New upstream version 1.27.1+dfsg1

[rustc.git] / src / doc / book / second-edition / src / ch19-01-unsafe-rust.md

## Unsafe Rust

All the code we’ve discussed so far has had Rust’s memory safety guarantees
enforced at compile time. However, Rust has a second language hidden inside it
that doesn’t enforce these memory safety guarantees: it’s called *unsafe Rust*
and works just like regular Rust, but gives us extra superpowers.

Unsafe Rust exists because, by nature, static analysis is conservative. When
the compiler tries to determine whether or not code upholds the guarantees,
it’s better for it to reject some valid programs rather than accepting some
invalid programs. Although the code might be okay, as far as Rust is able to
tell, it’s not! In these cases, we can use unsafe code to tell the compiler,
“trust me, I know what I’m doing.” The downside is that we use it at our own
risk: if we use unsafe code incorrectly, problems due to memory unsafety, such
as null pointer dereferencing, can occur.

Another reason Rust has an unsafe alter ego is that the underlying computer
hardware is inherently unsafe. If Rust didn’t let us do unsafe operations, we
couldn’t do certain tasks. Rust needs to allow us to do low-level systems
programming, such as directly interacting with the operating system or even
writing our own operating system. Working with low-level systems programming is
one of the goals of the language. Let’s explore what we can do with unsafe Rust
and how to do it.

### Unsafe Superpowers

To switch to unsafe Rust, we use the `unsafe` keyword, and then start a new
block that holds the unsafe code. We can take four actions in unsafe Rust,
which we call *unsafe superpowers*, that we can’t in safe Rust. Those
superpowers include the ability to:

* Dereference a raw pointer
* Call an unsafe function or method
* Access or modify a mutable static variable
* Implement an unsafe trait

It’s important to understand that `unsafe` doesn’t turn off the borrow checker
or disable any other of Rust’s safety checks: if you use a reference in unsafe
code, it will still be checked. The `unsafe` keyword only gives us access to
these four features that are then not checked by the compiler for memory
safety. We still get some degree of safety inside of an unsafe block.

In addition, `unsafe` does not mean the code inside the block is necessarily
dangerous or that it will definitely have memory safety problems: the intent is
that as the programmer, we’ll ensure the code inside an `unsafe` block will
access memory in a valid way.

People are fallible, and mistakes will happen, but by requiring these four
unsafe operations to be inside blocks annotated with `unsafe` we’ll know that
any errors related to memory safety must be within an `unsafe` block. Keep
`unsafe` blocks small; you’ll be thankful later when you investigate memory
bugs.

To isolate unsafe code as much as possible, it’s best to enclose unsafe code
within a safe abstraction and provide a safe API, which we’ll discuss later in
the chapter when we examine unsafe functions and methods. Parts of the standard
library are implemented as safe abstractions over unsafe code that has been
audited. Wrapping unsafe code in a safe abstraction prevents uses of `unsafe`
from leaking out into all the places that you or your users might want to use
the functionality implemented with `unsafe` code, because using a safe
abstraction is safe.

Let’s look at each of the four unsafe superpowers in turn: we’ll also look at
some abstractions that provide a safe interface to unsafe code.

### Dereferencing a Raw Pointer

In Chapter 4, in the “Dangling References” section, we mentioned that the
compiler ensures references are always valid. Unsafe Rust has two new types
called *raw pointers* that are similar to references. As with references, raw
pointers can be immutable or mutable and are written as `*const T` and `*mut
T`, respectively. The asterisk isn’t the dereference operator; it’s part of the
type name. In the context of raw pointers, “immutable” means that the pointer
can’t be directly assigned to after being dereferenced.

Different from references and smart pointers, keep in mind that raw pointers:

* Are allowed to ignore the borrowing rules by having both immutable and
  mutable pointers or multiple mutable pointers to the same location
* Aren’t guaranteed to point to valid memory
* Are allowed to be null
* Don’t implement any automatic cleanup

By opting out of having Rust enforce these guarantees, we can make the
trade-off of giving up guaranteed safety to gain performance or the ability to
interface with another language or hardware where Rust’s guarantees don’t apply.

