rust/uninitialized.md

% Working With Uninitialized Memory

All runtime-allocated memory in a Rust program begins its life as uninitialized. In this state the value of the memory is an indeterminate pile of bits that may or may not even reflect a valid state for the type that is supposed to inhabit that location of memory. Attempting to interpret this memory as a value of any type will cause Undefined Behaviour. Do Not Do This.

Like C, all stack variables in Rust are uninitialized until a value is explicitly assigned to them. Unlike C, Rust statically prevents you from ever reading them until you do:

fn main() {
	let x: i32;
	println!("{}", x);
}

src/main.rs:3:20: 3:21 error: use of possibly uninitialized variable: `x`
src/main.rs:3     println!("{}", x);
                                 ^

This is based off of a basic branch analysis: every branch must assign a value to x before it is first used. Interestingly, Rust doesn't require the variable to be mutable to perform a delayed initialization if every branch assigns exactly once. However the analysis does not take advantage of constant analysis or anything like that. So this compiles:

fn main() {
	let x: i32;

	if true {
		x = 1;
	} else {
		x = 2;
	}

    println!("{}", x);
}

but this doesn't:

fn main() {
	let x: i32;
	if true {
		x = 1;
	}
	println!("{}", x);
}

src/main.rs:6:17: 6:18 error: use of possibly uninitialized variable: `x`
src/main.rs:6 	println!("{}", x);

while this does:

fn main() {
	let x: i32;
	if true {
		x = 1;
		println!("{}", x);
	}
	// Don't care that there are branches where it's not initialized
	// since we don't use the value in those branches
}

If a value is moved out of a variable, that variable becomes logically uninitialized if the type of the value isn't Copy. That is:

fn main() {
	let x = 0;
	let y = Box::new(0);
	let z1 = x; // x is still valid because i32 is Copy
	let z2 = y; // y is now logically uninitialized because Box isn't Copy
}

However reassigning y in this example would require y to be marked as mutable, as a Safe Rust program could observe that the value of y changed. Otherwise the variable is exactly like new.

This raises an interesting question with respect to Drop: where does Rust try to call the destructor of a variable that is conditionally initialized? It turns out that Rust actually tracks whether a type should be dropped or not at runtime. As a variable becomes initialized and uninitialized, a drop flag for that variable is set and unset. When a variable goes out of scope or is assigned a value, it evaluates whether the current value of the variable should be dropped. Of course, static analysis can remove these checks. If the compiler can prove that a value is guaranteed to be either initialized or not, then it can theoretically generate more efficient code! As such it may be desirable to structure code to have static drop semantics when possible.

As of Rust 1.0, the drop flags are actually not-so-secretly stashed in a hidden field of any type that implements Drop. The language sets the drop flag by overwriting the entire struct with a particular value. This is pretty obviously Not The Fastest and causes a bunch of trouble with optimizing code. As such work is currently under way to move the flags out onto the stack frame where they more reasonably belong. Unfortunately this work will take some time as it requires fairly substantial changes to the compiler.

So in general, Rust programs don't need to worry about uninitialized values on the stack for correctness. Although they might care for performance. Thankfully, Rust makes it easy to take control here! Uninitialized values are there, and Safe Rust lets you work with them, but you're never in danger.

One interesting exception to this rule is working with arrays. Safe Rust doesn't permit you to partially initialize an array. When you initialize an array, you can either set every value to the same thing with let x = [val; N], or you can specify each member individually with let x = [val1, val2, val3]. Unfortunately this is pretty rigid, especially if you need to initialize your array in a more incremental or dynamic way.

Unsafe Rust gives us a powerful tool to handle this problem: mem::uninitialized. This function pretends to return a value when really it does nothing at all. Using it, we can convince Rust that we have initialized a variable, allowing us to do trickier things with conditional and incremental initialization.

Unfortunately, this opens us up to all kinds of problems. Assignment has a different meaning to Rust based on whether it believes that a variable is initialized or not. If it's uninitialized, then Rust will semantically just memcopy the bits over the uninitialized ones, and do nothing else. However if Rust believes a value to be initialized, it will try to Drop the old value! Since we've tricked Rust into believing that the value is initialized, we can no longer safely use normal assignment.

This is also a problem if you're working with a raw system allocator, which returns a pointer to uninitialized memory.

To handle this, we must use the ptr module. In particular, it provides three functions that allow us to assign bytes to a location in memory without evaluating the old value: write, copy, and copy_nonoverlapping.

• ptr::write(ptr, val) takes a val and moves it into the address pointed to by ptr.
• ptr::copy(src, dest, count) copies the bits that count T's would occupy from src to dest. (this is equivalent to memmove -- note that the argument order is reversed!)
• ptr::copy_nonoverlapping(src, dest, count) does what copy does, but a little faster on the assumption that the two ranges of memory don't overlap. (this is equivalent to memcopy -- note that the argument order is reversed!)

It should go without saying that these functions, if misused, will cause serious havoc or just straight up Undefined Behaviour. The only things that these functions themselves require is that the locations you want to read and write are allocated. However the ways writing arbitrary bits to arbitrary locations of memory can break things are basically uncountable!

Putting this all together, we get the following:

fn main() {
	use std::mem;

	// size of the array is hard-coded but easy to change. This means we can't
	// use [a, b, c] syntax to initialize the array, though!
	const SIZE = 10;

	let x: [Box<u32>; SIZE];

	unsafe {
		// convince Rust that x is Totally Initialized
		x = mem::uninitialized();
		for i in 0..SIZE {
			// very carefully overwrite each index without reading it
			// NOTE: exception safety is not a concern; Box can't panic
			ptr::write(&mut x[i], Box::new(i));
		}
	}

	println!("{}", x);
}

It's worth noting that you don't need to worry about ptr::write-style shenanigans with types which don't implement Drop or contain Drop types, because Rust knows not to try to Drop them. Similarly you should be able to assign to fields of partially initialized structs directly if those fields don't contain any Drop types.

However when working with uninitialized memory you need to be ever-vigilant for Rust trying to Drop values you make like this before they're fully initialized. Every control path through that variable's scope must initialize the value before it ends, if has a destructor. This includes code panicking.

And that's about it for working with uninitialized memory! Basically nothing anywhere expects to be handed uninitialized memory, so if you're going to pass it around at all, be sure to be really careful.