Here's a way weirder example:
volatile int x = 5;
printf("%d in hex is 0x%x.\n", x, x);
So in common parlance, a "data race" is any concurrent accesses to the same object from different threads, at least one of which is a write. In C, we can have a data race on a single thread and without any writes!
> It barely scratches the surface.
I agree. The point of the post is not to enumerate and explain the implications of all 283 uses of the word "undefined" in the standard. Nor enumerate all the things that are undefined by omission.
The point of the post is to say it's not possible to avoid them. Or at least, no human since the invention of C in 1972 has.
And if it's not succeeded for 54 years, "try harder", or "just never make a mistake", is at least not the solution.
The (one!) exploitable flaw found by Mythos in OpenBSD was an impressive endorsement of the OpenBSD developers, and yet as the post says, I pointed it at the simplest of their code and found a heap of UB.
Now, is it exploitable that `find` also reads the uninitialized auto variable `status` (UB) from a `waitpid(&status)` before checking if `waitpid()` returned error? (not reported) I can't imagine an architecture or compiler where it would be, no.
FTA:
> The following is not an attempt at enumerating all the UB in the world. It’s merely making the case that UB is everywhere, and if nobody can do it right, how is it even fair to blame the programmer? My point is that ALL nontrivial C and C++ code has UB.
> And if it's not succeeded for 54 years, "try harder", or "just never make a mistake", is at least not the solution.
And I 100% agree. UB is way overused by these standards for how dangerous it is, and as a consequence using C (and C++) for anything nontrivial amounts to navigating a minefield.
What are you talking about? UB was coined only in the first C standard, in 1989. Prior to that there was no "If you do this, anything can happen". It was "If you do this, that will happen".
It's fair to blame the programmer for the choice of programming in a language like this, if it was in fact their choice. As you've so eloquently put, choosing those languages is essentially equivalent to choosing UB, so starting a new project with one of them is 100% blameworthy when the UB is inevitably found.
The reason for the hack is that very early C compilers just always spill, so you can write MMIO driver code by setting a pointer to point at the MMIO hardware and it actually works because every time you change x the CPU instruction performs a memory write.
Once C compilers got some basic optimisations that obvious "clever" trick stops working because the compiler can see that we're just modifying x over, and over and over, and so it doesn't spill x from a register and the driver doesn't work properly. C's "volatile" keyword is a hack saying "OK compiler, forget that optimisation" which was presumably a few minutes work to implement, whereas the correct fix, providing MMIO intrinsics in the associated library, was a lot of work.
Why should you want intrinsics here? Intrinsics let you actually spell out what's possible and what isn't. On some targets we can actually do a 1-byte 2-byte and 4-byte write, those are distinct operations and the hardware knows, so e.g. maybe some device expects a 4-byte RGBA write and so if you emit four 1-byte writes that's very confusing and maybe it doesn't work, don't do that. On some targets bit-level writes are available, you can say OK, MMIO write to bit 4 of address 0x1234 and it will write a single bit. If you only have volatile there's no way to know what happens or what it means.
As a nit pick, I don't think this is correct use of "spill". Register spilling refers to when a compiler's code generator runs out of registers and needs to store variables in memory instead. In the MMIO case you are reading/writing via a pointer, so this is unrelated to registers and spilling behavior.
Volatile on a non pointer value is not for MMIO, though, that’s typically for concurrency like with interrupts.
Source?
You need to distinguish between a UB and a race, and I think that's something that discussions of UB miss. Take any C program and compile it. Then disassemble it. You end up with an Assembly program that doesn't have any UB, because Assembly doesn't have UB.
UB is a property of a source program, not the executable. It means that the spec for the language in which the source is written doesn't assign it any meaning. But the executable that's the result of compiling the program does have a meaning assigned to it by the machine's spec, as machine code doesn't have UB.
A race is a property of the behaviour of a program. So it's true to say that your C program has UB, but the executable won't actually have a race. Of course, a C compiler can compile a program with UB in any way it likes so it's possible it will introduce a race, but if it chooses to compile the program in a way that doesn't introduces another thread, then there won't be a race.
