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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" class="active"><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"><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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<h2><a class="header" href="#using-threads-to-run-code-simultaneously" id="using-threads-to-run-code-simultaneously">Using Threads to Run Code Simultaneously</a></h2>
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<p>In most current operating systems, an executed program’s code is run in a
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<em>process</em>, and the operating system manages multiple processes at once. Within
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your program, you can also have independent parts that run simultaneously. The
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features that run these independent parts are called <em>threads</em>.</p>
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<p>Splitting the computation in your program into multiple threads can improve
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performance because the program does multiple tasks at the same time, but it
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also adds complexity. Because threads can run simultaneously, there’s no
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inherent guarantee about the order in which parts of your code on different
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threads will run. This can lead to problems, such as:</p>
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<ul>
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<li>Race conditions, where threads are accessing data or resources in an
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inconsistent order</li>
|
||
<li>Deadlocks, where two threads are waiting for each other to finish using a
|
||
resource the other thread has, preventing both threads from continuing</li>
|
||
<li>Bugs that happen only in certain situations and are hard to reproduce and fix
|
||
reliably</li>
|
||
</ul>
|
||
<p>Rust attempts to mitigate the negative effects of using threads, but
|
||
programming in a multithreaded context still takes careful thought and requires
|
||
a code structure that is different from that in programs running in a single
|
||
thread.</p>
|
||
<p>Programming languages implement threads in a few different ways. Many operating
|
||
systems provide an API for creating new threads. This model where a language
|
||
calls the operating system APIs to create threads is sometimes called <em>1:1</em>,
|
||
meaning one operating system thread per one language thread.</p>
|
||
<p>Many programming languages provide their own special implementation of threads.
|
||
Programming language-provided threads are known as <em>green</em> threads, and
|
||
languages that use these green threads will execute them in the context of a
|
||
different number of operating system threads. For this reason, the
|
||
green-threaded model is called the <em>M:N</em> model: there are <code>M</code> green threads per
|
||
<code>N</code> operating system threads, where <code>M</code> and <code>N</code> are not necessarily the same
|
||
number.</p>
|
||
<p>Each model has its own advantages and trade-offs, and the trade-off most
|
||
important to Rust is runtime support. <em>Runtime</em> is a confusing term and can
|
||
have different meanings in different contexts.</p>
|
||
<p>In this context, by <em>runtime</em> we mean code that is included by the language in
|
||
every binary. This code can be large or small depending on the language, but
|
||
every non-assembly language will have some amount of runtime code. For that
|
||
reason, colloquially when people say a language has “no runtime,” they often
|
||
mean “small runtime.” Smaller runtimes have fewer features but have the
|
||
advantage of resulting in smaller binaries, which make it easier to combine the
|
||
language with other languages in more contexts. Although many languages are
|
||
okay with increasing the runtime size in exchange for more features, Rust needs
|
||
to have nearly no runtime and cannot compromise on being able to call into C to
|
||
maintain performance.</p>
|
||
<p>The green-threading M:N model requires a larger language runtime to manage
|
||
threads. As such, the Rust standard library only provides an implementation of
|
||
1:1 threading. Because Rust is such a low-level language, there are crates that
|
||
implement M:N threading if you would rather trade overhead for aspects such as
|
||
more control over which threads run when and lower costs of context switching,
|
||
for example.</p>
|
||
<p>Now that we’ve defined threads in Rust, let’s explore how to use the
|
||
thread-related API provided by the standard library.</p>
|
||
<h3><a class="header" href="#creating-a-new-thread-with-spawn" id="creating-a-new-thread-with-spawn">Creating a New Thread with <code>spawn</code></a></h3>
|
||
<p>To create a new thread, we call the <code>thread::spawn</code> function and pass it a
|
||
closure (we talked about closures in Chapter 13) containing the code we want to
|
||
run in the new thread. The example in Listing 16-1 prints some text from a main
|
||
thread and other text from a new thread:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::thread;
|
||
use std::time::Duration;
|
||
|
||
fn main() {
|
||
thread::spawn(|| {
|
||
for i in 1..10 {
|
||
println!("hi number {} from the spawned thread!", i);
|
||
thread::sleep(Duration::from_millis(1));
|
||
}
|
||
});
|
||
|
||
for i in 1..5 {
|
||
println!("hi number {} from the main thread!", i);
|
||
thread::sleep(Duration::from_millis(1));
|
||
}
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 16-1: Creating a new thread to print one thing
|
||
while the main thread prints something else</span></p>
|
||
<p>Note that with this function, the new thread will be stopped when the main
|
||
thread ends, whether or not it has finished running. The output from this
|
||
program might be a little different every time, but it will look similar to the
|
||
following:</p>
|
||
<pre><code class="language-text">hi number 1 from the main thread!
