- cross-posted to:
- cpp@programming.dev
- cross-posted to:
- cpp@programming.dev
From Russ Cox
Lumping both non-portable and buggy code into the same category was a mistake. As time has gone on, the way compilers treat undefined behavior has led to more and more unexpectedly broken programs, to the point where it is becoming difficult to tell whether any program will compile to the meaning in the original source. This post looks at a few examples and then tries to make some general observations. In particular, today’s C and C++ prioritize performance to the clear detriment of correctness.
I am not claiming that anything should change about C and C++. I just want people to recognize that the current versions of these sacrifice correctness for performance. To some extent, all languages do this: there is almost always a tradeoff between performance and slower, safer implementations. Go has data races in part for performance reasons: we could have done everything by message copying or with a single global lock instead, but the performance wins of shared memory were too large to pass up. For C and C++, though, it seems no performance win is too small to trade against correctness.
I’m definitely open to the idea that C and C++ have problems, but the things listed in this article aren’t them. He lists some very weird behavior by the clang compiler, and then blames it on C despite the fact that in my mind they’re clearly misfeatures of clang. He talks about uncertainty of arithmetic overflow… unless I’ve missed something, every chip architecture that 99% of programmers will ever encounter uses two’s complement, so the undefined behavior he talks about is in practice defined.
He says:
This is the same thing. He’s taking something that’s been a non-issue in practice for decades and deciding it’s an issue again. Yes, programming in C has some huge and unnecessary difficulties on non-memory-protected systems. The next time I’m working on that MS-DOS project, I’ll be sure to do it in Python to avoid those difficulties. OH WAIT
Etc etc. C++ actually has enough big flaws to fill an essay ten times this long about things that cause active pain to working programmers every day… but no, we’re unhappy that arithmetic overflow depends on the machine’s reliably-predictable behavior, instead of being written into the C standard regardless overriding the machine architecture. It just seems like a very weird and esoteric list of things to complain about.
Edit: Actually, I thought about it, and I don’t think clang’s behavior is wrong in the examples he cites. Basically, you’re using an uninitialized variable, and choosing to use compiler settings which make that legal, and the compiler is saying “Okay, you didn’t give me a value for this variable, so I’m just going to pick one that’s convenient for me and do my optimizations according to the value I picked.” Is that the best thing for it to do? Maybe not; it certainly violates the principle of least surprise. But, it’s hard for me to say it’s the compiler’s fault that you constructed a program that does something surprising when uninitialized variables you’re using happen to have certain values.
You got it correct in this edit. But the important part is that gcc will also do this, and they both are kinda expected to do so. The article cites some standard committee discussions: somebody suggested ensuring that signed integer overflow in C++20 will not UB, and the committee decided against it. Also, somebody suggested not allowing to optimize out the infinite loops like 13 years ago, and then the committee decided that it should be allowed. Therefore, these optimisations are clearly seen as features.
And these are not theoretical issues by any means, there has been this vulnerability in the kernel for instance: https://lwn.net/Articles/342330/ which happened because the compiler just removed a null pointer check.
Right, exactly. If you’re using C in this day and age, that means you want to be one step above assembly language. Saying C should attempt to emulate a particular specific architecture – for operations as basic as signed integer add and subtract – if you’re on some weird other architecture, is counter to the whole point. From the point of view of the standard, the behavior is “undefined,” but from the point of view of the programmer it’s very defined; it means whatever those operations are in reality on my current architecture.
That example of the NULL pointer use in the kernel was pretty fascinating. I’d say that’s another exact example of the same thing: Russ Cox apparently wants the behavior to be “defined” by the standard, but that’s just not how C works or should work. The behavior is defined; the behavior is whatever the processor does when you read memory from address 0. Trying to say it should be something else just means you’re wanting to use a language other than C – which again is fine, but for writing a kernel, I think you’re going to have a hard time saying that the language need to introduce an extra layer of semantics between the code author and the CPU.
If that were true, there would be no problem. Unfortunately, what actually happens is that compilers use the undefined behavior as an excuse to mangle your program far beyond what mere variation in processor behavior could cause, in the name of optimization. In the kernel bug, the issue wasn’t that the null pointer dereference was undefined per se, the real issue was that the subsequent null check got optimized out because of the previous undefined behavior.
Well… I partially agree with you. The final step in the failure-chain was the optimizer assuming that dereferencing NULL would have blown up the program, but (1) that honestly seems like a pretty defensible choice, since it’s accurate 99.999% of the time (2) that’s nothing to do with the language design. It’s just an optimizer bug. It’s in that same category as C code that’s mucks around with its own stack, or single-threaded code that has to have stuff marked
volatilebecause of crazy pointer interactions; you just find complex problems sometimes when your language starts getting too close to machine code.I guess where I disagree is that I don’t think a NULL pointer dereference is undefined. In the spec, it is. In a running program, I think it’s fair to say it should dereference 0. Like e.g. I think it’s safe for an implementation of assert() to do that to abort the program, and I would be unhappy if a compiler maker said “well the behavior’s undefined, so it’s okay if the program just keeps going even though you dereferenced NULL to abort it.”
The broader assertion that C is a badly-designed language because it has these important things undefined, I would disagree with; I think there needs to be a category of “not nailed down in the spec because it’s machine-dependent,” and any effort to make those things defined machine-independently would mean C wouldn’t fulfill the role it’s supposed to fulfill as a language.