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.
int increment(int x) {
return x + 1;
}
If you want to be standards correct, yes you have to know the standard well. True. And you can always slip, and learn another gotcha. Also true. But it's still extremely flexible.
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 bother, vs. learning C, assembly of given architecture, or one of the new and fancy compiled languages.
Well, sure, that's what volatile means - that the value may be changed by something else. If it's a global variable then the something else might be an interrupt or signal handler, not just another thread. If it's a pointer to something (i.e. read from a specific address) then that could be a hardware device register who's value is changing.
The concept of a volatile variable isn't the problem - any language that is going to support writing interrupt routines and memory mapped I/O needs to have some way of telling the compiler "don't optimize this out" since reading from the same hardware device register twice isn't like reading from the same memory location twice.
I think the problem here is more that not all of the interactions between language features and restrictions have been fully thought out. It's pretty stupid to be able to explicity tell the language "this value can change at any time", and for it to still consider certain uses of that value as UB since it can change at any time! There should have been a carve out in the "unsequenced side effect" definitions for volatile variables.
As noted, there’s almost 300 usages of the word undefined in the standard. Believing that it’s possible to correctly define all the carve outs necessary correctly and have the compiler implement the carve outs successfully is about as logical as believing UB is humanly avoidable in written code.
That said, your “common parlance” definition of “data race” is not the definition used by the C standard, so your last sentence is at best misleading in a discussion of standard C.
> The execution of a program contains a data race if it contains two conflicting actions in different threads, at least one of which is not atomic, and neither happens before the other. Any such data race results in undefined behavior.
(Here “conflicting” and “happens before” are defined in the preceding text.)
However, this is not at all what UB means in C (or C++). The compiler is free to optimize away the entire block of code where this printf() sequence occurs, by the logic that it would be UB if the program were to ever reach it.
For example, the following program:
int y = rand();
if (y != 8) {
volatile int x;
printf("%d: %d", x, x) ;
} else {
printf("y is 8");
}
>unsequenced side effects on the same scalar object are UB
>6.5.3.3.8 tells us that the evaluations of function arguments are indeterminately sequenced w.r.t. each other.
Read 5.1.2.4.3:
"If A is not sequenced before or after B, then A and B are unsequenced."
"Evaluations A and B are indeterminately sequenced when A is sequenced either before or after B, but it is unspecified which."
With a footnote saying this:
"9)The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations cannot interleave, but can be executed in any order."
I.e the standard makes a distinction between "unsequenced" and "indeterminately sequenced". And with no mention of side effects on "indeterminately sequenced" being UB it leads me to conclude that your example is not UB.
Well, yes; but when the C standard authors wrote like this, they surely had in mind "the reads could be in either order, therefore the output could display the polled values in either order". Not C++ nasal demons.
And yeah, being able to say "reading is a side effect" is important when for example you interact with certain memory-mapped devices.
Edit: thread=thread of execution. I’m not making a point about thread safety within a program.
I'm also not convinced (yet) that the example really is UB: I agree reading a volatile is "a side effect" in some sense, and GP cited a paragraph that says just that. But GP doesn't clearly quote that it's a side effect on the object (or how a side effect on an object is defined). Reading an object doesn't mutate it after all.
But whatever language lawyer things, the code is obviously broken, with an obvious fix, so I'm not so interested in what its semantics should be. Here is the fix:
volatile int x;
// ...
int val = x; // volatile read
printf("%x %d\n", val, val);See my comment here - https://news.ycombinator.com/item?id=48205760
If you are using volatile you are reading from a device port mapped to that address.
Since C doesn't mandate in which order function arguments are evaluated, you don't know which argument will be read from port first.
How can that be anything but UB?
The lack of argument sequencing feels utterly petty however.
It's reasonable that such people would also be interested in design aspects of languages, and UB in C is in that field. Though I would argue that a lot of it was originally accommodating old CPU architectures without compromising performance too badly, and about as much a "design choice" as wheels being round...
