use a <code>Mutex<T></code> to have each
|
||
thread update the final result with its part.</p>
|
||
<h3><a class="header" href="#similarities-between-refcelltrct-and-mutextarct" id="similarities-between-refcelltrct-and-mutextarct">Similarities Between <code>RefCell<T></code>/<code>Rc<T></code> and <code>Mutex<T></code>/<code>Arc<T></code></a></h3>
|
||
<p>You might have noticed that <code>counter</code> is immutable but we could get a mutable
|
||
reference to the value inside it; this means <code>Mutex<T></code> provides interior
|
||
mutability, as the <code>Cell</code> family does. In the same way we used <code>RefCell<T></code> in
|
||
Chapter 15 to allow us to mutate contents inside an <code>Rc<T></code>, we use <code>Mutex<T></code>
|
||
to mutate contents inside an <code>Arc<T></code>.</p>
|
||
<p>Another detail to note is that Rust can’t protect you from all kinds of logic
|
||
errors when you use <code>Mutex<T></code>. Recall in Chapter 15 that using <code>Rc<T></code> came
|
||
with the risk of creating reference cycles, where two <code>Rc<T></code> values refer to
|
||
each other, causing memory leaks. Similarly, <code>Mutex<T></code> comes with the risk of
|
||
creating <em>deadlocks</em>. These occur when an operation needs to lock two resources
|
||
and two threads have each acquired one of the locks, causing them to wait for
|
||
each other forever. If you’re interested in deadlocks, try creating a Rust
|
||
program that has a deadlock; then research deadlock mitigation strategies for
|
||
mutexes in any language and have a go at implementing them in Rust. The
|
||
standard library API documentation for <code>Mutex<T></code> and <code>MutexGuard</code> offers
|
||
useful information.</p>
|
||
<p>We’ll round out this chapter by talking about the <code>Send</code> and <code>Sync</code> traits and
|
||
how we can use them with custom types.</p>
|
||
|
||
</main>
|
||
|
||
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