Listing 19-1 shows how to create an immutable and a mutable raw pointer from
references.

```rust
let mut num = 5;

let r1 = &num as *const i32;
let r2 = &mut num as *mut i32;
```

<span class="caption">Listing 19-1: Creating raw pointers from references</span>

Notice that we don’t include the `unsafe` keyword in this code. We can create
raw pointers in safe code; we just can’t dereference raw pointers outside an
unsafe block, as you’ll see in a bit.

We’ve created raw pointers by using `as` to cast an immutable and a mutable
reference into their corresponding raw pointer types. Because we created them
directly from references guaranteed to be valid, we know these particular raw
pointers are valid, but we can’t make that assumption about just any raw
pointer.

Next, we’ll create a raw pointer whose validity we can’t be so certain of.
Listing 19-2 shows how to create a raw pointer to an arbitrary location in
memory. Trying to use arbitrary memory is undefined: there might be data at
that address or there might not, the compiler might optimize the code so there
is no memory access, or the program might error with a segmentation fault.
Usually, there is no good reason to write code like this, but it is possible:

```rust
let address = 0x012345usize;
let r = address as *const i32;
```

<span class="caption">Listing 19-2: Creating a raw pointer to an arbitrary
memory address</span>

Recall that we can create raw pointers in safe code, but we can’t *dereference*
raw pointers and read the data being pointed to. In Listing 19-3, we use the
dereference operator `*` on a raw pointer that requires an `unsafe` block.

```rust
let mut num = 5;

let r1 = &num as *const i32;
let r2 = &mut num as *mut i32;

unsafe {
    println!("r1 is: {}", *r1);
    println!("r2 is: {}", *r2);
}
```

<span class="caption">Listing 19-3: Dereferencing raw pointers within an
`unsafe` block</span>

Creating a pointer does no harm; it’s only when we try to access the value that
it points at that we might end up dealing with an invalid value.

Note also that in Listing 19-1 and 19-3 we created `*const i32` and `*mut i32`
raw pointers that both pointed to the same memory location, where `num` is
stored. If we instead tried to create an immutable and a mutable reference to
`num`, the code would not have compiled because Rust’s ownership rules don’t
allow a mutable reference at the same time as any immutable references. With
raw pointers, we can create a mutable pointer and an immutable pointer to the
same location, and change data through the mutable pointer, potentially
creating a data race. Be careful!

With all of these dangers, why would we ever use raw pointers? One major use
case is when interfacing with C code, as you’ll see in the next section,
“Calling an Unsafe Function or Method.” Another case is when building up safe
abstractions that the borrow checker doesn’t understand. We’ll introduce unsafe
functions and then look at an example of a safe abstraction that uses unsafe
code.

### Calling an Unsafe Function or Method

The second type of operation that requires an unsafe block is calls to unsafe
functions. Unsafe functions and methods look exactly like regular functions and
methods, but they have an extra `unsafe` before the rest of the definition. The
`unsafe` keyword in this context indicates the function has requirements we
need to uphold when we call this function, because Rust can’t guarantee we’ve
met these requirements. By calling an unsafe function within an `unsafe` block,
we’re saying that we’ve read this function’s documentation and take
responsibility for upholding the function’s contracts.

Here is an unsafe function named `dangerous` that doesn’t do anything in its
body:

```rust
unsafe fn dangerous() {}

unsafe {
    dangerous();
}
```

We must call the `dangerous` function within a separate `unsafe` block. If we
try to call `dangerous` without the `unsafe` block, we’ll get an error:

```text
error[E0133]: call to unsafe function requires unsafe function or block
 -->
  |
4 |     dangerous();
  |     ^^^^^^^^^^^ call to unsafe function
```

By inserting the `unsafe` block around our call to `dangerous`, we’re asserting
to Rust that we’ve read the function’s documentation, we understand how to use
it properly, and we’ve verified that we’re fulfilling the contract of the
function.

