Everything in C is undefined behavior

Post by pjmlp » Wed, May 20, 2026, 5:41 PM UTC

3form wrote: post #218

simonask wrote: post #115

Earlier quoted context omitted.

I think the article's point is that you don't actually have to get weird at all to run into UB. Lots of people mistakenly think that C and C++ are "really flexible" because they let you do "what you want". The truth of the matter is that almost every fancy, powerful thing you think you can do is an absolute minefield of UB.

At which point it feels like some sort of high-level assembly-like language, which is simple enough to compile efficiently and stay crossplatform, with some primitives for calls, jumps, etc. could find a nice niche. Maybe this already exists, even? A stripped down version of C? A more advanced LLVM IR? I feel like this is a problem that could use a resolution, just maybe not with enough of a scale for anyone to bothe…

Post by pjmlp » Wed, May 20, 2026, 5:50 PM UTC

matheusmoreira wrote: post #153

quelsolaar wrote: post #62

The 5 stages of learning about UB in C: -Denial: "I know what signed overflow does on my machine." -Anger: "This compiler is trash! why doesn't it just do what I say!?" -Bargaining: "I'm submitting this proposal to wg14 to fix C..." -Depression: "Can you rely on C code for anything?" -Acceptance: "Just dont write UB."

What stage is the "just make the compiler define the undefined" stage? Unaligned access? Packed structs. Compiler will magically generate the correct code, as if it had always known how to do it right all along! Because it has, in fact, always known how to do it right. It just didn't. Strict aliasing? Union type punning. Literally documented to work in any compiler that matters, despite the holy C standard never sayi…

Post by tardedmeme » Wed, May 20, 2026, 5:51 PM UTC

munch117 wrote: post #574

marcosdumay wrote: post #558

Earlier quoted context omitted.

Except that UB doesn't mean that. UB means "the developer must never write this".

Both are wrong. It means "this standard does not constrain the behaviour of code that does this". It's entirely legal for implementations to have predictable behaviour, documented or not, for code that is undefined by the standard. In their quest for maxxing benchmark performance they generally choose not to, but there's really nothing in any standard that stops you from making an implementation that prioritises safe…

Post by pjmlp » Wed, May 20, 2026, 5:55 PM UTC

dns_snek wrote: post #109

Ygg2 wrote: post #85

Earlier quoted context omitted.

> -Acceptance: "Just dont write UB." Just switch to a saner language. And before I get attacked for being a Rust shill, I meant Java :P The bar is so low it's floating near the center of the Earth.

> And before I get attacked for being a Rust shill, I meant Java :P If all you want is C but less insane then the obvious answer here is Zig.

Post by pjmlp » Wed, May 20, 2026, 6:03 PM UTC

account42 wrote: post #166

parasti wrote: post #142

I have never in my 20 years of writing C heard so much about undefined behavior as I have in the past 6 months on Hacker News. It has never entered the conversation. You write the code. If it doesn't work, you debug it and apply a fix or a workaround. Why does the idea of undefined behavior in C get to the front page so consistently?

There are a lot of Rust/whatever hipsters here that have defined their whole identity around hating C and C++.