From 1a691352b57b7338388ff568403495ecb44272eb Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sun, 19 Jul 2015 12:54:44 +0200 Subject: [PATCH] finish part 16 --- README.rst | 7 +++++ src/main.rs | 2 +- src/part15.rs | 4 +-- src/part16.rs | 60 ++++++++++++++++++++++------------------- workspace/src/part16.rs | 17 ++++++------ 5 files changed, 51 insertions(+), 39 deletions(-) diff --git a/README.rst b/README.rst index 3d4188f..db3e742 100644 --- a/README.rst +++ b/README.rst @@ -28,3 +28,10 @@ details. .. _git repository: http://www.ralfj.de/git/rust-101.git .. _on GitHub: https://github.com/RalfJung/rust-101 .. _CC-BY-SA 4.0: https://creativecommons.org/licenses/by-sa/4.0/ + +Contact +------- + +If you found a bug, or want to leave a comment, please +`send me a mail `_. I'm also happy about pull +requests :) diff --git a/src/main.rs b/src/main.rs index 19abe4e..d0069e6 100644 --- a/src/main.rs +++ b/src/main.rs @@ -85,7 +85,7 @@ // * [Part 13: Concurrency, Arc, Send](part13.html) // * [Part 14: Slices, Arrays, External Dependencies](part14.html) // * [Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync](part15.html) -// * (to be continued) +// * [Part 16: Unsafe Rust, Drop](part16.html) // #![allow(dead_code, unused_imports, unused_variables, unused_mut, unreachable_code)] mod part00; diff --git a/src/part15.rs b/src/part15.rs index ef2564a..99eb3be 100644 --- a/src/part15.rs +++ b/src/part15.rs @@ -119,7 +119,7 @@ pub fn main() { //@ //@ 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) +//@ `&mut` allows moving values back and forth, it is even possible to [`swap`](http://doc.rust-lang.org/stable/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 //@ sending full ownership, in the sense that it can be used to move the object to another thread. //@ @@ -144,4 +144,4 @@ pub fn main() { //@ [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) | [next](part16.html) diff --git a/src/part16.rs b/src/part16.rs index a613430..fefccdf 100644 --- a/src/part16.rs +++ b/src/part16.rs @@ -1,11 +1,11 @@ -// Rust-101, Part 16: Unsafe, Drop (WIP) -// =============================== +// Rust-101, Part 16: Unsafe Rust, Drop +// ==================================== use std::ptr; use std::mem; use std::marker::PhantomData; -//@ As we saw, the rules Rust imposes can get us pretty far: A surprising amount of programming patterns +//@ As we saw, the rules Rust imposes to ensure memory safety can get us pretty far. A surprising amount of programming patterns //@ can be written within safe Rust, and, more importantly, library code like iterators or threads can make //@ use of the type system to ensure some level of correctness beyond basic memory safety. //@ @@ -14,16 +14,18 @@ use std::marker::PhantomData; //@ as we saw with `RefCell` - overhead which may not be acceptable. In such a situation, it is possible to //@ use *unsafe* Rust: That's a part of the language that is *known* to open the gate to invalid pointer access //@ and all other sorts of memory safety. It is typically disabled, guarded by the keyword `unsafe`. Of course, -//@ `unsafe` also means "Here Be Dragons": You are on your own now. Types like `Rc` and `Vec` are implemented -//@ `using unsafe Rust. +//@ `unsafe` also means "Here Be Dragons": You are on your own now. +//@ +//@ The goal in these cases is to confine unsafety to the local module. Types like `Rc` and `Vec` are implemented +//@ using unsafe Rust, but *using* them as we did is (believed to be) perfectly safe. //@ //@ ## Unsafe Code //@ As an example, let us write a doubly-linked list. Clearly, such a data-structure involves aliasing and mutation: //@ Every node in the list is pointed to by its left and right neighbor, but still we will want to modify the nodes -//@ (either to change the value at that place, or to insert new nodes). We could now try some clever combination of +//@ (either to change the value at that place, or to insert/delete nodes). We could now try some clever combination of //@ `Rc` and `RefCell`, but this would end up being quite annoying - and it would incur some over-head. For a low-level //@ data-structure like a doubly-linked list, it makes sense to implement an efficient version *once*, that is unsafe -//@ internally, but taht can be used without any risk by safe client code. +//@ internally, but that can be used without any risk by safe client code. //@ As usually, we start by defining the types. Everything is parameterized by the type `T` of the data stored in the list. // A node of the list consists of the data, and two node pointers for the predecessor and successor. @@ -32,9 +34,9 @@ struct Node { prev: NodePtr, data: T, } -// A node pointer is a *mutable raw point* to a node. +// A