1. The document discusses Rust concepts like ownership, borrowing, cloning, copying, mutable references, threads and channels for parallelism, and shared memory using Arc and Mutex. 2. It provides examples of using ownership and borrowing rules to manage memory, spawning threads and using channels for inter-thread communication, and using Arc and Mutex for shared mutable memory across threads. 3. The document presents these concepts as part of an overview of Rust's approach for safe systems programming without fear of crashes or data races through its ownership and borrowing model.

1. 1
Hack without fear!
Nicholas Matsakis!
Mozilla Research

2. 2
Systems programming without the hassle
crashes!
heisenbugs!
fear
Parallel!

3. The Plan
3
1. Sequential search
2. Spawn threads
3. Channels
4. Shared data
0. Hello, world!
5. Mutex

4. Hello, world!
4
smallcultfollowing.com/20151202/
!
Example “Hello World”
fn main() {
println!(“Hello, world!”);
}

5. Ownership!
!
n. The act, state, or right of possessing something.
5
Borrow!
!
v. To receive something with the promise of returning it.

6. Ownership/Borrowing
Memory
safety
Data-race
freedom
No need for
a runtime
GCC++
6

7. Ownership
7

8. fn main() {
let name = format!(“…”);
helper(name);
helper(name);
}
fn helper(name: String) {
println!(..);
}
!
!
!
Ownership
Take ownership
of a String
8
Error: use of moved value: `name`

9. void main() {
Vector name = …;
helper(name);
helper(name);
}
void helper(Vector name) {
…
}
!
!
!
“Ownership” in Java
Take reference
to Vector
9
new Thread(…);

10. Borrowing
10

11. fn main() {
let name = format!(“…”);
helper(&name);
helper(&name);
}
Shared borrow
Take a reference
to a String
11
Lend the string
fn helper(name: &String) {
println!(..);
}
!
!
!

12. Borrowing Exercise
12
Example “Borrowing”

13. 13
fn main() {
let name = format!(“…”);
helper(&name);
helper(&name);
}
fn helper(name: &String) {
thread::spawn(…);
}
http://is.gd/cEeyzx
However: see crossbeam,
simple_parallel, etc on
crates.io

14. Clone
14
fn main() {
let name = format!(“…”);
helper(name.clone());
helper(name);
}
fn helper(name: String) {
println!(..);
}
!
!
!
Copy the String

15. Copy (auto-Clone)
15
fn main() {
let name = 22;
helper(name);
helper(name);
}
fn helper(name: i32) {
println!(..);
}
!
!
!
i32 is a Copy type

16. 16
Default: Type cannot be copied.
Values move from place to place.
Example: File descriptor.
!
Clone: Type is expensive to copy,
so make it explicit by calling clone().
Examples: Vector, hashtable.!
!
Copy: Type is implicitly copied
whenever it is referenced.
Examples: u32, i32, (f32, i32).

17. The Plan
17
1. Sequential search
2. Spawn threads
0. Hello, world!
3. Channels
4. Shared data
5. Mutex

18. 18
Shop till you drop!
By QuentinUK (Own work), via Wikimedia Commons

19. 19
$5.0
$25.5
$81.5
———
$112.0
$5.0
$20.5
$81.5
———
$107.0
$5.0
$23.5
$81.5
———
$110.0

20. Declaring a structure
20
use std::collections::HashMap;
!
struct Store {
name: String,
prices: HashMap<String, f32>,
}
Store
name
prices
String
HashMap
String f32
String f32

21. Standard traits
21
#[derive(Clone, Debug)]
struct Store {
name: String,
prices: HashMap<String, f32>,
}
Clone create explicit copies by writing `foo.clone()`
Copy create implicit copies (requires Clone)
Debug debug printing with `println!(“{:?}”, foo)`
PartialEq equality comparisons (`foo == bar`)
PartialOrd inequality comparisons (`foo > bar` etc)
…
Hash hashing for a hashmap

