Struct show_notes::e022::Script

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Full script for the episode.

e022: Send and Sync

Note: In the original (recorded) version of the episode, I said “deadlock” everywhere I meant “data race.” Throughout the transcript, I’ve made the substitution accordingly, but if you’re listening to the episode, you’ll have to do it mentally. Sorry about that!


Hello, I’m Chris Krycho and this is New Rustacean: a show about the Rust Programming Language and the people who use it. This is episode 22: TODO

There are a few major parts of Rust we haven’t covered yet, including one of the most important parts of Rust’s promise: safe concurrency. So today we’re going to dive in and talk about the two traits that are the most important part of Rust’s story for enabling safe concurrency: Send and Sync.


Multithreaded processing has become increasingly important over the last couple decades, as our ability to speed up single-threaded processing has effectively hit a wall. We can make things go faster, but it takes a lot of power and it generates a lot of heat, and that makes it hard to speed up our programs just by running the processor faster. Starting in the early 2000s, the computer industry began adapting to that by allowing for multi-threaded and multi-processor approaches, which at least in principle lets us parallelize parts of whatever we’re working on. But there’s a problem: parallelizing things effectively is hard. And it’s hard in at least two ways—one of which Rust really can’t help us with, but one of which it really can.

  • Rust cannot help us with designing algorithms well for parallelization, or even with identifying which specific problems are amenable to parallelization. We still need good old computer science reasoning for that side of things.

  • On the other hand, Rust can hep us make sure the design we implement is safe once we have identified candidates for parallelization. In fact, it can do more than help. Rust can actually just eliminate whole classes of parallelization bugs common in other languages, and do so at compile time.

This is basically true of all the ways Rust helps us, to be fair: it doesn’t tell you how to write an algorithm that performs linearly instead of quadratically. It makes it easier to not shoot yourself in the foot with memory errors while you write your algorithm. The same thing goes for parallelism.

So now we know why we care about Send and Sync—what are they and how do they work?

Marker traits

The first thing we need to understand is that both Send and Sync are marker traits. This is one of several things I passed over in my previous discussion of traits back in episode 8; we’ll be coming back to many of them in the next few months.

A marker trait is a trait which does not have behavior of its own; it simply specifies that certain kinds of invariants are enforced, which the compiler can then require types to implement before it will allow certain kinds of behavior. These are not special in that they’re not restricted to language built-ins or anything like that. You can implement your own and define what makes that trait hold or not in your own code, and get the same kinds of guarantees. (For an example tied to a recent episode, Diesel actually uses marker traits internally – for example, to segregate certain kinds of SQL expressions from each other.)

In the standard library, though, there are four of these market traits: Copy, Send, Sized, and Sync. There’s also the PhantomData struct type. We’ve already seen Copy, in episode 14’s discussion of strings, and we’ll come back to Sized and PhantomData in the future. For today, we’re just sticking with Send and Sync.

There are two fundamental things you need to understanding about Send and Sync.

  1. They are automatically implemented for every type they can be by the compiler—they have default implementations, and you have to explicitly opt out of those if you’re somehow doing unsafe behavior yourself. For example, the Rc type expressly indicates in its own type definition that it is not Send or Sync.
  2. Closely related to that first point: Send and Sync are unsafe traits. This means that you cannot implement them “safely” – and that’s fine because your safe types will basically get them for free. You need to write the unsafe implementations of any unsafe type machinery you intend to be Send and Sync very carefully, to guarantee that the relevant invariants hold (i.e. that you can’t get data races etc.).

We’re not going to dig further into the details of how you set up those invariants on your types today—it really only matters when you’re building your own implementations which are necessarily in the “trust me, I know what I’m doing category” of unsafe code, because they’re unsafe traits. Instead, we’re just going to talk about what the two traits are and then look at how we can put them to use together. However, you should note that if you do need to implement a new, low-level type which is Send or Sync, the responsibility is all on you to get the implementation right, as it is with unsafe code in general. (We’ll cover unsafe probably in early April.)

Send and Sync

Send is the trait that indicates it’s safe to move data across threads.

Sync is the trait that indicates it’s safe to share data across threads.

Put another way: Send is about cross-thread ownership, and Sync is about cross-thread borrows. A type T needs to be Send to be handed to a cross-thread function with a signature like fn do_something(with_a_t: T). A type T needs to be Sync to be handed to a cross-thread function with a signature like fn do_something(with_a_t_ref: &T).

Once that distinction is in place, the normal rules about mutability and access apply, just as they would in a single-threaded context.

So what makes a type Send-able and Sync-able?

Send rules

For a thread to be Send, it has to be

For more complex types – structs, enums, and smart pointer wrappers around them – as long as all the pieces that make up the type are also Send, the type is automatically Send. If any of the struct members or enum variants are not send, the type as a whole not Send either.

Sync rules

The briefest definition of Sync – and here I’m stealing straight from the standard library docs, because they’re really good! is that a type T is Sync when the type of a reference to T, &T, is Send. (This is why we’re talking about the two in the same episode: not only are they closely related conceptually, they’re closely related in terms of their implementation.)

