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Everything in C is undefined behavior

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Re: Everything in C is undefined behavior

#141

Post by account42 »

IshKebab wrote: post #86
flohofwoe wrote: post #78

Earlier quoted context omitted.

> 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…

There are still modern CPUs that don't support misaligned access. It would be insane for C to mandate that misaligned accesses are supported. 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…

For x86 SSE there are aligned instructions that will trap on unaligned access.

Re: Everything in C is undefined behavior

#142

Post by parasti »

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?

Re: Everything in C is undefined behavior

#144

Post by simonask »

leni536 wrote: post #134
flohofwoe wrote: post #78

Earlier quoted context omitted.

> 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…

Undefined means that the ISO C doesn't define the behavior. An implementation is free to do so.

If they do, that is no longer an implementation of C. It is a dialect of C, and there are many (GNU C being the most popular), but there are real drawbacks to using dialects.

This is in contrast to the other category that exists, which is "implementation-defined".

Re: Everything in C is undefined behavior

#145

Post by account42 »

bestouff wrote: post #8

The problem of UB is not really that it may crash in some architecture. The real problem is that the compiler expects UB code to NOT happen, so if you write UB code anyway the compiler (and especially the optimizer) is allowed to translate that to anything that's convenient for its happy path. And sometimes that "anything" can be really unexpected (like removing big chunks of code).

That's a feature, not a problem.

Re: Everything in C is undefined behavior

#146

Post by thomashabets2 »

beeforpork wrote: post #106

The UB in unaligned pointers is even worse: an unaligned pointer in itself is UB, not only an access to it. So even implicit casting a void*v to an int*i (like 'i=v' in C or 'f(v)' when f() accepts an int*) is UB if the cast pointer is not aligned to int. It is important to understand that this is a C level problem: if you have UB in your C program, then your C program is broken, i.e., it is formally invalid and wron…

Author here.

> an unaligned pointer in itself is UB

Yup. Per the "Actually, it was UB even before that" section in the post.

> UB is not on the HW, it has nothing to do with crashes or faults

Yeah. I tried to convey this too, but I'm also addressing the people who say "but it's demonstrably fine", by giving examples. Because it's not.

Re: Everything in C is undefined behavior

#148

Post by psychoslave »

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.

If all somebody want is a programming language than C/C++ on these matter, there are plentiful options of the shelf to pick from.

If all somebody want is a turn key replacement to C/C++ ecosystem, then there is nothing like that in the world that I’m aware of.

Re: Everything in C is undefined behavior

#149

Post by Etheryte »

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?

Because the production environment might be a completely different architecture, these details matter a lot. Works on my machine is not useful if your actual target is a small embedded system on top of a cell tower in the middle of nowhere. Granted, most people don't work on stuff like that, I imagine the vast majority of devs here are web developers, but even still it's an interesting discussion even if you haven't run into it yourself. Maybe even more so in that case.

Re: Everything in C is undefined behavior

#150

Post by thomashabets2 »

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."

Author here.

> -Acceptance: "Just dont write UB."

The point of my article is that this is not possible. This cannot be our end state, as long as humans are the ones writing the code. No human can avoid writing UB in C/C++.

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