Bodies of unsafe functions are effectively `unsafe` blocks, so to perform other
unsafe operations within an unsafe function, we don’t need to add another
`unsafe` block.

#### Creating a Safe Abstraction over Unsafe Code

Just because a function contains unsafe code doesn’t mean we need to mark the
entire function as unsafe. In fact, wrapping unsafe code in a safe function is
a common abstraction. As an example, let’s study a function from the standard
library, `split_at_mut`, that requires some unsafe code and explore how we
might implement it. This safe method is defined on mutable slices: it takes one
slice and makes it two by splitting the slice at the index given as an
argument. Listing 19-4 shows how to use `split_at_mut`.

```rust
let mut v = vec![1, 2, 3, 4, 5, 6];

let r = &mut v[..];

let (a, b) = r.split_at_mut(3);

assert_eq!(a, &mut [1, 2, 3]);
assert_eq!(b, &mut [4, 5, 6]);
```

<span class="caption">Listing 19-4: Using the safe `split_at_mut`
function</span>

We can’t implement this function using only safe Rust. An attempt might look
something like Listing 19-5, which won’t compile. For simplicity, we’ll
implement `split_at_mut` as a function rather than a method and only for slices
of `i32` values rather than for a generic type `T`.

```rust,ignore
fn split_at_mut(slice: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
    let len = slice.len();

    assert!(mid <= len);

    (&mut slice[..mid],
     &mut slice[mid..])
}
```

<span class="caption">Listing 19-5: An attempted implementation of
`split_at_mut` using only safe Rust</span>

This function first gets the total length of the slice, then it asserts that
the index given as a parameter is within the slice by checking that it’s less
than or equal to the length. The assertion means that if we pass an index that
is greater than the index to split the slice at, the function will panic before
it attempts to use that index.

Then we return two mutable slices in a tuple: one from the start of the
original slice to the `mid` index and another from `mid` to the end of the
slice.

When we try to compile the code in Listing 19-5, we’ll get an error:

```text
error[E0499]: cannot borrow `*slice` as mutable more than once at a time
 -->
  |
6 |     (&mut slice[..mid],
  |           ----- first mutable borrow occurs here
7 |      &mut slice[mid..])
  |           ^^^^^ second mutable borrow occurs here
8 | }
  | - first borrow ends here
```

Rust’s borrow checker can’t understand that we’re borrowing different parts of
the slice; it only knows that we’re borrowing from the same slice twice.
Borrowing different parts of a slice is fundamentally okay because the two
slices aren’t overlapping, but Rust isn’t smart enough to know this. When we
know code is okay, but Rust doesn’t, it’s time to reach for unsafe code.

Listing 19-6 shows how to use an `unsafe` block, a raw pointer, and some calls
to unsafe functions to make the implementation of `split_at_mut` work.

```rust
use std::slice;

fn split_at_mut(slice: &mut [i32], mid: usize) -> (&mut [i32], &mut [i32]) {
    let len = slice.len();
    let ptr = slice.as_mut_ptr();

    assert!(mid <= len);

    unsafe {
        (slice::from_raw_parts_mut(ptr, mid),
         slice::from_raw_parts_mut(ptr.offset(mid as isize), len - mid))
    }
}
```

<span class="caption">Listing 19-6: Using unsafe code in the implementation of
the `split_at_mut` function</span>

Recall from “The Slice Type” section in Chapter 4 that slices are a pointer to
some data and the length of the slice. We use the `len` method to get the
length of a slice and the `as_mut_ptr` method to access the raw pointer of a
slice. In this case, because we have a mutable slice to `i32` values,
`as_mut_ptr` returns a raw pointer with the type `*mut i32`, which we’ve stored
in the variable `ptr`.

We keep the assertion that the `mid` index is within the slice. Then we get to
the unsafe code: the `slice::from_raw_parts_mut` function takes a raw pointer
and a length, and creates a slice. We use this function to create a slice that
starts from `ptr` and is `mid` items long. Then we call the `offset` method on
`ptr` with `mid` as an argument to get a raw pointer that starts at `mid`, and
we create a slice using that pointer and the remaining number of items after
`mid` as the length.

