81–90 of 748 posts
I stoped reading about here: > bool parse_packet(const uint8_t* bytes) { > const int* magic_intp = (const int*)bytes; // UB! Author, if you are reading this, please cite the spec section explaining that this is UB. Dereferencing the produced pointer may be UB, but casting itself is not, since uint8_t is ~ char and char* can be cast to and from any type. you might try to argue that uint8_t is not necessarily char, and…
Byte and int has different alignment requirements. It is UB the moment you make such a ptr. Great way to demonstrate the point of the article.
I don’t see what spec part would prohibit that cast from validly compiling to
BIC r3, r0, #3
The examples aren't really undefined behavior. They are examples that could become UB based on input/circumstances. Which if you are going to be that generous, every function call is UB because it could exceed stack space. Which is basically true in any language (up to the equivalent def of UB in that language). I feel like c has enough actual rough edges that deserve attention that sensationalism like this muddies f…
UB based on input can be an exploit vector.
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."
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.
Earlier quoted context omitted.
The examples are unequivocally UB. Full stop. How to think of this properly is that when you have UB, you are no longer under the auspices of a language standard. Things may work fine for a time, indefinitely even. But what happens instead is you unknowingly become subject to whimsies of your toolchain (swap/upgrade compilers), architecture, or runtime (libc version differences). You end up building a foundation on q…
> The examples are unequivocally UB. Full stop. Tbh, already the first example (unaligned pointer access) is bogus and the C standard should be fixed (in the end the list of UB in the C standard is entirely "made up" and should be adapted to modern hardware, a lot of UB was important 30 years ago to allow optimizations on ancient CPUs, but a lot of those hardware restrictions are long gone). In the end it's the CPU a…
However I do agree that just saying "the behaviour is undefined" is an unhelpful cop-out. They could easily say something like "non-atomic misaligned accesses either succeed or trap" or something like that.
> In the end it's the CPU and not the compiler which decides whether an unaligned access is a problem or not.
Not just the CPU - memory decides as well. MMIO devices often don't support misaligned accesses.
Earlier quoted context omitted.
The irony is unmistakable.
There is nothing ironic in letting an llm have a pass at identifying potential UB and other correctness issues in C code. I say this as an experienced C developer.
> A problem with this is that in order to confirm the findings, you’ll need an expert human. But generally expert humans are busy doing other things. The article suggests using LLMs to identify and fix UB. However as per the above, I think the issue is that we need more expert humans. LLM generated code will eventually contain UB. EDIT: added "eventually"
It would already help a lot when the C and C++ standards start to clean up the list of Undefined Behaviour (e.g. there's a lot of nonsense UB currently in the C standard which could easily become Defined Behaviour - like the "file doesn't end in a new-line character" thing): https://gist.github.com/Earnestly/7c903f481ff9d29a3dd1
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."
> -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 where's the fun in that?