|
||
hi number 1 from the spawned thread!
|
||
hi number 2 from the main thread!
|
||
hi number 2 from the spawned thread!
|
||
hi number 3 from the main thread!
|
||
hi number 3 from the spawned thread!
|
||
hi number 4 from the main thread!
|
||
hi number 4 from the spawned thread!
|
||
hi number 5 from the spawned thread!
|
||
</code></pre>
|
||
<p>The calls to <code>thread::sleep</code> force a thread to stop its execution for a short
|
||
duration, allowing a different thread to run. The threads will probably take
|
||
turns, but that isn’t guaranteed: it depends on how your operating system
|
||
schedules the threads. In this run, the main thread printed first, even though
|
||
the print statement from the spawned thread appears first in the code. And even
|
||
though we told the spawned thread to print until <code>i</code> is 9, it only got to 5
|
||
before the main thread shut down.</p>
|
||
<p>If you run this code and only see output from the main thread, or don’t see any
|
||
overlap, try increasing the numbers in the ranges to create more opportunities
|
||
for the operating system to switch between the threads.</p>
|
||
<h3><a class="header" href="#waiting-for-all-threads-to-finish-using-join-handles" id="waiting-for-all-threads-to-finish-using-join-handles">Waiting for All Threads to Finish Using <code>join</code> Handles</a></h3>
|
||
<p>The code in Listing 16-1 not only stops the spawned thread prematurely most of
|
||
the time due to the main thread ending, but also can’t guarantee that the
|
||
spawned thread will get to run at all. The reason is that there is no guarantee
|
||
on the order in which threads run!</p>
|
||
<p>We can fix the problem of the spawned thread not getting to run, or not getting
|
||
to run completely, by saving the return value of <code>thread::spawn</code> in a variable.
|
||
The return type of <code>thread::spawn</code> is <code>JoinHandle</code>. A <code>JoinHandle</code> is an owned
|
||
value that, when we call the <code>join</code> method on it, will wait for its thread to
|
||
finish. Listing 16-2 shows how to use the <code>JoinHandle</code> of the thread we created
|
||
in Listing 16-1 and call <code>join</code> to make sure the spawned thread finishes before
|
||
<code>main</code> exits:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::thread;
|
||
use std::time::Duration;
|
||
|
||
fn main() {
|
||
let handle = thread::spawn(|| {
|
||
for i in 1..10 {
|
||
println!("hi number {} from the spawned thread!", i);
|
||
thread::sleep(Duration::from_millis(1));
|
||
}
|
||
});
|
||
|
||
for i in 1..5 {
|
||
println!("hi number {} from the main thread!", i);
|
||
thread::sleep(Duration::from_millis(1));
|
||
}
|
||
|
||
handle.join().unwrap();
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 16-2: Saving a <code>JoinHandle</code> from <code>thread::spawn</code>
|
||
to guarantee the thread is run to completion</span></p>
|
||
<p>Calling <code>join</code> on the handle blocks the thread currently running until the
|
||
thread represented by the handle terminates. <em>Blocking</em> a thread means that
|
||
thread is prevented from performing work or exiting. Because we’ve put the call
|
||
to <code>join</code> after the main thread’s <code>for</code> loop, running Listing 16-2 should
|
||
produce output similar to this:</p>
|
||
<pre><code class="language-text">hi number 1 from the main thread!
|
||
hi number 2 from the main thread!
|
||
hi number 1 from the spawned thread!
|
||
hi number 3 from the main thread!
|
||
hi number 2 from the spawned thread!
|
||
hi number 4 from the main thread!
|
||
hi number 3 from the spawned thread!
|
||
hi number 4 from the spawned thread!
|
||
hi number 5 from the spawned thread!
|
||
hi number 6 from the spawned thread!
|
||
hi number 7 from the spawned thread!
|
||
hi number 8 from the spawned thread!
|
||
hi number 9 from the spawned thread!
