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Int a = 5; a = a++ + ++a; a =? (2011)

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Re: Int a = 5; a = a++ + ++a; a =? (2011)

#71

Post by AnimalMuppet »

wat10000 wrote: post #61
jcranmer wrote: post #48

Earlier quoted context omitted.

The short answer is because C was designed to give leeway to really dumb compilers on really diverse hardware. This isn't quite the same case, but it's a good illustration of the effect: on gcc, if you have an expression f(a(), b()), the order that a and b get evaluated is [1] dependent on the architecture and calling-convention of f. If the calling convention wants you to push arguments from right to left, then b is…

I'd say it's more like C was designed from really dumb compilers on really diverse hardware. The standard, at least the early versions of it, was more to codify what was out there than to declare what was correct. For most things like this in the standard, you can point to two pre-standardization compilers that did it differently.

Kind of both? There were pre-standard compilers, but when they created the standard, they tried to make it so that one could write really dumb compilers and still fulfill the standard.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#72

Post by gynvael »

Boxxed wrote: post #14

> The interesting thing here is the Undefined Behavior (UB), well... actually two UBs, thanks to which there are three possible correct answers: 11, 12 and 13. No, if you invoke undefined behavior any result at all is possible.

Hey! Author here :)

So let me start by saying that that blog post was written was 15 years ago and I don't even remember the details of it and what I've written there. But, I have a hot-take on this topic you've touched on!

From a programmer perspective, you are absolutely right. The behaviour is undefined, end of discussion. A programmer should never rely on what they observe as the effective behaviour of an UB. A programmer must avoid creating situations in code that could result in the execution flow venturing into the areas of UB. And - per C and C++ standards - results of UB can be anything (insert the old joke about UB formatting one's disk being a formally correct behaviour).

However, I'm a security researcher, and from the security point of view - especially on the offensive side - we need to know and understand the effective behaviours of UBs. This is because basically all "low-level" vulnerabilities in C/C++ are formally effects of UBs. As such, for the security crowd, it still makes sense to investigate, understand, and discuss the actual observed effects of UBs, especially why a compiler does this, what are the real-world actual variants of generated code (if any) for a given UB for this and other compilers, how can this be abused and exploited, and so on.

My point being - there are two sides to this coin.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#73

Post by gynvael »

Someone wrote: post #13

> The interesting thing here is the Undefined Behavior (UB), well... actually two UBs, thanks to which there are three possible correct answers: 11, 12 and 13. There’s UB, so any answer is possible, isn’t it?

Hey! Author here :)

I'm going top-to-bottom through comments, and there was a similar question, so I'll link my answer here: https://news.ycombinator.com/item?id=48140821 (TL;DR: you are right, but there's another perspective on this)

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#74

Post by tzs »

kentm wrote: post #23
susam wrote: post #5

The code in the post seems very similar to the one in my own post from 2010: https://susam.net/sequence-points.html int a = 5; a += a++ + a++; I do remember that this particular code snippet (with a = 5, even) used to be popular as an interview question. I found such questions quite annoying because most interviewers who posed them seemed to believe that whatever output they saw with their compiler version was the co…

IMO, The only reasonable answer if asked this in an interview is “I would not write code where I have to know the answer to this question” These sorts of things are neat trivia to learn about things like sequence points but 99.9% of the time if it matters in your codebase you're writing something unmaintainable.

> IMO, The only reasonable answer if asked this in an interview is “I would not write code where I have to know the answer to this question”

That's half of a reasonable answer. The other half is "but I do know the answer so if I see it when reviewing or working on someone else's code I can flag it or rewrite it, and explain to them why it is bad".

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#76

Post by fc417fc802 »

kibwen wrote: post #58
fc417fc802 wrote: post #47

Earlier quoted context omitted.

When used well it makes for compact readable code. I don't see what it has to do with for loops or operators specifically. For example you can do the same in scheme while iterating by means of tail recursion.

> I don't see what it has to do with for loops or operators specifically. The reason that these operators pull their weight in C is because iteration over arrays is achieved by manual incrementation (usually via the leading clauses of the for-loop) followed by direct indexing. Languages with a first-class notion of iteration don't directly index in this way, which overwhelmingly eliminates not only the vast majority…

For loops are hardly the only usecase and built in iteration constructs frequently fall short. For example any mildly complex loop that involves pointer juggling can benefit.

> which doesn't have the footguns of `++` due to assignment being a statement rather than expression,

So then I implement the local equivalent of inc( v ) and ... same issue, right? Plus with rust macros is there any technical reason you can't trivially implement ++ for yourself? That's the case for most lisps that I touched on earlier.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#77

Post by gynvael »

magicalhippo wrote: post #4

Perhaps I'm just naive and/or have forgotten too much C, not that I knew that much, but I'm a bit perplexed as to why this is UB. It seems like something that should trigger a "we should specify this" reaction when adding these operators, and there is at least one reasonable way to define it which is fairly trivial and easily implementable.

Yeah, like left-to-right as in JS for example.

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#78

Post by dhosek »

kibwen wrote: post #38
saghm wrote: post #20

Earlier quoted context omitted.

Honestly, having increment in expressions rather than a statement feels like more of a footgun than a benefit. Expressions shouldn't mutate things.

In any language where the practice of iteration isn't achieved via C-style for-loops, having an operator devoted to increment just doesn't make sense (let alone four operators, for each of pre/post-increment/decrement). This is one of those backwards things that just needs to be chucked in the bin for any language developed post-2010.

I always hate C-style for-loops because even thought I learned C over 40 years ago, I can never remember whether the increment comes before the test or the test comes after the increment. Fortunately, modern IDEs let me continue to be ignorant on those occasions when they’re necessary (usually because I need the index for some reason).

Re: Int a = 5; a = a++ + ++a; a =? (2011)

#79

Post by gynvael »

syngrog66 wrote: post #54

The smart nerd will know precisely how to decode that line's results. The wise nerd will not allow lines like it in their codebase, in the first place and, having seen one, will refactor it (probably involving more lines or parentheses) to make it more clear and easier to maintain. The latter approach scales better, in long run.

This is true. What's also true, is that if that smart name works in cybersec, they'll feel right at home :)

(this is related to my other comment here https://news.ycombinator.com/item?id=48140821)

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