22. Methods
22
struct Store { .. }
!
impl Store {
fn add_item(&mut self, name: String, price: f32) {
self.prices.insert(name, price);
}
!
fn price(&self, item_name: &str) -> f32 {
self.prices[item_name]
}
}
store.add_item(…); // must be let mut
store.price(…); // let OR let mut
itself an &mut method

23. Methods
23
struct Store { .. }
!
impl Store {
fn new(name: String) -> Store {
Store {
name: name,
prices: HashMap::new(),
}
}
}
Store::new(some_name)

24. 24
fn build_stores() -> Vec<Store> {
let mut stores = vec![];
!
let mut store = Store::new(format!("R-mart"));
store.add_item(format!("chocolate"), 5.0);
store.add_item(format!("doll"), 22.0);
store.add_item(format!("bike"), 150.0);
stores.push(store);
!
…
!
stores // or `return stores`, as you prefer
}

25. Basic Loops
25
let mut i = 0;
while i < 3 {
println!(“{:?}”, i);
i += 1;
}
println!(“Once you enter…”);
loop {
println!(“…you can never leave”);
!
}
!
!
!
break; // continue works too
!
println!(“…oh, I guess that works. nm.”);

26. For Loops
26
fn main() {
let v = vec![format!(“Alpha”),
format!(“Beta”),
format!(“Gamma”)];
for s in v {
println!(“{:?}”, s);
}
}
http://is.gd/6kJc0O
“Alpha”
“Beta”
“Gamma”
v: s:
String

27. For Loops
27
fn main() {
let v = vec![format!(“Alpha”),
format!(“Beta”),
format!(“Gamma”)];
!
for s in &v {
println!(“{:?}”, s);
}
!
for s in &v {
println!(“{:?}”, s);
}
}
“Alpha”
“Beta”
“Gamma”
v:
s:
&String

28. Iterators
28
for s in v.iter()
.filter(|s| s.len() > 2) {
// only strings greater than length 2
println!(“{:?}”, s);
}
(IntoIterator trait)

29. Options and Enums
29
enum Option<T> {
Some(T),
None
}
fn main() {
let v: Option<i32> = Some(22);
match v {
Some(x) => println!(“v = {}”, x),
None => println!(“v = None”),
}
println!(“v = {}”, v.unwrap()); // risky
}
http://is.gd/Gfum32

30. Exercise: Sequential Search
30
for s in v { … }
for s in &v { … }
enum Option<T> {
Some(T),
None
}
x.unwrap() is:
match x {
Some(v) => v,
None => panic!()
}
std::f32::INFINITY
doc.rust-lang.org/std/
if x != 0 { … }
while x != 0 { … }
println!(“{:?}”, x)
let x = Some(22);
let x = None;

31. The Plan
31
1. Sequential search
2. Spawn threads
0. Hello, world!
3. Channels
4. Shared data
5. Mutex

32. 32
for store in stores {
let sum = compute_sum(&store, shopping_list);
if sum < best_price {
best = Some(store.name);
best_price = sum;
}
}

33. for store in stores {
let sum = compute_sum(&store, shopping_list);
if sum < best_price {
best = Some(store.name);
best_price = sum;
}
}
33

34. 34
use std::thread;
…
!
for store in stores {
let handle =
thread::spawn(
move || compute_sum(&store, shopping_list));
…
}
Closure
takes ownership
of variables it uses.
Variables used by
this closure.

35. 35
use std::thread;
…
!
for store in stores {
let handle =
thread::spawn(
move || compute_sum(&store, shopping_list));
let sum = handle.join().unwrap();
…
}
Handle to the
thread we spawned.
Wait for thread
to finish and
get return value.
Thread may have
panicked. Propagate.
Result<f32, Error>

36. 36
use std::thread;
…
!
for store in stores {
let handle =
thread::spawn(…);
let sum = handle.join().unwrap();
…
}
R-Mart
Bullseye
Woolmart
22
44

37. Exercise: Parallel Search
37
let mut v = vec![];
v.push(…);
for item in v { }
enum Option<T> {
Some(T),
None
}
x.unwrap() is:
match x {
Some(v) => v,
None => panic!()
}
doc.rust-lang.org/std/
if x != 0 { … }
while x != 0 { … }
println!(“{:?}”, x)
let x = Some(22);
let x = None;

38. The Plan
38
1. Sequential search
2. Spawn threads
0. Hello, world!
3. Channels
4. Shared data
5. Mutex

39. 39
Joining a thread allows
thread to send one result.
What if we wanted
multiple results?
Or if we wanted
a response?

