undefined | Better HN

0 pointssteveklabnik1mo ago0 comments

> You can tune a C compiler as well to have a very specific defined behaviour for integer overflow. You can add -fwrapv or you can add UBSAN.

This is an example of a compiler flag that adds definition to undefined behavior, which is of course, legal to do. That doesn't change that in the standard, it is undefined behavior, and in Rust, it is not.

> To say that overflow would be defined in Rust is at least half a lie.

In the context of "undefined behavior", it is not a lie at all.

> So the user has to constrain the set of valid inputs, and do manual sanitization, just like in C.

No, because the consequences of how the two languages define these behaviors are very, very different.

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jstimpfle1mo ago· 2 in thread

As you said, overflow is defined in Cargo, given a specific build type and/or specific build flags. It's not defined in Rust.

Just saying that it's defined and then not saying what the definition is, is no different from saying it's undefined.

steveklabnikOP1mo ago

No, “release mode vs debug mode” is defined in Cargo. What’s defined in Rust is the debug_assertions flag, which is one of the things that Cargo will set by default as part of the debug mode by default.

> Just saying that it's defined and then not saying what the definition is, is no different from saying it's undefined.

It actually is, because, as I said earlier, “undefined behavior” is a term of art with very specific meaning. Regardless, it is defined: there are two possible behaviors, with one guaranteed with that flag and the other chosen by implementations.

jstimpfle1mo ago

I think people make up way too much of it. What is the actual term of art? What is the meaning of UB? If you look in the standard, UB is basically what its name says, it is behaviour (or state) that is not defined. It can be anything. And that makes sense in many cases: What if you construct a random pointer, and read it or write it? It's not useful or practically possible to define the behaviour from then on. So the behaviour is left undefined, simple as that.

Now are there many cases of UB in C, many more than strictly need to exist on contemporary platforms? For sure there are. But does it affect me? Not unless I need a specific behaviour common to most contemporary platforms that I can't get within the confines of C, even considering compiler specific extensions. Honestly I can't come up with any of the top of my head. Maybe some integer-shifting stuff or such, if the compiler was able to prove I'm doing sth undefined, it can leave out that code (or delete my mail, for the doomers). Personally, it hasn't happened to me, and it's on the compiler authors to not do stupid things too.

Leaving all the semantic hair-splitting aside. What is the practical difference in how you write a Rust program compared to a C program, given that integer overflow is "defined" in Rust?

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