There were a few high-profile "scandals" around GCC 3.2 (IIRC) because the compiler finally started much more aggressively using UB in optimizations, which was a reason that lots of people stayed on GCC 2.95 for a very long time. GCC 3.2 came out in 2002.
Every company keep harping on about safety and being exposed (being in the news): so the narrative against 'unsafe' is up the wazoo.
The new world is basically a bunch of city dwellers who haven't seen raw nature and you show them a lawn mower, they freak out. Blades that spin?!?!?! Madness!!
Can't talk about C without CVE.
I have lots of my code running day-in, day-out on literally hundreds of millions of machines. The approach to "getting it working" is exactly OP's.
I'll admit to being pretty defensive and anal in checking values and return-codes (more so than most, I suspect), and I'm a firm believer in KISS principles in software engineering ("solving hard problems with complicated code is easy, solving them with simple, understandable algorithms is the hard bit") but generally there's no real difference in approach to the code I write to work on my workstation, and the code I write to work in the field.
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy
Understanding three important concepts properly in C allows one to easily identify what can/cannot result in UB viz. 1) Expressions 2) Statements 3) Sequence Points and "Single Update Rule". It is not that hard at all.
I wrote about it here with links to further reading provided - https://news.ycombinator.com/item?id=48144734
Compilers might have bugs where the spec is supposed to work but it doesn't, and many extensions without standard equivalents, or implementation-specific behaviour where undefined things in the standard do get assigned a meaningful outcome.
The real answer is that proponents of languages like C seem to completely disregard the dangers/difficulty of hitting/difficulty of fixing UB. Proponents of languages like Rust overstate it instead. Pointless wars/drama is fun to read and gets clicks.
2. You don't really appreciate the issue. Signed integer overflow is undefined. If you check for that overflow after the fact the compiler can, and demonstrably has pretended that the overflow can't happen and optimised away your overflow check.
You may not even come across that failure mode to know to 'fix' it. And good luck finding the issue unless you know about UB and what the compiler can and will do in such situations.
Although I haven’t noticed a spike the last 6 months, just a slowly increasing realization that C isn’t fit for humans and should go the way of asbest: Don’t use it for anything new, and remove it where it already exists, unless doing so would be too expensive or disruptive.
Personally I like C because you should have a good idea of what it's going to do. Other languages feel like a black box, and I start having to fight them far too often. But I say that as a hacker of low level stuff, not as someone who's paid and working on higher level stuff, so that is probably a niche view.
tl;dr: C defined language semantics, and leaves some behavior undefined. Each system that C is ported to has the ability to define the behavior however it wants.
This blows the mind of PL folks every decade or so.
It’s cool that we have portable methods and formal language semantics for stuff like memory fences and atomics now, but that sort of thing worked fine in C back in 1970 (or else unix would not have worked). You just needed to read the target machine’s manual when porting stuff.
The modern version is arguably better, but also arguably worse. Does anyone else remember when the JVM got this stuff wrong, making safe multithreaded code impossible, and then later had to break compatibility with the language spec?
You could claim that we can’t trust hardware folks to get instruction semantics right (this is demonstrably true), but duplicating and slightly modifying the specs in your language spec doesn’t actually fix the underlying hardware bugs.
Yeah, getting old… I’ll go find a cloud to yell at.
After switching to Rust five years ago I agree with all the Rust hipsters as far as disliking those languages go.
I just don't talk about it a lot. If every Rust person I know that was a C/C++ developer before was as outspoken about what they think of the latter, you'd see that these people are a majority.
We're just old hands who like to use stuff that works. And most of us don't get attached to code or languages.
It's also difficult to admint to yourself that you were never in command of a language as far as UB/other footguns go, as much as you thought. Or ever, for your enire career. For me that self-realization about C/C++ (enabled by Rust) was a turning point.
Lately you can read about the dichotomy re. AI use.
I.e. developers who define them themselves through what they build/ideas are embracing LLMs; for what they can do.
I.e.: I am what I build.
