How many of these 200+ undefined behaviours exist in more modern low-level languages, such as Rust and D?
Dereferencing null or dangling pointers
Reading uninitialized memory
Breaking the pointer aliasing rules
Producing invalid primitive values:
dangling/null references
a bool that isn't 0 or 1
an undefined enum discriminant
a char outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
a non-utf8 str
Unwinding into another language
Causing a data race
It's true that unsafe is needed to get these problems, but they can also occur outside of unsafe blocks. See an example here: https://gankro.github.io/blah/only-in-rust/#unbound-lifetime...
Your own code need not use "unsafe" at all, but the program may still crash in your code if you called some function that internally does unsafe things to mess up your memory.
EDIT: I should say that the linked code does not crash, it only reads uninitialized memory. It seems to me like the same hole could be used to make things crash, but I don't have a ready-made example.
Yes!
> If you mix in unsafe code, bad things may happen
Yes!
My point was only that those bad things may also happen after the unsafe block that is their root cause. This is in reply to parent's "all of these [UBs] are inside of unsafe blocks". They aren't. They are caused by something inside unsafe blocks, but they may also materialize after.
> unsafe does more than pollute a whole function: it pollutes a whole module. Generally, the only bullet-proof way to limit the scope of unsafe code is at the module boundary with privacy.
> the only bullet-proof way to limit the scope of unsafe code is at the module boundary with privacy
I understand how this is meant in the context of that page, and it is true that modules protect against users messing with modules' internal invariants.
But does that also work the other way around? In the example in https://gankro.github.io/blah/only-in-rust/#unbound-lifetime... it's not the caller messing with the callee's state, it's the callee messing up the caller's state. Do modules help at all here? That is, would the function
fn foo<'a>(input: *const u32) -> &'a u32 {
unsafe {
return &*input
}
}
I agree. But then this shows that it's possible to write "safe" Rust code that only calls "safe" external code and still (to a first approximation) have the possibility of undefined behavior showing up at any point. In other words, Rust's famous static, compiler-enforced guarantees are not guarantees at all, more like firm promises.
Nothing wrong with that; it's easy to write memory-corrupting code in other safe-by-default languages like Haskell or OCaml as well. But it seems like Rust's marketing materials do try to suggest otherwise, and many people get wrong impressions (look at the first post in this thread, and the other comments on this article saying that Rust's "safe" code is 100% free from undefined behavior).
Your point about other languages is exactly what I was going to say; unsafe is like an FFI layer. Nobody says that Ruby isn't memory safe because it can call into C code, and if someone messes up the C, well, it's at fault. "It's memory safe except for FFI" is a mouthful, and so people generally let the exceptions slide. Same with Rust.
Correct me if I'm wrong but I don't think such a strong universal statement can be true (even outside unsafe blocks) because LLVM has corner cases of "undefined behavior". (And since Rust is relies on LLVM, ...)
Maybe it's more accurate to say that Rust minimizes undefined behavior as a design goal, or it doesn't have intentional undefined behavior.
Sure and I believe that adding a conditional qualification such as "if one does not invoke UB of LLVM" restates my point: one can't make a universal statement that "safe Rust has zero undefined behavior."
E.g., as of this writing, the following "safe Rust" UB issue (3+ years ago) last had comments 21 days ago and I believe it's still open:
A conditional qualification which is intended to be unconditionally true of safe Rust code, outside bugs in the compiler. The universal statement is totally possible, because your conditional is equivalent to saying "if you write valid code".
I emphasized "intended" because it seems like we're talking past each other.
You: re-emphasizing Rust's specified design goal.
Me: emphasizing the current state of Rust compiler as reality which makes the statement "safe Rust has no undefined behavior" as not true.
(In other words, I emphasize the unintentional UB whereas you do not.)
>, because your conditional is equivalent to saying "if you write valid code".
If you look at the github issue, "1.04E+17 as u8" is valid safe Rust code which invokes UB.
Yes I get what you're saying but I'll try to emphasize again that I'm not trying to play semantic games to irritate everyone. (Yes, we can play word games such as "a tank is an armored military vehicle -- unless it is just a cardboard facade to fool Germans that the Allies are invading a different a part of France's coastline or acting as a movie prop for special effects work.") Every "thing" can be defined with endless cumbersome qualifiers that nobody actually says in real life.
That said, I felt the context in this thread warranted a different threshold to qualify Rust's UB because one example of John Regehr 200 UB bullet points is:
- Demotion of one real floating type to another produces a value outside the range that can be represented (6.3.1.5).
Rust developers should be aware of this - they're almost always incredibly trivial patterns to avoid, but only if you know about them.
And I am only mentioning the most known ones during the 10 years before C was born until early 80's.
Sure some of them also had safety issues like use-after-free, but not in the same amount as C.
Regarding those things that actually matter for programmer, following would be well-behaved in Rust (including unsafe blocks): conversion between types, integer arithmetic, pointer arithmetic (there are generally two variants of operations, one that essentially treats pointers as unsigned integers, and another one that behaves like in C with more opportunities for optimization). On the other side of the coin, in Rust mutable references cannot be aliased.
unspecified != undefined behavior
See this Stack overflow answer for an explanation: https://stackoverflow.com/questions/2397984/undefined-unspec...
Ie function arguments are still evaluated in some (possibly changing) order. C++ undefined behavior allows to eg format your hard disk or launch the nukes.