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 could 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, where threads are accessing data or resources in an inconsistent order
- Deadlocks, where two threads are waiting for each other, preventing both threads from continuing
- Bugs that happen only 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 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 tradeoffs to the 1:1 model.
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:
Filename: src/main.rs
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)); } }
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:
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.
Waiting for All Threads to Finish Using join
Handles
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 ending prematurely
by saving the return value of thread::spawn
in a variable. The return type of
thread::spawn
is JoinHandle
. A JoinHandle
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
of the thread we created in Listing 16-1 and
call join
to make sure the spawned thread finishes before main
exits:
Filename: src/main.rs
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(); }
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:
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:
Filename: src/main.rs
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)); } }
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:
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 the “Capturing the Environment with Closures” section of 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 yet work, as you’ll see in a moment.
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().unwrap();
}
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:
$ cargo run
Compiling threads v0.1.0 (file:///projects/threads)
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
|
note: function requires argument type to outlive `'static`
--> src/main.rs:6:18
|
6 | let handle = thread::spawn(|| {
| __________________^
7 | | println!("Here's a vector: {:?}", v);
8 | | });
| |______^
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 || {
| ++++
For more information about this error, try `rustc --explain E0373`.
error: could not compile `threads` due to previous error
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 if 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:
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().unwrap();
}
If Rust allowed us 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 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:
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:
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().unwrap(); }
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:
$ cargo run
Compiling threads v0.1.0 (file:///projects/threads)
error[E0382]: use of moved value: `v`
--> src/main.rs:10:10
|
4 | let v = vec![1, 2, 3];
| - move occurs because `v` has type `Vec<i32>`, which does not implement the `Copy` trait
5 |
6 | let handle = thread::spawn(move || {
| ------- value moved into closure here
7 | println!("Here's a vector: {:?}", v);
| - variable moved due to use in closure
...
10 | drop(v); // oh no!
| ^ value used here after move
For more information about this error, try `rustc --explain E0382`.
error: could not compile `threads` due to previous error
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 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.
With a basic understanding of threads and the thread API, let’s look at what we can do with threads.