Threads - The Rust Programming Language

## Using Threads to Run Code Simultaneously

In most operating systems in use today, when your program executes, the context in which the operating system runs your code is called a process. The operating system runs many processes, and the operating system managing these processes is what lets multiple programs execute at the same time on your computer.

We can take the idea of processes each running a program down one level of abstraction: your program can also have independent parts that run simultaneously within the context of your program. The feature that enables this functionality is called threads.

Splitting up the computation your program needs to do into multiple threads can improve performance, since the program will be doing multiple things at the same time. Programming with threads can add complexity, however. Since threads run simultaneously, there's no inherent guarantee about the order in which the parts of your code on different threads will run. This can lead to race conditions where threads are accessing data or resources in an inconsistent order, deadlocks where two threads both prevent each other from continuing, or bugs that only happen in certain situations that are hard to reproduce reliably. Rust lessens the effect of these and other downsides of using threads, but programming in a multithreaded context still takes thought and code structured differently than for programs only expected to run in a single thread.

There are a few different ways that programming languages implement threads. Many operating systems provide an API for creating new threads. In addition, many programming languages provide their own special implementation of threads. Programming language provided threads are sometimes called lightweight or green threads. These languages take a number of green threads and execute them in the context of a different number of operating system threads. For this reason, the model where a language calls the operating system APIs to create threads is sometimes called 1:1, one OS thread per one language thread. The green threaded model is called the M:N model, `M` green threads per `N` OS threads, where `M` and `N` are not necessarily the same number.

Each model has its own advantages and tradeoffs. The tradeoff that's most important to Rust is runtime support. Runtime is a confusing term; it can have different meaning in different contexts. Here, we mean some code included by the language in every binary. For some languages, this code is large, and for others, this code is small. Colloquially, "no runtime" is often what people will say when they mean "small runtime", since every non-assembly language has some amount of runtime. Smaller runtimes have fewer features but have the advantage of resulting in smaller binaries. Smaller binaries make it easier to combine the language with other languages in more contexts. While many languages are okay with increasing the runtime in exchange for more features, Rust needs to have nearly no runtime, and cannot compromise on being able to call into C in order to maintain performance.

The green threading model is a feature that requires a larger language runtime in order to manage the 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.

Now that we've defined what threads are in Rust, let's explore how to use the thread-related API that the standard library provides for us.

### 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 new thread and other text from the main thread:

Filename: src/main.rs

    use std::thread;

    fn main() {
        thread::spawn(|| {
            for i in 1..10 {
                println!("hi number {} from the spawned thread!", i);
            }
        });

        for i in 1..5 {
            println!("hi number {} from the main thread!", i);
        }
    }

Listing 16-1: Creating a new thread to print one thing while the main thread is printing something else

Note that the way this function is written, when the main thread ends, it will stop the new thread too. The output from this program might be a little different every time, but it will look similar to this:

    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 threads will probably take turns, but that's not guaranteed. In this run, the main thread printed first, even though the print statement from the spawned thread appears first in the code we wrote. 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 always only see one thread, or if you don't see any overlap, try increasing the numbers in the ranges to create more opportunities for a thread to take a break and give the other thread a turn.

#### Waiting for All Threads to Finish Using `join` Handles

Not only does the code in Listing 16-1 not allow the spawned thread to finish most of the time since the main thread ends before the spawned thread is done, there's actually no guarantee that the spawned thread will get to run at all! We can fix this by saving the return value of `thread::spawn`, which is a `JoinHandle`. That looks like Listing 16-2:

Filename: src/main.rs

    use std::thread;

    fn main() {
        let handle = thread::spawn(|| {
            for i in 1..10 {
                println!("hi number {} from the spawned thread!", i);
            }
        });

        for i in 1..5 {
            println!("hi number {} from the main thread!", i);
        }

        handle.join();
    }

Listing 16-2: Saving a `JoinHandle` from `thread::spawn` to guarantee the thread is run to completion

A `JoinHandle` is an owned value that can wait for a thread to finish, which is what the `join` method does. By calling `join` on the handle, the current thread will block until the thread that the handle represents terminates. Since we've put the call to `join` after the main thread's `for` loop, running this example should produce output that looks something like this:

    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 are still alternating, but the main thread waits because of the call to `handle.join()` and does not end until the spawned thread is finished.

