Undefined vs Unsafe in Rust

Recently Julia Evans wrote an excellent post about debugging a segfault in Rust. (Go read it, it’s good)

One thing it mentioned was

I think “undefined” and “unsafe” are considered to be synonyms.

This is … incorrect. However, we in the Rust community have never really explicitly outlined the distinction, so that confusion is on us! This blog post is an attempt to clarify the difference of terminology as used within the Rust community. It’s a very useful but subtle distinction and I feel we’d be able to talk about safety more expressively if this was well known.

Unsafe means two things in Rust, yay

So, first off, the waters are a bit muddied by the fact that Rust uses unsafe to both mean “within an unsafe {} block” and “something Bad is happening here”. It’s possible to have safe code within an unsafe block; indeed this is the primary function of an unsafe block. Somewhat counterintutively, the unsafe block’s purpose is to actually tell the compiler “I know you don’t like this code but trust me, it’s safe!” (where “safe” is the negation of the second meaning of “unsafe”, i.e. “something Bad is not happening here”).

Similarly, we use “safe code” to mean “code not using unsafe{} blocks” but also “code that is not unsafe”, i.e. “code where nothing bad happens”.

This blog post is primarily about the “something bad is happening here” meaning of “unsafe”. When referring to the other kind I’ll specifically say “code within unsafe blocks” or something like that.

Undefined behavior

In languages like C, C++, and Rust, undefined behavior is when you reach a point where the compiler is allowed to do anything with your code. This is distinct from implementation-defined behavior, where usually a given compiler/library will do a deterministic thing, however they have some freedom from the spec in deciding what that thing is.

Undefined behavior can be pretty scary. This is usually because in practice it causes problems when the compiler assumes “X won’t happen because it is undefined behavior”, and X ends up happening, breaking the assumptions. In some cases this does nothing dangerous, but often the compiler will end up doing wacky things to your code. Dereferencing a null pointer will sometimes cause segfaults (which is the compiler generating code that actually dereferences the pointer, making the kernel complain), but sometimes it will be optimized in a way that assumes it won’t and moves around code such that you have major problems.

Undefined behavior is a global property, based on how your code is used. The following function in C++ or Rust may or may not exhibit undefined behavior, based on how it gets used:

As long as you always call it with a valid pointer to an integer, there is no undefined behavior involved.

But in either language, if you use it with some pointer conjured out of thin air (like 0x01), that’s probably undefined behavior.

As it stands, UB is a property of the entire program and its execution. Sometimes you may have snippets of code that will always exhibit undefined behavior regardless of how they are called, but in general UB is a global property.

Unsafe behavior

Rust’s concept of “unsafe behavior” (I’m coining this term because “unsafety” and “unsafe code” can be a bit confusing) is far more scoped. Here, fn deref is “unsafe”1, even if you always call it with a valid pointer. The reason it is still unsafe is because it’s possible to trigger UB by only changing the “safe” caller code. I.e. “changes to code outside unsafe blocks can trigger UB if they include calls to this function”.

Basically, in Rust a bit of code is “safe” if it cannot exhibit undefined behavior under all circumstances of that code being used. The following code exhibits “safe behavior”:

We dereferenced a raw pointer, but we knew it was valid. Of course, actual unsafe blocks will usually be “actually totally safe” for less obvious reasons, and part of this is because unsafe blocks sometimes can pollute the entire module.

Basically, “safe” in Rust is a more local property. Code isn’t safe just because you only use it in a way that doesn’t trigger UB, it is safe because there is literally no way to use it such that it will do so. No way to do so without using unsafe blocks, that is2.

This is a distinction that’s possible to draw in Rust because it gives us the ability to compartmentalize safety. Trying to apply this definition to C++ is problematic; you can ask “is std::unique_ptr<T> safe?”, but you can always use it within code in a way that you trigger undefined behavior, because C++ does not have the tools for compartmentalizing safety. The distinction between “code which doesn’t need to worry about safety” and “code which does need to worry about safety” exists in Rust in the form of “code outside of unsafe {}” and “code within unsafe {}”, whereas in C++ it’s a lot fuzzier and based on expectations (and documentation/the spec).

So C++’s std::unique_ptr<T> is “safe” in the sense that it does what you expect but if you use it in a way counter to how it’s supposed to be used (constructing one from an invalid pointer, for example) it can blow up. This is still a useful sense of safety, and is how one regularly reasons about safety in C++. However it’s not the same sense of the term as used in Rust, which can be a bit more formal about what the expectations actually are.

So unsafe in Rust is a strictly more general concept – all code exhibiting undefined behavior in Rust is also “unsafe”, however not all “unsafe” code in Rust exhibits undefined behavior as written in the current program.

Rust furthermore attempts to guarantee that you will not trigger undefined behavior if you do not use unsafe {} blocks. This of course depends on the correctness of the compiler (it has bugs) and of the libraries you use (they may also have bugs) but this compartmentalization gets you most of the way there in having UB-free programs.

1. Once again in we have a slight difference between an “unsafe fn”, i.e. a function that needs an unsafe block to call and probably is unsafe, and an “unsafe function”, a function that exhibits unsafe behavior.

2. This caveat and the confusing dual-usage of the term “safe” lead to the rather tautological-sounding sentence “Safe Rust code is Rust code that cannot cause undefined behavior when used in safe Rust code”