The function `slice::from_raw_parts_mut` is unsafe because it takes a raw
pointer and must trust that this pointer is valid. The `offset` method on raw
pointers is also unsafe, because it must trust that the offset location is also
a valid pointer. Therefore, we had to put an `unsafe` block around our calls to
`slice::from_raw_parts_mut` and `offset` so we could call them. By looking at
the code and by adding the assertion that `mid` must be less than or equal to
`len`, we can tell that all the raw pointers used within the `unsafe` block
will be valid pointers to data within the slice. This is an acceptable and
appropriate use of `unsafe`.

Note that we don’t need to mark the resulting `split_at_mut` function as
`unsafe`, and we can call this function from safe Rust. We’ve created a safe
abstraction to the unsafe code with an implementation of the function that uses
`unsafe` code in a safe way, because it creates only valid pointers from the
data this function has access to.

In contrast, the use of `slice::from_raw_parts_mut` in Listing 19-7 would
likely crash when the slice is used. This code takes an arbitrary memory
location and creates a slice ten thousand items long:

```rust
use std::slice;

let address = 0x012345usize;
let r = address as *mut i32;

let slice = unsafe {
    slice::from_raw_parts_mut(r, 10000)
};
```

<span class="caption">Listing 19-7: Creating a slice from an arbitrary memory
location</span>

We don’t own the memory at this arbitrary location, and there is no guarantee
that the slice this code creates contains valid `i32` values. Attempting to use
`slice` as though it’s a valid slice results in undefined behavior.

#### Using `extern` Functions to Call External Code

Sometimes, your Rust code might need to interact with code written in another
language. For this, Rust has a keyword, `extern`, that facilitates the creation
and use of a *Foreign Function Interface (FFI)*. An FFI is a way for a
programming language to define functions and enable a different (foreign)
programming language to call those functions.

Listing 19-8 demonstrates how to set up an integration with the `abs` function
from the C standard library. Functions declared within `extern` blocks are
always unsafe to call from Rust code. The reason is that other languages don’t
enforce Rust’s rules and guarantees, and Rust can’t check them, so
responsibility falls on the programmer to ensure safety.

<span class="filename">Filename: src/main.rs</span>

```rust
extern "C" {
    fn abs(input: i32) -> i32;
}

fn main() {
    unsafe {
        println!("Absolute value of -3 according to C: {}", abs(-3));
    }
}
```

<span class="caption">Listing 19-8: Declaring and calling an `extern` function
defined in another language</span>

Within the `extern "C"` block, we list the names and signatures of external
functions from another language we want to call. The `"C"` part defines which
*application binary interface (ABI)* the external function uses: the ABI
defines how to call the function at the assembly level. The `"C"` ABI is the
most common and follows the C programming language’s ABI.

> #### Calling Rust Functions from Other Languages
>
> We can also use `extern` to create an interface that allows other languages
> to call Rust functions. Instead of an `extern` block, we add the `extern`
> keyword and specify the ABI to use just before the `fn` keyword. We also need
> to add a `#[no_mangle]` annotation to tell the Rust compiler not to mangle
> the name of this function. *Mangling* is when a compiler changes the name
> we’ve given a function to a different name that contains more information for
> other parts of the compilation process to consume but is less human readable.
> Every programming language compiler mangles names slightly differently, so
> for a Rust function to be nameable by other languages, we must disable the
> Rust compiler’s name mangling.
>
> In the following example, we make the `call_from_c` function accessible from
> C code, after it’s compiled to a shared library and linked from C:
>
> ```rust
> #[no_mangle]
> pub extern "C" fn call_from_c() {
>     println!("Just called a Rust function from C!");
> }
> ```
>
> This usage of `extern` does not require `unsafe`.

### Accessing or Modifying a Mutable Static Variable

Until now, we’ve not talked about *global variables*, which Rust does support
but can be problematic with Rust’s ownership rules. If two threads are
accessing the same mutable global variable, it can cause a data race.

