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Ch16 01 Threads

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Using Threads to Run Code Simultaneously

In most current operating systems, an executed program’s code is run in a process, and the operating system will manage multiple processes at once. Within a program, you can also have independent parts that run simultaneously. The features that run these independent parts are called threads. For example, a web server could have multiple threads so that it can respond to more than one request at the same time.

Splitting the computation in your program into multiple threads to run multiple tasks at the same time can improve performance, but it also adds complexity. Because threads can run simultaneously, there’s no inherent guarantee about the order in which parts of your code on different threads will run. This can lead to problems, such as:

  • Race conditions, in which threads are accessing data or resources in an inconsistent order
  • Deadlocks, in which two threads are waiting for each other, preventing both threads from continuing
  • Bugs that only happen in certain situations and are hard to reproduce and fix reliably

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.

Programming languages implement threads in a few different ways, and many operating systems provide an API the programming language can call for creating new threads. The Rust standard library uses a 1:1 model of thread implementation, whereby a program uses one operating system thread per one language thread. There are crates that implement other models of threading that make different trade-offs to the 1:1 model. (Rust’s async system, which we will see in the next chapter, provides another approach to concurrency as well.)

Creating a New Thread with spawn

To create a new thread, we call the thread::spawn 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.

<Listing number="16-1" file-name="src/main.rs" caption="Creating a new thread to print one thing while the main thread prints something else">
rust
{{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-01/src/main.rs}}
</Listing>

Note that when the main thread of a Rust program completes, all spawned threads are shut down, whether or not they have finished running. The output from this program might be a little different every time, but it will look similar to the following:

<!-- Not extracting output because changes to this output aren't significant; the changes are likely to be due to the threads running differently rather than changes in the compiler -->
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!

The calls to thread::sleep 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 i is 9, it only got to 5 before the main thread shut down.

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.

<!-- Old headings. Do not remove or links may break. -->

<a id="waiting-for-all-threads-to-finish-using-join-handles"></a>

Waiting for All Threads to Finish

The code in Listing 16-1 not only stops the spawned thread prematurely most of the time due to the main thread ending, but because there is no guarantee on the order in which threads run, we also can’t guarantee that the spawned thread will get to run at all!

We can fix the problem of the spawned thread not running or of it ending prematurely by saving the return value of thread::spawn in a variable. The return type of thread::spawn is JoinHandle<T>. A JoinHandle<T> is an owned value that, when we call the join method on it, will wait for its thread to finish. Listing 16-2 shows how to use the JoinHandle<T> of the thread we created in Listing 16-1 and how to call join to make sure the spawned thread finishes before main exits.

<Listing number="16-2" file-name="src/main.rs" caption="Saving a `JoinHandle<T>` from `thread::spawn` to guarantee the thread is run to completion">
rust
{{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-02/src/main.rs}}
</Listing>

Calling join on the handle blocks the thread currently running until the thread represented by the handle terminates. Blocking a thread means that thread is prevented from performing work or exiting. Because we’ve put the call to join after the main thread’s for loop, running Listing 16-2 should produce output similar to this:

<!-- Not extracting output because changes to this output aren't significant; the changes are likely to be due to the threads running differently rather than changes in the compiler -->
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!

The two threads continue alternating, but the main thread waits because of the call to handle.join() and does not end until the spawned thread is finished.

But let’s see what happens when we instead move handle.join() before the for loop in main, like this:

<Listing file-name="src/main.rs">
rust
{{#rustdoc_include ../listings/ch16-fearless-concurrency/no-listing-01-join-too-early/src/main.rs}}
</Listing>

The main thread will wait for the spawned thread to finish and then run its for loop, so the output won’t be interleaved anymore, as shown here:

<!-- Not extracting output because changes to this output aren't significant; the changes are likely to be due to the threads running differently rather than changes in the compiler -->
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!

Small details, such as where join is called, can affect whether or not your threads run at the same time.

Using move Closures with Threads

We’ll often use the move keyword with closures passed to thread::spawn because the closure will then take ownership of the values it uses from the environment, thus transferring ownership of those values from one thread to another. In “Capturing References or Moving Ownership”<!-- ignore --> in Chapter 13, we discussed move in the context of closures. Now we’ll concentrate more on the interaction between move and thread::spawn.

Notice in Listing 16-1 that the closure we pass to thread::spawn 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 work yet, as you’ll see in a moment.

<Listing number="16-3" file-name="src/main.rs" caption="Attempting to use a vector created by the main thread in another thread">
rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-03/src/main.rs}}
</Listing>

The closure uses v, so it will capture v and make it part of the closure’s environment. Because thread::spawn runs this closure in a new thread, we should be able to access v inside that new thread. But when we compile this example, we get the following error:

console
{{#include ../listings/ch16-fearless-concurrency/listing-16-03/output.txt}}

Rust infers how to capture v, and because println! only needs a reference to v, the closure tries to borrow v. However, there’s a problem: Rust can’t tell how long the spawned thread will run, so it doesn’t know whether the reference to v will always be valid.

Listing 16-4 provides a scenario that’s more likely to have a reference to v that won’t be valid.

<Listing number="16-4" file-name="src/main.rs" caption="A thread with a closure that attempts to capture a reference to `v` from a main thread that drops `v`">
rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-04/src/main.rs}}
</Listing>

If Rust allowed us to run this code, there’s a possibility that the spawned thread would be immediately put in the background without running at all. The spawned thread has a reference to v inside, but the main thread immediately drops v, using the drop function we discussed in Chapter 15. Then, when the spawned thread starts to execute, v is no longer valid, so a reference to it is also invalid. Oh no!

To fix the compiler error in Listing 16-3, we can use the error message’s advice:

<!-- manual-regeneration after automatic regeneration, look at listings/ch16-fearless-concurrency/listing-16-03/output.txt and copy the relevant part -->
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 || {
  |                                ++++

By adding the move 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.

<Listing number="16-5" file-name="src/main.rs" caption="Using the `move` keyword to force a closure to take ownership of the values it uses">
rust
{{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-05/src/main.rs}}
</Listing>

We might be tempted to try the same thing to fix the code in Listing 16-4 where the main thread called drop by using a move closure. However, this fix will not work because what Listing 16-4 is trying to do is disallowed for a different reason. If we added move to the closure, we would move v into the closure’s environment, and we could no longer call drop on it in the main thread. We would get this compiler error instead:

console
{{#include ../listings/ch16-fearless-concurrency/output-only-01-move-drop/output.txt}}

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 v for the thread, which meant the main thread could theoretically invalidate the spawned thread’s reference. By telling Rust to move ownership of v to the spawned thread, we’re guaranteeing to Rust that the main thread won’t use v anymore. If we change Listing 16-4 in the same way, we’re then violating the ownership rules when we try to use v in the main thread. The move keyword overrides Rust’s conservative default of borrowing; it doesn’t let us violate the ownership rules.

Now that we’ve covered what threads are and the methods supplied by the thread API, let’s look at some situations in which we can use threads.