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 accept some invalid programs. Although the code might be okay, as far as Rust is able to tell, it’s not! In these cases, you can use unsafe code to tell the compiler, “Trust me, I know what I’m doing.” The downside is that you use it at your own risk: if you 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 you do unsafe operations, you couldn’t do certain tasks. Rust needs to allow you to do low-level systems programming, such as directly interacting with the operating system or even writing your 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, use the unsafe keyword and then start a new block that holds the unsafe code. You can take four actions in unsafe Rust, called unsafe superpowers, that you 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 you access to these four features that are then not checked by the compiler for memory safety. You’ll 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, you’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 you’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, 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, you can give up guaranteed safety in exchange for greater 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.


# #![allow(unused_variables)]
#fn main() {
let mut num = 5;

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

Listing 19-1: Creating raw pointers from references

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.


# #![allow(unused_variables)]
#fn main() {
let address = 0x012345usize;
let r = address as *const i32;
#}

Listing 19-2: Creating a raw pointer to an arbitrary memory address

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.


# #![allow(unused_variables)]
#fn main() {
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);
}
#}

Listing 19-3: Dereferencing raw pointers within an unsafe block

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 you 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:


# #![allow(unused_variables)]
#fn main() {
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:

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.


# #![allow(unused_variables)]
#fn main() {
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]);
#}

Listing 19-4: Using the safe split_at_mut function

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.

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..])
}

Listing 19-5: An attempted implementation of split_at_mut using only safe Rust

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 whether 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.

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.


# #![allow(unused_variables)]
#fn main() {
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))
    }
}
#}

Listing 19-6: Using unsafe code in the implementation of the split_at_mut function

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 it 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 10,000 items long.


# #![allow(unused_variables)]
#fn main() {
use std::slice;

let address = 0x01234usize;
let r = address as *mut i32;

let slice : &[i32] = unsafe {
    slice::from_raw_parts_mut(r, 10000)
};
#}

Listing 19-7: Creating a slice from an arbitrary memory location

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. If we would not have taken care to align address to 4 (the alignment of i32), then even just calling slice::from_raw_parts_mut would already be undefined behavior -- slices must always be aligned, even if they are not used (and even if they are empty).

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.

Filename: src/main.rs

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

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

Listing 19-8: Declaring and calling an extern function defined in another language

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:


# #![allow(unused_variables)]
#fn main() {
#[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.

Filename: src/main.rs

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

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

Listing 19-9: Defining and using an immutable static variable

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.

Filename: src/main.rs

static mut COUNTER: u32 = 0;

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

fn main() {
    add_to_count(3);

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

Listing 19-10: Reading from or writing to a mutable static variable is unsafe

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 works only 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 and marking the implementation of the trait as unsafe too, as shown in Listing 19-11.


# #![allow(unused_variables)]
#fn main() {
unsafe trait Foo {
    // methods go here
}

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

Listing 19-11: Defining and implementing an unsafe trait

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.