this post was submitted on 15 Nov 2025
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Is it possible to cause undefined behaviour with static mut inside a function? (anarchist.nexus)
submitted 9 months ago by Val@anarchist.nexus to c/rust@programming.dev
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For example, is there any problems with doing this?

fn main() {  
	static mut BUF: [u8; 0x400] = [0; 0x400];  
	let buf = &mut unsafe { BUF };  
}

and is this code the same as just using an array directly? From my understanding local variables get put on the stack but do the static variables do too?

I'm essentially trying to find the most performant way to get a simple read/write buffer.

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[–] calcopiritus@lemmy.world 1 points 9 months ago* (last edited 9 months ago)

The problem with static mut is that it allows you to create multiple mutable references. And also mix mutable and immutable references. Additionally, it is accessible by any thread. So, as long as you don't do any of that, it should be safe. Maybe I'm missing something.

If this is the entire program, it's not unsafe. But if it is just a fraction of the program, it may be unsafe. For example if 2 threads call the function at the same time. Since you would have 2 mutable references to BUF. Well, not actually unsafe since you don't use the mutable reference, only create it.

As to the other question, static variables are not in the stack. They have their own region of memory. If they were in the stack, they couldn't be accessed across threads, since each thread has its own stack.

EDIT: for completeness sake. For your last question. Yes, using a static buffer is probably more performant, since it doesn't need to be set to 0 each time it's called. However, that's not what statics are for. If what you want is just to avoid that setting to 0, there are ways to get initialized arrays. For example MybeUninit. Which would be way better.

[–] TehPers@beehaw.org 1 points 9 months ago

Another commenter already explained why this is unsound, so I'll skip that, though static mut is almost universally unsound.

Note, of course, that main() won't be called more than once, so if you can, I would honestly just make this a stack variable containing a Box<[u8; 0x400]> instead. Alternatively, a Box<[u8]> can make it simpler to pass around, and a Vec<u8> that is pre-allocated with Vec::with_capacity lets you track the current length as well with the buffer (if it's going to have a variable length of actually useful data).

If you want to make it a static for some reason, I'd recommend making it just static and thread_local, then wrapping it in some kind of cell. Making it thread local will mean you don't need to lock to access it safely.