Removed warning of rustc 1.7.0 complaining about E0252. Trait Print has to be public...

[rust-101.git] / src / part15.rs

diff --git a/src/part15.rs b/src/part15.rs
index 9ae5aaf93dd62c9508bc5119a33381490a7c3d19..79fd9b4fb843b80c59b92b871aa8f39633677558 100644 (file)
--- a/src/part15.rs
+++ b/src/part15.rs
@@ -10,7 +10,8 @@ use std::thread;
 //@ there are no data races. In Rust, shared-memory concurrency is obtained through *interior mutability*,
 //@ which we already discussed in a single-threaded context in part 12.
 //@ 
 //@ there are no data races. In Rust, shared-memory concurrency is obtained through *interior mutability*,
 //@ which we already discussed in a single-threaded context in part 12.
 //@ 

-//@ The most basic type for interior mutability that supports concurrency is [`Mutex<T>`](http://doc.rust-lang.org/stable/std/sync/struct.Mutex.html).
+//@ ## `Mutex`
+//@ The most basic type for interior mutability that supports concurrency is [`Mutex<T>`](https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html).

 //@ This type implements *critical sections* (or *locks*), but in a data-driven way: One has to specify
 //@ the type of the data that's protected by the mutex, and Rust ensures that the data is *only* accessed
 //@ through the mutex. In other words, "lock data, not code" is actually enforced by the type system, which
 //@ This type implements *critical sections* (or *locks*), but in a data-driven way: One has to specify
 //@ the type of the data that's protected by the mutex, and Rust ensures that the data is *only* accessed
 //@ through the mutex. In other words, "lock data, not code" is actually enforced by the type system, which


@@ -44,7 +45,7 @@ impl ConcurrentCounter {
         *counter = *counter + by;
         //@ At the end of the function, `counter` is dropped and the mutex is available again.
         //@ This can only happen when full ownership of the guard is given up. In particular, it is impossible for us
         *counter = *counter + by;
         //@ At the end of the function, `counter` is dropped and the mutex is available again.
         //@ This can only happen when full ownership of the guard is given up. In particular, it is impossible for us

-        //@ to borrow some of its content, release the lock of the mutex, and subsequently access the protected data without holding
+        //@ to take a reference to some of its content, release the lock of the mutex, and subsequently access the protected data without holding

         //@ the lock. Enforcing the locking discipline is expressible in the Rust type system, so we don't have to worry
         //@ about data races *even though* we are mutating shared memory!
         //@ 
         //@ the lock. Enforcing the locking discipline is expressible in the Rust type system, so we don't have to worry
         //@ about data races *even though* we are mutating shared memory!
         //@ 


@@ -102,45 +103,45 @@ pub fn main() {
 // the data inside the lock. Change the code above to do that. Try using `unwrap_or_else` for this job.
 
 //@ ## `RwLock`
 // the data inside the lock. Change the code above to do that. Try using `unwrap_or_else` for this job.
 
 //@ ## `RwLock`

-//@ Besides `Mutex`, there's also [`RwLock`](http://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which
+//@ Besides `Mutex`, there's also [`RwLock`](https://doc.rust-lang.org/stable/std/sync/struct.RwLock.html), which

 //@ provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one
 //@ provides two ways of locking: One that grants only read-only access, to any number of concurrent readers, and another one

-//@ for exclusive write access. Notice that this is the same pattern we already saw with shared vs. mutable borrows. Hence
+//@ for exclusive write access. Notice that this is the same pattern we already saw with shared vs. mutable references. Hence

 //@ another way of explaining `RwLock` is to say that it is like `RefCell`, but works even for concurrent access. Rather than
 //@ panicking when the data is already borrowed, `RwLock` will of course block the current thread until the lock is available.
 //@ In this view, `Mutex` is a stripped-down version of `RwLock` that does not distinguish readers and writers.
 
 // **Exercise 15.3**:  Change the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
 
 //@ another way of explaining `RwLock` is to say that it is like `RefCell`, but works even for concurrent access. Rather than
 //@ panicking when the data is already borrowed, `RwLock` will of course block the current thread until the lock is available.
 //@ In this view, `Mutex` is a stripped-down version of `RwLock` that does not distinguish readers and writers.
 