Whereas developers for whom software engineering is a craft that defines them hate them openly.
I.e.: I am how I build.
Now this seems to suggest to me that maybe Rust developers who openly hate C/C++ squarely belong to the latter group whereas the silent ones belong to the former. It's builders vs programmers. Just different world views.
Also you can not dislike something and still not speak about it. Because you decided to not care.
Just like TypeScript can't get rid of JavaScript WATs.
I trust your historical C usage was more productive than that..
-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."
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 saying so. Alternatively, just disable it straight up: -fno-strict-aliasing. Enjoy reinterpreting memory as you see fit. You might hit some sharp edges here and there but they sure as hell aren't gonna be coming from the compiler.
Overflow? Just make it defined: -fwrapv. Replace +, -, * with __builtin_*_overflow while you're at it, and you even get explicit error checking for free. Nice functional interface. Generates efficient code too.
The "acceptance" stage is really "nobody sane actually cares about the C standard". The standard is garbage, only the compilers matter. And it turns out that compilers have plenty of extremely useful functions that let you side step most if not all of this. People just don't use this because they want to write "portable" "standard" C. The real acceptance is to break out of that mindset.
Somehow I built an entire lisp interpreter in freestanding C that actually managed to pass UBSan just by following the above logic. I was actually surprised at first: I expected it to crash and burn, but it didn't. So if I can do it, then anyone can do it too.
A better way to think about UB is as a contract between developer and implementation, so that the implementations can more easily reason about the code. How would you optimize:
(x * 2) / 2
An optimizer can optimize this out for a signed integer, because it doesn't have to consider overflow, but with a unsigned integer it can not. UB is a big reason why C is the most power efficient high level language.
Packed structs are dangerous. You can do unaligned accesses through a packed type, but once you take the address of your misaligned int field, then you are back into UB territory. Very annoying in C++ when you try to pass the a misaligned field through what happens to be generic code that takes a const reference, as it will trigger a compiler warning. Unary operator+ is your friend.
It does say so, actually, since C99 TC3 (DR 283).
It can be left as implementation defined, which means that the compiler can't simply do arbitrary things, it needs to document what it would do.
Take, for example, signed-integer overflow: currently a compiler can simply refuse to emit the code in one spot while emitting it in another spot in the same compilation unit! Making it IB means that the compiler vendor will be forced to define what happens when a signed-integer overflows, rather than just saying, as they do now, "you cannot do that, and if you do we can ignore it, correct it, replace it or simply travel back in time and corrupt your program".
> Somehow I built an entire lisp interpreter in freestanding C that actually managed to pass UBSan just by following the above logic. I was actually surprised at first: I expected it to crash and burn, but it didn't. So if I can do it, then anyone can do it too.
Same here; I built a few non-trivial things that passed the first attempt at tooling (valgrind, UBsan with tests, fuzzing, etc) with no UB issues found.
> -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++.
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.
If all you want is C but less insane then the obvious answer here is Zig.
Because the last time I looked it appeared to need some godawful slow bytecode interpreter that took up thousands of kilobytes of RAM.
Or you just not skip the introductory pages, that tell you what the language philosophy of C is, and why there is UB. Yes, UB can be a struggle, but the first four steps are entirely unnecessary. It means that you do not actually understand the core concepts of the very same language you are using, which is kinda stupid.
When that started happened people became alarmed (oMG UB iS TeH BAD!) and since some old UB machines still had industry support (of organisations that actually participated in ISO meetings instead of arguing online) there was never any movement on defining de-facto usage as de-jure and the alarmist position became the default.
Personally I think the industry would've benefited from a Boring C (as described by DJB) push by people that would've created a public parallell "de-jure" standard that would've had a chance to be adopted by compiler creators.
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 wrong, because it is against the C language spec. UB is not on the HW, it has nothing to do with crashes or faults. That cast from void* to int* most likely corresponds to no code on the HW at all -- types are in C only, not on the HW, so a cast is a reinterpretation at C level -- and no HW will crash on that cast (because there is not even code for it). You may think that an integer value in a register must be fine, right? No, because it's not about pointers actually being integers in registers on your HW, but your C program is broken by definition if the cast pointer is unaligned.