node pointer is a *mutable raw pointer* to a node. //@ Raw pointers (`*mut T` and `*const T`) are the Rust equivalent of pointers in C. Unlike borrows, they do not come with -//@ any guarantees: Raw pointers can be null, or they can point to garbage. They don't have a lifetime. +//@ any guarantees: Raw pointers can be null, or they can point to garbage. They don't have a lifetime, either. type NodePtr = *mut Node; // The linked list itself stores pointers to the first and the last node. In addition, we tell Rust that this type @@ -53,7 +55,9 @@ pub struct LinkedList { //@ Before we get to the actual linked-list methods, we write two short helper functions converting between //@ mutable raw pointers, and owned pointers (aka `Box`). Both employ `mem::transmute`, which is Rust's //@ `reinterpret_cast`: It can convert anything to anything, by just re-interpreting the bytes. Clearly, -//@ that's an unsafe operation. +//@ that's an unsafe operation and must only be used with great care. If at all possible, its use should be avoided.
+//@ We are making the assumption here that a `Box` and a raw pointer have the same representation in memory. In the future, +//@ Rust will [provide](http://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#method.from_raw) such [operations](http://doc.rust-lang.org/beta/alloc/boxed/struct.Box.html#method.into_raw) in the standard library, but the exact API is still being fleshed out. //@ We declare `raw_into_box` to be an `unsafe` function, telling Rust that calling this function is not generally safe. //@ The caller will have to ensure that `r` is a valid pointer, and that nobody else has a pointer to this data. @@ -62,8 +66,7 @@ unsafe fn raw_into_box(r: *mut T) -> Box { } //@ The case is slightly different for `box_into_raw`: Converting a `Box` to a raw pointer is always safe. I just drops some //@ information. Hence we keep the function itself safe, and use an *unsafe block* within the function. This is an (unchecked) -//@ promise to the Rust compiler, saying that even though the code inside that block *could* go wrong, we actually know that -//@ it will not. +//@ promise to the Rust compiler, saying that a safe invocation of `box_into_raw` cannot go wrong. fn box_into_raw(b: Box) -> *mut T { unsafe { mem::transmute(b) } } @@ -74,7 +77,7 @@ impl LinkedList { LinkedList { first: ptr::null_mut(), last: ptr::null_mut(), _marker: PhantomData } } - // Add a new node to the end of the list. + // This function adds a new node to the end of the list. pub fn push_back(&mut self, t: T) { // Create the new node, and make it a raw pointer. //@ Calling `box_into_raw` gives up ownership of the box, which is crucial: We don't want the @@ -89,7 +92,7 @@ impl LinkedList { } else { debug_assert!(!self.first.is_null()); // We have to update the `next` pointer of the tail node. - //@ Since Rust does not know that a raw pointer actually to anything, dereferencing such a pointer is + //@ Since Rust does not know that a raw pointer actually points to anything, dereferencing such a pointer is //@ an unsafe operation. So this unsafe block promises that the pointer will actually be valid. unsafe { (*self.last).next = new; } /*@*/ } @@ -101,7 +104,7 @@ impl LinkedList { // Add testcases for `push_back` and all of your functions. The `pop` functions should take `&mut self` // and return `Option`. - // Of course, we will also want to provide an iterator. + // Next, we are going to provide an iterator. //@ This function just creates an instance of `IterMut`, the iterator type which does the actual work. pub fn iter_mut(&self) -> IterMut { IterMut { next: self.first, _marker: PhantomData } @@ -115,7 +118,7 @@ impl LinkedList { //@ borrow used for `iter_mut`. We will thus give `IterMut` two parameters: A type parameter specifying //@ the type of the data, and a lifetime parameter specifying for how long the list was borrowed to the //@ iterator. -//@ + //@ For Rust to accept the type, we have to add two more annotations. First of all, we have to ensure that //@ the data in the list lives at least as long as the iterator: If you drop the `T: 'a`, Rust will tell //@ you to add it back. And secondly, Rust will complain if `'a` is not actually used in the struct. @@ -136,14 +139,14 @@ impl<'a, T> Iterator for IterMut<'a, T> { fn next(&mut self) -> Option { // The actual iteration is straight-forward: Once we reached a null pointer, we are done. if self.next.is_null() { - None + None } else { // Otherwise, we can convert the next pointer to a borrow, get a borrow to the data // and update the iterator. let next = unsafe { &mut *self.next }; let ret = &mut next.data; - self.next = next.next; - Some(ret) + self.next = next.next; /*@*/ + Some(ret) /*@*/ } } } @@ -160,13 +163,13 @@ impl<'a, T> Iterator for IterMut<'a, T> { // Add testcases for both kinds of iterators. // ## `Drop` -//@ The linked list we wrote is already working quite nicely, but there is one problem: When the list is removed, +//@ The linked list we wrote is already working quite nicely, but there is one problem: When the list is dropped, //@ nobody bothers to deallocate the remaining nodes. Even worse, if `T` itself has a destructor that needs to //@ clean up, it is not called for the element remaining in the list. We need to take care of that ourselves. //@ //@ In Rust, adding a destructor for a type is done by implementing the `Drop` trait. This is a very special trait. //@ It can only be implemented for *nominal types*, i.e., you cannot implement `Drop` for `&mut T`. You also cannot -//@ restrict the type and lifetime parameters further - the `Drop` implementation has to apply to *all* instances +//@ restrict the type and lifetime parameters further than the type does - the `Drop` implementation has to apply to *all* instances //@ of `LinkedList`. impl Drop for LinkedList { // The destructor itself is a method which takes `self` in mutably borrowed form. It cannot own `self`, because then @@ -185,12 +188,13 @@ impl Drop for LinkedList { } } -//@ ## The End -//@ Congratulations! You complete Rust-101. This was the last example of the last part. I hope you enjoyed it. -//@ If you have feedback, please head to the [Rust-101](https://www.ralfj.de/projects/rust-101/) website -//@ and let me know how you liked it. The entire course is open-source (under CC-BY-SA 4.0), and contributions are welcome! +// ## The End +//@ Congratulations! You completed Rust-101. This was the last part of the course. I hope you enjoyed it. +//@ If you have feedback or want to contribute yourself, please head to the [Rust-101](https://www.ralfj.de/projects/rust-101/) website +//@ fur further information. The entire course is open-source (under [CC-BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/)). //@ -//@ The [index](main.html) contains some more links to additional resources you may find useful. With that, there's -//@ only one thing left to say: Happy Rust Hacking! +//@ If you want to do more, the examples you saw in this course provide lots of playground for coming up with your own little +//@ extensions here and there. The [index](main.html) contains some more links to additional resources you may find useful. +//@ With that, there's only one thing left to say: Happy Rust Hacking! -//@ [index](main.html) | [previous](part15.html) +//@ [index](main.html) | [previous](part15.html) | next diff --git a/workspace/src/part16.rs b/workspace/src/part16.rs index a804f8e..d0b173a 100644 --- a/workspace/src/part16.rs +++ b/workspace/src/part16.rs @@ -1,5 +1,5 @@ -// Rust-101, Part 16: Unsafe, Drop (WIP) -// =============================== +// Rust-101, Part 16: Unsafe Rust, Drop +// ==================================== use std::ptr; use std::mem; @@ -12,7 +12,7 @@ struct Node { prev: NodePtr, data: T, } -// A node pointer is a *mutable raw point* to a node. +// A node pointer is a *mutable raw pointer* to a node. type NodePtr = *mut Node; // The linked list itself stores pointers to the first and the last node. In addition, we tell Rust that this type @@ -37,7 +37,7 @@ impl LinkedList { LinkedList { first: ptr::null_mut(), last: ptr::null_mut(), _marker: PhantomData } } - // Add a new node to the end of the list. + // This function adds a new node to the end of the list. pub fn push_back(&mut self, t: T) { // Create the new node, and make it a raw pointer. let new = Box::new( Node { data: t, next: ptr::null_mut(), prev: self.last } ); @@ -60,12 +60,13 @@ impl LinkedList { // Add testcases for `push_back` and all of your functions. The `pop` functions should take `&mut self` // and return `Option`. - // Of course, we will also want to provide an iterator. + // Next, we are going to provide an iterator. pub fn iter_mut(&self) -> IterMut { IterMut { next: self.first, _marker: PhantomData } } } + pub struct IterMut<'a, T> where T: 'a { next: NodePtr, _marker: PhantomData<&'a mut LinkedList>, @@ -77,14 +78,13 @@ impl<'a, T> Iterator for IterMut<'a, T> { fn next(&mut self) -> Option { // The actual iteration is straight-forward: Once we reached a null pointer, we are done. if self.next.is_null() { - None + None } else { // Otherwise, we can convert the next pointer to a borrow, get a borrow to the data // and update the iterator. let next = unsafe { &mut *self.next }; let ret = &mut next.data; - self.next = next.next; - Some(ret) + unimplemented!() } } } @@ -110,4 +110,5 @@ impl Drop for LinkedList { } } +// ## The End -- 2.30.2