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 begin their life as uninitialized until a value is explicitly assigned to them. Unlike C, Rust statically prevents you from ever reading them until you do:

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

```text
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:

```rust
fn main() {
	let x: i32;
	let y: i32;

	y = 1;

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

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

but this doesn't:

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

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

while this does:

```rust
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:

```rust
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 has once more become logically uninitialized, since Box is not 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 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 secret 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 trouble.

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: `std::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 raises a tricky problem. 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 uninit 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 of course returns a
pointer to uninitialized memory.

To handle this, we must use the `std::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 bit
patterns to arbitrary locations of memory can break things are basically uncountable!

Putting this all together, we get the following:

```rust
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
			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
Plain Old Data (POD; types which don't implement Drop, nor contain Drop types),
because Rust knows not to try to Drop them. Similarly you should be able to assign the POD
fields of partially initialized structs directly.

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. So every control path through
that variable's scope must initialize the value before it ends. *This includes code panicking*.
Again, POD types need not worry.

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.