 // **Exercise 15.3**:  Change the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
 

-//@ ## Sync
+//@ ## `Sync`

 //@ Clearly, if we had used `RefCell` rather than `Mutex`, the code above could not work: `RefCell` is not prepared for
 //@ multiple threads trying to access the data at the same time. How does Rust make sure that we don't accidentally use
 //@ `RefCell` across multiple threads?
 //@ 
 //@ In part 13, we talked about types that are marked `Send` and thus can be moved to another thread. However, we did *not*
 //@ Clearly, if we had used `RefCell` rather than `Mutex`, the code above could not work: `RefCell` is not prepared for
 //@ multiple threads trying to access the data at the same time. How does Rust make sure that we don't accidentally use
 //@ `RefCell` across multiple threads?
 //@ 
 //@ In part 13, we talked about types that are marked `Send` and thus can be moved to another thread. However, we did *not*

-//@ talk about the question whether a borrow is `Send`. For `&mut T`, the answer is: It is `Send` whenever `T` is send.
-//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](http://doc.rust-lang.org/beta/std/mem/fn.swap.html)
-//@ the contents of two mutably borrowed values. So in terms of concurrency, sending a mutable borrow is very much like
+//@ talk about the question whether a reference is `Send`. For `&mut T`, the answer is: It is `Send` whenever `T` is send.
+//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](https://doc.rust-lang.org/stable/std/mem/fn.swap.html)
+//@ the contents of two mutable references. So in terms of concurrency, sending a mutable, unique reference is very much like

 //@ sending full ownership, in the sense that it can be used to move the object to another thread.
 //@ 
 //@ sending full ownership, in the sense that it can be used to move the object to another thread.
 //@ 

-//@ But what about `&T`, a shared borrow? Without interior mutability, it would always be all-right to send such values.
+//@ But what about `&T`, a shared reference? Without interior mutability, it would always be all-right to send such values.

 //@ After all, no mutation can be performed, so there can be as many threads accessing the data as we like. In the
 //@ presence of interior mutability though, the story gets more complicated. Rust introduces another marker trait for
 //@ this purpose: `Sync`. A type `T` is `Sync` if and only if `&T` is `Send`. Just like `Send`, `Sync` has a default implementation
 //@ and is thus automatically implemented for a data-structure *if* all its members implement it.
 //@ 
 //@ After all, no mutation can be performed, so there can be as many threads accessing the data as we like. In the
 //@ presence of interior mutability though, the story gets more complicated. Rust introduces another marker trait for
 //@ this purpose: `Sync`. A type `T` is `Sync` if and only if `&T` is `Send`. Just like `Send`, `Sync` has a default implementation
 //@ and is thus automatically implemented for a data-structure *if* all its members implement it.
 //@ 

-//@ Since `Arc` provides multiple threads with a shared borrow of its content, `Arc<T>` is only `Send` if `T` is `Sync`.
+//@ Since `Arc` provides multiple threads with a shared reference to its content, `Arc<T>` is only `Send` if `T` is `Sync`.

 //@ So if we had used `RefCell` above, which is *not* `Sync`, Rust would have caught that mistake. Notice however that
 //@ `RefCell` *is* `Send`: If ownership of the entire cell is moved to another thread, it is still not possible for several
 //@ threads to try to access the data at the same time.
 //@ 
 //@ So if we had used `RefCell` above, which is *not* `Sync`, Rust would have caught that mistake. Notice however that
 //@ `RefCell` *is* `Send`: If ownership of the entire cell is moved to another thread, it is still not possible for several
 //@ threads to try to access the data at the same time.
 //@ 

-//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a shared borrow is good enough
-//@ for cloning, and we don't want other threads to clone our local `Rc`, so it must not be `Sync`. The rule of `Mutex`
-//@ is to enforce synchronization, so it should not be entirely surprising that `Mutex<T>` is `Send` *and* `Sync` provided that
-//@ `T` is `Send`.
+//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a shared reference is good enough
+//@ for cloning, and we don't want other threads to clone our local `Rc` (they would race for updating the reference count),
+//@ so it must not be `Sync`. The rule of `Mutex` is to enforce synchronization, so it should not be entirely surprising that
+//@ `Mutex<T>` is `Send` *and* `Sync` provided that `T` is `Send`.

 //@ 
 //@ You may be curious whether there is a type that's `Sync`, but not `Send`. There are indeed rather esoteric examples
 //@ of such types, but that's not a topic I want to go into. In case you are curious, there's a
 //@ [Rust RFC](https://github.com/rust-lang/rfcs/blob/master/text/0458-send-improvements.md), which contains a type `RcMut` that would be `Sync` and not `Send`.
 //@ You may also be interested in [this blog post](https://huonw.github.io/blog/2015/02/some-notes-on-send-and-sync/) on the topic.
 
 //@ 
 //@ You may be curious whether there is a type that's `Sync`, but not `Send`. There are indeed rather esoteric examples
 //@ of such types, but that's not a topic I want to go into. In case you are curious, there's a
 //@ [Rust RFC](https://github.com/rust-lang/rfcs/blob/master/text/0458-send-improvements.md), which contains a type `RcMut` that would be `Sync` and not `Send`.
 //@ You may also be interested in [this blog post](https://huonw.github.io/blog/2015/02/some-notes-on-send-and-sync/) on the topic.
 

-//@ [index](main.html) | [previous](part14.html) | [next](main.html)
+//@ [index](main.html) | [previous](part14.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part15.rs) | [next](part16.html)