> 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.
It's perfectly reasonable to expect any load through `int*` to just load 4 bytes from memory, done and done. They get surprised that it is far from the whole story, and the result is UB.
Meanwhile, the actual computers we have been using for decades have no problems actually just loading 4 bytes through any arbitrary pointer with zero overhead. But no.
Can someone point to where the standard states this?
This type of UB is fine and nobody really complains about hardware differences leading to bugs.
However, over time aggressive readings of UB evolved C into an implicit "Design by Contract" language where the constraints have become invisible. This creates a similar problem to RAII, where the implicit destructor calls are invisible.
When you dereference a pointer in C, the compiler adds an implicit non-nullable constraint to the function signature. When you pass in a possibly nullable pointer into the function, rather than seeing an error that there is no check or assertion, the compiler silently propagates the non-nullable constraint onto the pointer. When the compiler has proven the constraints to be invalid, it marks the function as unreachable. Calls to unreachable functions make the calling function unreachable as well.
You're conflating undefined behavior with implementation-defined behavior. If it was only to do with what we think of as normal variance between processors, then it would be easy to make it implementation-defined behavior instead.
The differentiating factor of undefined behavior is that there are no constraints on program behavior at that point, and it was introduced to handle cases where processor or compiler behavior cannot be meaningfully constrained. One key class is of course hardware traps: in the presence of compiler optimizations, it is effectively impossible to make any guarantees about program state at the time of a trap (Java tried, and most people agreed they failed); but even without optimizations, there are processors that cannot deliver a trap at a precise point of execution and thus will continue to execute instructions after a trapping instruction.
It's not only C-level is it. There's no (guarantee across architectures for) machine code for that either.
You can, and the results are machine specific, clearly defined and well-documented. Ancient ARM raises an exception, modern ARM and x86 can do it with a performance penalty. It's only the C or C++ layer that is allowed to translate the code into arbitrary garbage, not the CPU.
In worse scenarios, your programme will silently continue with garbage, or format your hard disk or give attackers the key to the kingdom.
Another commenter suggested using LLMs, but I disagree. Having clangd emit warning squiggles for unchecked operations (like signed addition) would be a good start.
Dead code elimination is essential for performance, especially when using templates (this is basically what enables the fabled "zero cost abstraction" because complex template code may generate a lot of 'inactive' code which needs to be removed by the optimizer).
The actual issue is that the compiler is free to eliminate code paths after UB, but that's also not trivial to fix (and some optimizations are actually enabled by manually injecting UB (like `__builtin_unreachable()` which can make a measurable difference in the right places).
enum op_t{ add, mul };
int exec(op_t op, int a, int b) {
if(op == add) { return a+b; }
if(op == mul) { return a\*b; }
}
c = exec(add, a,b);
http://archive.adaic.com/standards/83lrm/html/lrm-11-01.html
"STORAGE_ERROR This exception is raised in any of the following situations: (...) or during the execution of a subprogram call, if storage is not sufficient."
It's not safe though because throwing an exception, panicking, etc, is still a denial of service. It's just more deterministic than silently overwriting the heap instead. If the program is critical then you need to be able to statically prove the full size of the stack, which you can do with C and C++ with the right tools and restrictions.
First, you can define what happens when stack space is exceeded. Second not all programs need an arbitrary amount of stack space, some only need a constant amount that can be calculated ahead of time. (And some languages don't use a stack at all in their implementations.)
Your language could also offer tools to probe how much stack space you have left, and make guarantees based on that. Or they could let you install some handlers for what to do when you run out of stack space.
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 quicksand. That's the danger of UB.
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 and not the compiler which decides whether an unaligned access is a problem or not. On most modern CPUs unaligned load/stores are no problem at all (not even a performance penalty unless you straddle a cache line). There's no point in restricting the entire C standard because of the behaviour of a few esoteric CPUs that are stuck in the past.