|
||
</code></pre>
|
||
<p>The two threads continue alternating, but the main thread waits because of the
|
||
call to <code>handle.join()</code> and does not end until the spawned thread is finished.</p>
|
||
<p>But let’s see what happens when we instead move <code>handle.join()</code> before the
|
||
<code>for</code> loop in <code>main</code>, like this:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::thread;
|
||
use std::time::Duration;
|
||
|
||
fn main() {
|
||
let handle = thread::spawn(|| {
|
||
for i in 1..10 {
|
||
println!("hi number {} from the spawned thread!", i);
|
||
thread::sleep(Duration::from_millis(1));
|
||
}
|
||
});
|
||
|
||
handle.join().unwrap();
|
||
|
||
for i in 1..5 {
|
||
println!("hi number {} from the main thread!", i);
|
||
thread::sleep(Duration::from_millis(1));
|
||
}
|
||
}
|
||
</code></pre></pre>
|
||
<p>The main thread will wait for the spawned thread to finish and then run its
|
||
<code>for</code> loop, so the output won’t be interleaved anymore, as shown here:</p>
|
||
<pre><code class="language-text">hi number 1 from the spawned thread!
|
||
hi number 2 from the spawned thread!
|
||
hi number 3 from the spawned thread!
|
||
hi number 4 from the spawned thread!
|
||
hi number 5 from the spawned thread!
|
||
hi number 6 from the spawned thread!
|
||
hi number 7 from the spawned thread!
|
||
hi number 8 from the spawned thread!
|
||
hi number 9 from the spawned thread!
|
||
hi number 1 from the main thread!
|
||
hi number 2 from the main thread!
|
||
hi number 3 from the main thread!
|
||
hi number 4 from the main thread!
|
||
</code></pre>
|
||
<p>Small details, such as where <code>join</code> is called, can affect whether or not your
|
||
threads run at the same time.</p>
|
||
<h3><a class="header" href="#using-move-closures-with-threads" id="using-move-closures-with-threads">Using <code>move</code> Closures with Threads</a></h3>
|
||
<p>The <code>move</code> closure is often used alongside <code>thread::spawn</code> because it allows
|
||
you to use data from one thread in another thread.</p>
|
||
<p>In Chapter 13, we mentioned we can use the <code>move</code> keyword before the parameter
|
||
list of a closure to force the closure to take ownership of the values it uses
|
||
in the environment. This technique is especially useful when creating new
|
||
threads in order to transfer ownership of values from one thread to another.</p>
|
||
<p>Notice in Listing 16-1 that the closure we pass to <code>thread::spawn</code> takes no
|
||
arguments: we’re not using any data from the main thread in the spawned
|
||
thread’s code. To use data from the main thread in the spawned thread, the
|
||
spawned thread’s closure must capture the values it needs. Listing 16-3 shows
|
||
an attempt to create a vector in the main thread and use it in the spawned
|
||
thread. However, this won’t yet work, as you’ll see in a moment.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::thread;
|
||
|
||
fn main() {
|
||
let v = vec![1, 2, 3];
|
||
|
||
let handle = thread::spawn(|| {
|
||
println!("Here's a vector: {:?}", v);
|
||
});
|
||
|
||
handle.join().unwrap();
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 16-3: Attempting to use a vector created by the
|
||
main thread in another thread</span></p>
|
||
<p>The closure uses <code>v</code>, so it will capture <code>v</code> and make it part of the closure’s
|
||
environment. Because <code>thread::spawn</code> runs this closure in a new thread, we
|
||
should be able to access <code>v</code> inside that new thread. But when we compile this
|
||
example, we get the following error:</p>
|
||
<pre><code class="language-text">error[E0373]: closure may outlive the current function, but it borrows `v`,
|
||
which is owned by the current function
|
||
--> src/main.rs:6:32
|
||
|
|
||
6 | let handle = thread::spawn(|| {
|
||
| ^^ may outlive borrowed value `v`
|
||
7 | println!("Here's a vector: {:?}", v);
|
||
| - `v` is borrowed here
|
||
|
|
||
help: to force the closure to take ownership of `v` (and any other referenced
|
||
variables), use the `move` keyword
|
||
|
|
||
6 | let handle = thread::spawn(move || {
|
||
| ^^^^^^^
|
||
</code></pre>
|
||
<p>Rust <em>infers</em> how to capture <code>v</code>, and because <code>println!</code> only needs a reference
|
||
to <code>v</code>, the closure tries to borrow <code>v</code>. However, there’s a problem: Rust can’t
|
||
tell how long the spawned thread will run, so it doesn’t know if the reference
|
||
to <code>v</code> will always be valid.</p>
|
||
<p>Listing 16-4 provides a scenario that’s more likely to have a reference to <code>v</code>
|
||
that won’t be valid:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use std::thread;
|
||
|
||
fn main() {
|
||
let v = vec![1, 2, 3];
|
||
|
||
let handle = thread::spawn(|| {
|
||
println!("Here's a vector: {:?}", v);
|
||
});
|
||
|
||
drop(v); // oh no!