In Rust, global variables are called *static* variables. Listing 19-9 shows an
example declaration and use of a static variable with a string slice as a
value.

<span class="filename">Filename: src/main.rs</span>

```rust
static HELLO_WORLD: &str = "Hello, world!";

fn main() {
    println!("name is: {}", HELLO_WORLD);
}
```

<span class="caption">Listing 19-9: Defining and using an immutable static
variable</span>

Static variables are similar to constants, which we discussed in the
“Differences Between Variables and Constants” section in Chapter 3. The names
of static variables are in `SCREAMING_SNAKE_CASE` by convention, and we *must*
annotate the variable’s type, which is `&'static str` in this example. Static
variables can only store references with the `'static` lifetime, which means
the Rust compiler can figure out the lifetime; we don’t need to annotate it
explicitly. Accessing an immutable static variable is safe.

Constants and immutable static variables might seem similar, but a subtle
difference is that values in a static variable have a fixed address in memory.
Using the value will always access the same data. Constants, on the other hand,
are allowed to duplicate their data whenever they’re used.

Another difference between constants and static variables is that static
variables can be mutable. Accessing and modifying mutable static variables is
*unsafe*. Listing 19-10 shows how to declare, access, and modify a mutable
static variable named `COUNTER`.

<span class="filename">Filename: src/main.rs</span>

```rust
static mut COUNTER: u32 = 0;

fn add_to_count(inc: u32) {
    unsafe {
        COUNTER += inc;
    }
}

fn main() {
    add_to_count(3);

    unsafe {
        println!("COUNTER: {}", COUNTER);
    }
}
```

<span class="caption">Listing 19-10: Reading from or writing to a mutable
static variable is unsafe</span>

As with regular variables, we specify mutability using the `mut` keyword. Any
code that reads or writes from `COUNTER` must be within an `unsafe` block. This
code compiles and prints `COUNTER: 3` as we would expect because it’s single
threaded. Having multiple threads access `COUNTER` would likely result in data
races.

With mutable data that is globally accessible, it’s difficult to ensure there
are no data races, which is why Rust considers mutable static variables to be
unsafe. Where possible, it’s preferable to use the concurrency techniques and
thread-safe smart pointers we discussed in Chapter 16, so the compiler checks
that data accessed from different threads is done safely.

### Implementing an Unsafe Trait

The final action that only works with `unsafe` is implementing an unsafe trait.
A trait is unsafe when at least one of its methods has some invariant that the
compiler can’t verify. We can declare that a trait is `unsafe` by adding the
`unsafe` keyword before `trait`; then implementation of the trait must be
marked as `unsafe` too, as shown in Listing 19-11.

```rust
unsafe trait Foo {
    // methods go here
}

unsafe impl Foo for i32 {
    // method implementations go here
}
```

<span class="caption">Listing 19-11: Defining and implementing an unsafe
trait</span>

By using `unsafe impl`, we’re promising that we’ll uphold the invariants that
the compiler can’t verify.

As an example, recall the `Sync` and `Send` marker traits we discussed in the
“Extensible Concurrency with the `Sync` and `Send` Traits” section in Chapter
16: the compiler implements these traits automatically if our types are
composed entirely of `Send` and `Sync` types. If we implement a type that
contains a type that is not `Send` or `Sync`, such as raw pointers, and we want
to mark that type as `Send` or `Sync`, we must use `unsafe`. Rust can’t verify
that our type upholds the guarantees that it can be safely sent across threads
or accessed from multiple threads; therefore, we need to do those checks
manually and indicate as such with `unsafe`.

### When to Use Unsafe Code

Using `unsafe` to take one of the four actions (superpowers) just discussed
isn’t wrong or even frowned upon. But it is trickier to get `unsafe` code
correct because the compiler can’t help uphold memory safety. When you have a
reason to use `unsafe` code, you can do so, and having the explicit `unsafe`
annotation makes it easier to track down the source of problems if they occur.