PS: we also need to stop with the "what if there is a CPU that..." discussions. The C standard should follow the current hardware, and not care about 40 year old CPUs or theoretical future CPU architectures. If esoteric CPUs need to be supported, compilers can do that with non-standard extensions.
An honest discussion would be something more like 'dereferencing pointers can lead to UB on invalid pointers. Here are N examples of that. Maybe avoid using pointers. Maybe consider how other languages avoid pointers. Maybe these shouldn't be UB and instead some other class of error.' And then even more honest discussion would present the upsides of having pointers and the upsides of having these errors be UB.
Instead, the article (and your comment) take this valid operation and presents it as invalid. Imagine you're a new programmer, you are just starting to wrap your head around pointers and you stumble across this article. You see the first example and it looks exactly what you would expect a dereference to look like. But the article claims it's wrong, and now you're confused. So you dig into the article more closely and are exposed to all these terms like UB, alignment, type coercion etc and come away more confused and scared and disinclined to understand pointers. This is classic FUD. This is a technique to manipulate, not educate.
Pointers have pros and cons. UB has pros and cons. Let's try to educate people about them.
Far from being just "C with classes", modern C++ is very different than C. The language is huge and complex, for sure, but nobody is forced to use all of it.
No HN comment can possibly cover all the use cases of C++ but in general, unless you have a very good reason not to:
- eschewing boomer loops in favor of ranges
- using RAII with smart pointers
- move semantics
- using STL containers instead of raw arrays
- borrowing using spans and string views
These things go a long way towards, shall we say, "safe-ish" code without UB. It is not memory-safe enforced at the language level, like Rust, but the upshot is you never need to deal with the Rust community :^)
>After all, C/C++ is not a memory safe language.
In the context of UB discussion, the arguments apply equally to C and C++.
How would you write that?
I entirely agree with all your points that C and C++ are completely different languages at this point. And yet I wanted to write this post about something that is true for both.
In the end, everything comes down to culture war.
However, that's obviously not the point? Ignoring the idea that people can/should just "git gud" and write perfect code in a language with lots of old traps, you can't control how everyone else writes their code, even on your own team once it gets big enough. And there will always be junior devs stumbling into the bear traps of c/c++ (even if the rest of the codebase is all modern c++). So no matter how many great new features get added to C++, until (never) they start taking away the bad ones, the danger inherent to writing in that language doesn't go away.
Also, safe != non-UB. TFA isn't so much about memory safety anyway.
As far as stdlib usage is concerned: that's just your opinion. The stdlib has a lot of footguns and terrible design decisions too, e.g. std::vector pulling in 20k lines of code into each compilation unit is simply bizarre.
Also:
- eschewing boomer loops in favor of ranges
Those "boomer loops" compile infinitely faster than the new ranges stuff (and they are arguably more readable too): https://aras-p.info/blog/2018/12/28/Modern-C-Lamentations/
- borrowing using spans and string views
Those are just as unsafe as raw pointers. It's not really "borrowing" when the referenced data can disappear while the "borrow" is active.
> bool parse_packet(const uint8_t* bytes) {
> const int* magic_intp = (const int*)bytes; // UB!
you might try to argue that uint8_t is not necessarily char, and while it is true that implementations of C can exist where CHAR_BIT > 8, but those do not have uint8_t defined (as per spec), so if you have uint8_t, then it is "unsigned char", which makes this cast perfectly safe and defined as far as i can tell. Of course CHAR_BIT is required to be >= 8, so if it is not >8, it is exactly 8. (In any case, whether uint8_t is literally a typedef of unsigned char is implementation-defined and not actually relevant to whether the cast itself is valid -- it is)
Of course, this exchange just demonstrates the larger point, that even a world-class expert in low level programming can easily make mistakes in spotting potential UB.
A "world-class expert in low level programming" knows that unaligned memory accesses are no problem anymore on most modern CPUs, and that this particular UB in the C standard is bogus and needs to fixed ;)
Pointer casts changing pointer bit sequences is common on weird platforms (eg: some TI DSPs, PIC, and aarch64+PAC). And it is valid as per spec. Pointer assignment is not required to be the same as memcpy-ing the pointer unto a pointer to another type.