|
||
|
||
handle.join().unwrap();
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 16-4: A thread with a closure that attempts to
|
||
capture a reference to <code>v</code> from a main thread that drops <code>v</code></span></p>
|
||
<p>If we were allowed to run this code, there’s a possibility the spawned thread
|
||
would be immediately put in the background without running at all. The spawned
|
||
thread has a reference to <code>v</code> inside, but the main thread immediately drops
|
||
<code>v</code>, using the <code>drop</code> function we discussed in Chapter 15. Then, when the
|
||
spawned thread starts to execute, <code>v</code> is no longer valid, so a reference to it
|
||
is also invalid. Oh no!</p>
|
||
<p>To fix the compiler error in Listing 16-3, we can use the error message’s
|
||
advice:</p>
|
||
<pre><code class="language-text">help: to force the closure to take ownership of `v` (and any other referenced
|
||
variables), use the `move` keyword
|
||
|
|
||
6 | let handle = thread::spawn(move || {
|
||
| ^^^^^^^
|
||
</code></pre>
|
||
<p>By adding the <code>move</code> keyword before the closure, we force the closure to take
|
||
ownership of the values it’s using rather than allowing Rust to infer that it
|
||
should borrow the values. The modification to Listing 16-3 shown in Listing
|
||
16-5 will compile and run as we intend:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">use std::thread;
|
||
|
||
fn main() {
|
||
let v = vec![1, 2, 3];
|
||
|
||
let handle = thread::spawn(move || {
|
||
println!("Here's a vector: {:?}", v);
|
||
});
|
||
|
||
handle.join().unwrap();
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 16-5: Using the <code>move</code> keyword to force a closure
|
||
to take ownership of the values it uses</span></p>
|
||
<p>What would happen to the code in Listing 16-4 where the main thread called
|
||
<code>drop</code> if we use a <code>move</code> closure? Would <code>move</code> fix that case? Unfortunately,
|
||
no; we would get a different error because what Listing 16-4 is trying to do
|
||
isn’t allowed for a different reason. If we added <code>move</code> to the closure, we
|
||
would move <code>v</code> into the closure’s environment, and we could no longer call
|
||
<code>drop</code> on it in the main thread. We would get this compiler error instead:</p>
|
||
<pre><code class="language-text">error[E0382]: use of moved value: `v`
|
||
--> src/main.rs:10:10
|
||
|
|
||
6 | let handle = thread::spawn(move || {
|
||
| ------- value moved (into closure) here
|
||
...
|
||
10 | drop(v); // oh no!
|
||
| ^ value used here after move
|
||
|
|
||
= note: move occurs because `v` has type `std::vec::Vec<i32>`, which does
|
||
not implement the `Copy` trait
|
||
</code></pre>
|
||
<p>Rust’s ownership rules have saved us again! We got an error from the code in
|
||
Listing 16-3 because Rust was being conservative and only borrowing <code>v</code> for the
|
||
thread, which meant the main thread could theoretically invalidate the spawned
|
||
thread’s reference. By telling Rust to move ownership of <code>v</code> to the spawned
|
||
thread, we’re guaranteeing Rust that the main thread won’t use <code>v</code> anymore. If
|
||
we change Listing 16-4 in the same way, we’re then violating the ownership
|
||
rules when we try to use <code>v</code> in the main thread. The <code>move</code> keyword overrides
|
||
Rust’s conservative default of borrowing; it doesn’t let us violate the
|
||
ownership rules.</p>
|
||
<p>With a basic understanding of threads and the thread API, let’s look at what we
|
||
can <em>do</em> with threads.</p>
|
||
|
||
</main>
|
||
|
||
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