40. MessageMessage
rx
tx
tx
m
fn parent() {
let (tx, rx) = channel();
spawn(move || {…});
let m = rx.recv().unwrap();
}
40
move || {
let m = Message::new();
…
tx.send(m).unwrap();
}

41. 41
rx0
tx0
let (tx0, rx0) = channel();
let tx1 = tx0.clone();
spawn(move || /* omitted */);
let tx2 = tx0.clone();
spawn(move || /* omitted */);
mem::drop(tx0);
tx1
tx2

42. 42
for value in rx {
use(value);
}
loop {
let value = match rx.recv() {
Ok(v) => v,
Err(mpsc::RecvError) => break,
};
use(value);
}
while let Ok(value) = rx.recv() {
use(value);
}

43. 43
let (tx1, rx1) = channel();
let message = …;
tx0.send((m, tx1)).unwrap();
let response = rx0.recv.unwrap();
rx0
tx0 rx1
message
tx1
response
let (tx0, rx0) = channel();
spawn(move || /* see lower-right corner below */);
let (message, tx1) = rx0.recv().unwrap();
tx1.send(response).unwrap();

44. Exercise: Channels
44
use std::mpsc::channel;
doc.rust-lang.org/std/
let (tx, rx) = channel();
tx.send(m).unwrap();
let m = rx.recv().unwrap();
let tx1 = tx.clone();
for m in rx { … }
use std::mem::drop;

45. The Plan
45
1. Sequential search
2. Spawn threads
0. Hello, world!
3. Channels
4. Shared data
5. Mutex

46. 46

47. 47
use std::sync::Arc;
let shopping_list: Vec<ShoppingList> = …;
let arc1 = Arc::new(shopping_list);
let arc2 = arc1.clone();
let data = &arc1[0];
arc1
arc2
data

48. Arc => Immutable
48
use std::sync::Arc;
let shopping_list: Vec<ShoppingList> = …;
let arc1 = Arc::new(shopping_list);
let data = &mut arc1[0];
<anon>:6:21: 6:24 error: cannot borrow immutable borrowed
content as mutable
<anon>:6 let data = &mut arc[0];
^~~
http://is.gd/nP3Pvb

49. 49
It’s a
dangling
pointer!
No, it’s a
data race!
No, it’s a
double free!
Simultaneous
Sharing and
Mutation
No,
it’s iterator
invalidation!

50. Exercise: Shared Memory
50
use std::sync::Arc;
doc.rust-lang.org/std/
let arc1 = Arc::new(…);
let arc2 = arc1.clone();

51. The Plan
51
1. Sequential search
2. Spawn threads
0. Hello, world!
3. Channels
4. Shared data
5. Mutex

52. 52
Start threads
Compute sums
Compare sums

53. 53https://www.flickr.com/photos/mabi/38871148

54. Data races
Sharing
Mutation
No ordering
Data race
Job of the API
54
The default path.

55. 0
55
let counter = Mutex::new(0);
{
let mut data = counter.lock().unwrap();
*data += 1;
}
counter
data
https://commons.wikimedia.org/wiki/File:No-DRM_lock.svg
1

56. 56
let counter = Mutex::new(0);
let arc1 = Arc::new(counter);
let arc2 = arc1.clone();
let mut data = arc1.lock();
*data += 1;
0
https://commons.wikimedia.org/wiki/File:No-DRM_lock.svg
arc1
arc2
data

57. Exercise: Mutex
57
doc.rust-lang.org/std/
let counter = Mutex::new(0);
let arc1 = Arc::new(counter);
let arc2 = arc1.clone();
let mut data = arc1.lock();
*data += 1;

58. 58
Thanks for listening!