You misunderstood the spec. No promises are made that that cast copies the pointer bit for bit (and thus creates an invalid pointer). Therefore, your objection to invalid pointers is null and void. :)
> A pointer to an object type may be converted to a pointer to a different object type. If the resulting pointer is not correctly aligned71) for the referenced type, the behavior is undefined.
C23 6.3.2.3p7.
Great way to demonstrate the point of the article.
On recent Intel and 64-bit ARM processors, data alignment does not make processing a lot faster. It is a micro-optimization. Data alignment for speed is a myth. // https://lemire.me/blog/2012/05/31/data-alignment-for-speed-m...
(while in the olden days, a program may crash on unaligned access, esp on RISC)
I don’t see what spec part would prohibit that cast from validly compiling to
BIC r3, r0, #3
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"
But don't misunderstand the goal of that: C and C++ will never get rid of UB. The result of dereferencing an invalid pointer is UB, will always remain UB, and really cannot be anything other than UB.
> 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.
Yup. But the point of the article is that even expert humans cannot do this alone. And as I wrote, LLM+junior won't suffice either. We need LLM+senior experts.
And it's a problem that we have way more existing UB than expert capacity.
Now, will LLMs and experts both miss UB in some cases? Of course. There's no 100% solution. But LLMs, I claim, will find orders of magnitude more, with low false positive, than any expert. Even if these expert humans (like in the OpenBSD case for the two bugs I found, one of which was UB) are given more than three decades to do it.
I didn't even use the best model, complex code target, or time. I just wanted to choose a target that has a high chance of having very good experts already having audited it.
Yes.
Even in languages other than C (i.e. you will get behaviour that nothing in the input specified).
When LLMs generate code, all languages have UB.
UB means literally no restrictions. So if you standard says 'you have to crash with an error message' that's already no longer UB.
struct foo {int i;};
int func(struct foo *x) {return x->i;}
int main() {
int (*funcptr)(void*) = (int (*)(void*)) &func;
struct foo foo = { 42 };
return funcptr(&foo);
}
struct foo {int i;};
int func(void *x) {return ((struct foo *)x)->i;}
int main() {
int (*funcptr)(void*) = &func;
struct foo foo = { 42 };
return funcptr(&foo);
}> If the function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function, the behavior is undefined.
Compatible types requires integrating texts from several different paragraphs, but the general notion is "identical type, in a frontend sense", not "same ABI." This means that "const void " and "void " are not compatible types, much less "void " and "struct foo ".
I can see the value in saying that struct x* isn't compatible with struct y*, because they could have different alignment or packing rules. But struct x* and void*, which is already special-cased to allow assignment without a cast? Why aren't these considered compatible in function pointer parameter definitions?
Is there any work involved in casting void* to struct* (on any architecture) that a plain function pointer would miss out?
- casting an object pointer to or from void*
- casting an object pointer to or from char*
You're not doing either of those things. A function pointer is not an object pointer (the standard does not guarantee that the two kinds of pointer even have the same size/representation, and in fact on some esoteric hardware they don't), and even if it were, you aren't casting to or from void* or char*. So it's UB for two separate reasons.
You can cast between pointer types freely so long as they can be representable in one another (some casts are undefined because the address would be unaligned in the target pointer type, and there's actually no guarantee that pointers to objects and pointers to functions have the same representation).
Strict aliasing rules don't kick in at pointer type casting, but rather kick in at lvalue access--when you dereference a pointer, in other words--and you've also given the list of strict aliasing rules completely incorrectly.
I guess enumerating all the possibility is just .. don't look right? make the standard too long and complex?
struct foo *x = malloc(sizeof(struct foo)); /* malloc returns void* */
3.16 undefined behavior: Behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or of indeterminately valued objects, for which this International Standard imposes no requirements. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message).