To expand that out a bit, let’s thing about String. Remember that a String is a smart pointer, which is ultimately just a Vec of bytes which has some guarantees in construction so you can be sure that it’s always valid UTF-8. (If you need to brush up on strings, you can go back and listen to episode 14, where I talked about them in some detail.) When you take a reference to a String, you get an &String, but most of the time you actually get a reference to an &str slice type. If &str or the under-the-hood &Vec which &String is a special case of were not safe for Send, then String would not be Sync. But since string slices and references to Vecs are carefully implemented by the standard library to make sure they are thread-safe, you can use them this way.

As a result, the exact same rule applies for Sync types as for Send types: as long as all components of the type are Sync-safe, so is the type itself, no matter how complex; but if any of the parts of the type are not Sync-safe, neither is the type itself.

Other data structures

Of course, this isn’t all there is to say. We can also combine these with other data structures and traits we’ve talked about in the past, when we need other kinds of behavior. The strategy is to build up our abstractions from the specific, lower-level combinations of types we need. For example, we might need a reference-counted piece of data that’s shared across threads, and which needs to be possibly owned by more than one thread. With today’s context in mind, we now know that we can take a piece of data that’s both Send and Sync and put it in an Arc—an atomically reference-counted smart pointer type, which we talked about briefly in an earlier episode.

Part of what makes Arc especially useful is the guarantee we talked about above: any type whose contents are all Send or Sync are themselves automatically Send or Sync. That means that any Arc which wraps a Send and Sync type is automatically safe to use across thread boundaries. And the fact that that’s not true for Rc—because it’s not an atomic type and doesn’t maintain the right invariants for thread safety itself—is what makes Arc appropriate for cross-thread work and Rc inappropriate for it. Put in the terms I used earlier in the episode: Arc’s (internal, unsafe) implementation maintains all the invariants we need; Rc’s implementation doesn’t.

Using Send and Sync data

Once you have a piece of data that is Send (and Sync) you can use them in contexts where you’re doing multi-threaded work. For a fairly easy-to-understand example, let’s imagine processing hundreds or thousands of Markdown files. Since each file is independent of the others, we decide this problem is ripe for parallelization.

Now, I want to say up front: what I’m about to desscribe is definitely a naïve approach, and in anything remotely approaching a real-world scenario we’d want something like a thread pool to avoid totally hammering our machine. There’s a great discussion of this in ch. 20 of the second edition of the Rust book, so if you want to see how to do this right! Props to Steve Klabnik and Carol Nichols-or-Goulding for their write-up there. Listeners who have followed along with previous episodes will note that we could also sort of cheat and just use Rayon for multi-threading parallelism, but that wouldn’t help us see how Send and Sync come into play more manually.

In this case, we might have a function markdown_to_html, which just takes a string buffer – an &str – as its argument. We could just iterate over a Vec of Strings representing the Markdown files and pass them to markdown_to_html in sequence. But we want to parallelize them, so instead we’d chunk up the vector into something like the number of processor cores on our machine, and then iterate over those chunks. With each of them, we’d then use the std::thread::spawn function to spin up a new thread, and pass it a closure to execute. That would read something like this, if we had a current Markdown item named md:

let first = std::thread::spawn(|| markdown_to_html(&md));

We’d do the same with the others, and push them into some other container to look them up later, and once we finished iterating through everything, we’d call join on the thread references we saved, and every time we did we’d get back

What’s interesting here isn’t so much the std::thread APIs – though it’s nice to have at least mentioned them explicitly on the show now! – as it is the fact that all of this can be guaranteed to be thread-safe by the compiler. If we tried to hand over a mutable reference to, say, the Vec itself on each pass through, we’d get the usual Rust complaints about there being more than one mutable reference at the same time – no different than any other context!

But if for some reason (and there are lots of times this might come up!) you were wrapping each Markdown buffer with a type which was thread-unsafe – say, a Cell or RefCell, with their non-atomic interior mutability – the Rust compiler would say, “NOPE. You can and will end up surprising yourself in a painful way with those via data races or overwriting mutable data or something like that, so you’re not allowed to do it.” When that circumstance comes up, you just have to switch to another strategy – if you need interior mutability, to something like Mutex or RWLock or one of the Atomic types if you’re getting really down into the nitty-gritty.


So now we have a pretty good starting idea how to deal with multi-threading and parallelism in Rust. Again, most of the time unless you’re implementing your own abstractions you’re actually better using one of the existing community libraries for parallelism – Rayon, which we’ve talked about in the past, or Futures and Tokio for async I/O, which we’ll talk about in the future – being the big ones. But now you know the machinery those use “under the hood,” and that’s an important part of understanding how they work and what their performance characteristics will be, and therefore when each is appropriate!


Thanks, as always, to all of this month’s sponsors. Contributors who gave at least $10 included:

  • Aaron Turon
  • Alexander Payne
  • Anthony Deschamps
  • Chris Palmer
  • Christopher Giffard
  • Behnam Esfahbod
  • Dan Abrams
  • Daniel Collin
  • David W. Allen
  • Guido Hoermann
  • Hans Fjällemark
  • Hendrik Sollich
  • John Rudnick
  • Matt Rudder
  • Nathan Sculli
  • Nick Stevens
  • Peter Tillemans
  • Olaf Leidinger
  • Oluseyi Sonaiya
  • Raph Levien
  • Shane Utt
  • and Vesa Khailavirta

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