From C to Rust

So let's start with something simple: we have some struct that we want to initialize with some values. The values in that struct don't require allocation themselves and we want to allow passing this final value around. Where it's allocated doesn't matter to us, let's just put it on the stack for this example. The idea is that after the initialization that thing can be passed around safely and printed.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

struct role {
    const char *name;
    bool disabled;
    int flag;
};

int main() {
    struct role r;
    r.name = "basic";
    r.flag = 1;
    r.disabled = false;
    printf("%s (%d, %s)\n", r.name, r.flag, r.disabled ? "true" : "false");
}

Now let's write this in Rust. Let's not read the docs too much, let's just do a 1:1 translation to more or less the same but by using unsafe. One note here before you read the code: we're purposefully trying to create an object that looks familiar to Rust programmers and can be seen as public API. So we use a &'static str here instead of a C string so there are some changes to the C code.

use std::mem;

struct Role {
    name: &'static str,
    disabled: bool,
    flag: u32,
}

fn main() {
    let role = unsafe {
        let mut role: Role = mem::zeroed();
        role.name = "basic";
        role.flag = 1;
        role.disabled = false;
        role
    };

    println!("{} ({}, {})", role.name, role.flag, role.disabled);
}

So immediately one will ask why unsafe is needed here and the answer is that of course you don't need it here. However this code is also using a suboptimal function: std::mem::zeroed. If you run this on a recent Rust compiler you will get this result:

thread 'main' panicked at 'attempted to zero-initialize type `Role`, which is invalid', src/main.rs:11:30

On older Rust compilers this code will run but it was never really correct. So how do we solve this? The compiler already tells us that we need to use something else:

warning: the type `Role` does not permit zero-initialization
  --> src/main.rs:11:30
   |
11 | let mut role: Role = mem::zeroed();
   |                      ^^^^^^^^^^^^^
   |                      |
   |                      this code causes undefined behavior when executed
   |                      help: use `MaybeUninit<T>` instead, and only call
   |                         `assume_init` after initialization is done
   |

So why does this type not support zero initialization? What do we have to change? Can zeroed not be used at all? Some of you might think that the answer is #[repr(C)] on the struct to force a C layout but that won't solve the problem. We in fact need to reach for MaybeUninit as the compiler indicates. So let's try that first and then afterwards we figure out why we need it:

use std::mem::MaybeUninit;

struct Role {
    name: &'static str,
    disabled: bool,
    flag: u32,
}

fn main() {
    let role = unsafe {
        let mut uninit = MaybeUninit::<Role>::uninit();
        let role = uninit.as_mut_ptr();
        (*role).name = "basic";
        (*role).flag = 1;
        (*role).disabled = false;
        uninit.assume_init()
    };

    println!("{} ({}, {})", role.name, role.flag, role.disabled);
}

By swapping out zeroed for MaybeUninit everything changes. We can no longer manipulate our struct directly, we now need to manipulate a raw pointer. Because that raw pointer does not implement deref and because Rust has no -> operator we now need to dereference the pointer permanently to assign the fields with that awkward syntax.

So first of all: why does this work now and what changed? The answer lies in the fact that any construct like a mutable reference (&mut) or value on the stack in itself (even in unsafe) that would be valid outside of unsafe code still needs to be in a valid state at all times. zeroed returns a zeroed struct and there is no guarantee that this is a valid representation of either the struct or the fields within it. So in particular our &'static str reference is definitely not valid all zeroed out.

A mutable reference must also never point to an invalid object, so doing let role = &mut uninit.as_mut_ptr() if that object is not fully initialized is also wrong.

So let's just accept that MaybeUninit is necessary and we need to deal with raw references here. It's somewhat cumbersome but it doesn't look too bad. Unfortunately we're still using it wrong. Remember how I mentioned that creating “safe things” that don't uphold the guarantees of that safe thing is not allowed, even in unsafe code? We're in fact having exactly this happen in our code. For instance (*role).name creates a &mut str behind the scenes which is illegal, even if we can't observe it because the memory where it points to is not initialized.

So now we have two new problems: we know that &mut X is not allowed, but *mut X is. How do we get this? Ironically until Rust 1.51 it was impossible to construct such a thing without breaking the rules. Today you can use the addr_of_mut! macro. So we can do this:

let name_ptr = std::ptr::addr_of_mut!((*role).name);

Great, so now we have this pointer. How do we write into it? Can't you just dereference and assign?

let name_ptr = std::ptr::addr_of_mut!((*role).name);
*name_ptr = "basic";

Again, dereferencing is illegal, so we need to do something else. We can use the write method instead:

addr_of_mut!((*role).name).write("basic");

Are we okay now? Remember how we used a regular struct? If we read the documentation we learn that there are no guarantees of such a struct at all. I'm pretty sure we can depend on things being aligned as even the original motivating GitHub issue only calls out #[repr(packed)] but let's be better safe than sorry. So we now either change to #[repr(C)] or we use write_unaligned instead which is legal if Rust were to pick for a member of the struct to be unaligned. So this could be the final version:

use std::mem::MaybeUninit;
use std::ptr::addr_of_mut;

struct Role {
    name: &'static str,
    disabled: bool,
    flag: u32,
}

fn main() {
    let role = unsafe {
        let mut uninit = MaybeUninit::<Role>::uninit();
        let role = uninit.as_mut_ptr();

        addr_of_mut!((*role).name).write_unaligned("basic");
        addr_of_mut!((*role).flag).write_unaligned(1);
        addr_of_mut!((*role).disabled).write_unaligned(false);

        uninit.assume_init()
    };

    println!("{} ({}, {})", role.name, role.flag, role.disabled);
}

Is my Unsafe Correct?

It's 2022 and I will admit that I no longer feel confident writing unsafe Rust code. The rules were probably always complex but I know from reading a lot of unsafe Rust code over many years that most unsafe code just did not care about those rules and just disregarded them. There is a reason that addr_of_mut! did not get added to the language until 1.53. Even today the docs both say there are no guarantees on the alignment on native rust struct reprs yet a lot of code assumes now that write rather than write_unaligned is legal.

Over the last few years it seem to have happened that the Rust developers has made writing unsafe Rust harder in practice and the rules are so complex now that it's very hard to understand for a casual programmer. This has made one of Rust's best features less and less approachable.

I'm no longer think this is good. In fact, I believe this is not at all a great trend. C interop is a bit part of what made Rust great, and that we're creating such massive barriers should be seen as undesirable. More importantly: the compiler is not helpful in pointing out when I'm doing something wrong. The compiler does not warn that not using addr_of_mut! is wrong. It also does not warn if I'm using write instead of write_unaligned and even consulting the docs does not clarify this.

Making unsafe more ergonomic is a hard problem for sure but it might be worth addressing. Because one thing is clear: people won't be stopping writing unsafe code any time soon.