If we instead move `handle.join()` before the `for` loop in main, like this:

Filename: src/main.rs

    use std::thread;

    fn main() {
        let handle = thread::spawn(|| {
            for i in 1..10 {
                println!("hi number {} from the spawned thread!", i);
            }
        });

        handle.join();

        for i in 1..5 {
            println!("hi number {} from the main thread!", i);
        }
    }

The main thread will wait for the spawned thread to finish before the main thread starts running its `for` loop, so the output won't be interleaved anymore:

    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!

Thinking about a small thing such as where to call `join` can affect whether your threads are actually running at the same time or not.

### Using `move` Closures with Threads

There's a feature of closures that we didn't cover in Chapter 13 that's often useful with `thread::spawn`: `move` closures. We said this in Chapter 13:

> Creating closures that capture values from their environment is mostly used in the context of starting new threads.

Now we're creating new threads, so let's talk about capturing values in closures!

Notice the closure that we pass to `thread::spawn` in Listing 16-1 takes no arguments: we're not using any data from the main thread in the spawned thread's code. In order to use data in the spawned thread that comes from the main thread, we need the spawned thread's closure to 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, which won't work the way this example is written:

Filename: src/main.rs

    use std::thread;

    fn main() {
        let v = vec![1, 2, 3];

        let handle = thread::spawn(|| {
            println!("Here's a vector: {:?}", v);
        });

        handle.join();
    }

Listing 16-3: Attempting to use a vector created by the main thread from another thread

The closure uses `v`, so the closure will capture `v` and make `v` part of the closure's environment. Because `thread::spawn` runs this closure in a new thread, we can access `v` inside that new thread.

When we compile this example, however, we'll get the following error:

    error[E0373]: closure may outlive the current function, but it borrows `v`,
    which is owned by the current function
     -->
      |
    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, as shown:
      |     let handle = thread::spawn(move || {

When we capture something in a closure's environment, Rust will try to infer how to capture it. `println!` only needs a reference to `v`, so the closure tries to borrow `v`. There's a problem, though: we don't know how long the spawned thread will run, so we don't know if the reference to `v` will always be valid.

Consider the code in Listing 16-4 that shows a scenario where it's more likely that the reference to `v` won't be valid:

Filename: src/main.rs

    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();
    }

Listing 16-4: A thread with a closure that attempts to capture a reference to `v` from a main thread that drops `v`

This code could be run, and the spawned thread could immediately get put in the background without getting a chance to run at all. The spawned thread has a reference to `v` inside, but the main thread is still running: it immediately drops `v`, using the `drop` function that we discussed in Chapter 15 that explicitly drops its argument. Then, the spawned thread starts to execute. `v` is now invalid, so a reference to it is also invalid. Oh no!

To fix this problem, we can listen to the advice of the error message:

    help: to force the closure to take ownership of `v` (and any other referenced
    variables), use the `move` keyword, as shown:
      |     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 inferring borrowing. This modification to the code from Listing 16-3 shown in Listing 16-5 will compile and run as we intend:

Filename: src/main.rs

    use std::thread;

    fn main() {
        let v = vec![1, 2, 3];

        let handle = thread::spawn(move || {
            println!("Here's a vector: {:?}", v);
        });

        handle.join();
    }

Listing 16-5: Using the `move` keyword to force a closure to take ownership of the values it uses

What about the code in Listing 16-4 where the main thread called `drop`? If we add `move` to the closure, we've moved `v` into the closure's environment, and we can no longer call `drop` on it. We get this compiler error instead:

    error[E0382]: use of moved value: `v`
      -->
       |
    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

Rust's ownership rules have saved us again!

Now that we have a basic understanding of threads and the thread API, let's talk about what we can actually do with threads.