By 'bounded', this obviously ignores the security consequences of e.g. buffer overflows, but just because UB can be exploited doesn't mean it's appropriate for e.g. the compiler to exploit it too, that clearly violates the intent of this paragraph.
Aren't "unpredictable results" and "no requirements" contrary to the idea that the behavior would be "somewhat bounded"?
I don't think you could sincerely argue that this definition intends to allow the compiler to totally rewrite your code because of one guaranteed UB detected on line 5, just that it would be good to print a diagnostic if it can be detected, and if not to do what's "characteristic of the environment". Does that make sense?
I touched on this in the "it's not about optimizations" section. It's not the compiler is out to get you. It's that you told it to do something it cannot express.
It's like if you slipped in a word in French, and not being programmed for French, it misheard the word as a false friend in English. The compiler had no way to represent the French word in it's parse tree.
So no, it's not overly legalistic. Like if the compiler knows that this hardware can do unaligned memory access, but not atomic unaligned access, should it check for alignment in std::atomic<int> ptr but not in int ptr? Probably not, right?
I've (fruitlessly) had this discussion on HN before - super-aggressive optimisations for diminishing rewards are the norm in modern compilers.
In old C compilers, dereferencing NULL was reliable - the code that dereferenced NULL will always be emitted. Now, dereferencing NULL is not reliable, because the compiler may remove that and the program may fail in ways not anticipated (i.e, no access is attempted to memory location 0).
The compiler authors are on the standard, and they tend to push for more cases of UB being added rather than removing what UB there is right now (for exampel, by replacing with Implementation Defined Behaviour).
- Creating a potentially troublesome misaligned int pointer is a precisely localized and completely explicit user mistake, not something that just happens because it's C.
- Passing signed char to character classification functions that expect an unsigned char (disguised as an int) is a very specific dumb user error. The C standard could specify that all negative inputs, including EOF and invalid signed char values, are classified as not belonging to the character class, but I doubt the current undefined behaviour in isxdigit() etc. implementations ever went beyond accepting invalid inputs.
- Casting floating point values to integer values in general requires taking care of whether the FP values are small enough to be represented and what to do with NaN and Inf values: not the language's responsibility. C offers a toolbox of tests, not ready-made application specific error handling.
- Expecting C to handle "address zero" in physical memory in ways that conflict with NULL in source code denotes a complete lack of understanding of what a program is. Where stuff in an executable is loaded in memory, in the rare cases when it matters, can surely be affected with platform specific extensions, possibly at the level of linker commands with nothing appearing in the C source code. i = i++edit: I'm not sure it's even undefined in C.
Although many newer languages are safer (with the exclusion of Rust, primarily by being slower) the same kinds of issues that are there in C are there in these languages, their effects are just harder to see.
People complain about C as though they know how to fix it.
Brainfuck is "simple" by any other definition as well, but that's not a useful quality.
That doesn't mean the C is a safer language than Swift, or a less-capable language than Swift. But in terms of "easy to understand along the happy-path", it's a lot easier to get going in C.
Swift, for example, bakes a whole load of CS-degree-level ideas and concepts into the basic language with its optionals, unwrapping, type-inference, async/await, existential types, ... ... ... . C doesn't do any of that. There are (many!) more footguns in C, but the language is less complex as a result.
Brainfuck is not at all simple, from that point of view. This is a valid Brainfuck program:
>+++++++++[<++++++++>-]<.>+++++++[<++++>-]<+.+++++++..+++.[-]>++++++++[<++++>-]<. >+++++++++++[<+++++>-]<.>++++++++[<+++>-]<.+++.------.--------.[-]>++++++++[<++++ >-]<+.[-]++++++++++.
This is the equivalent C program
#include <stdio.h> int main() { printf("Hello world!\n"); }
One of these is far simpler than the other.
[edit: changed to make the examples do the same thing]
It's slower than Fortran and, depending on the platform, cobol. It's a bigger minefield than any language that came after it barring C++.
The only real advantage I can ascribe to C is that it's actually still being used after all these years, and it mostly works similarly on most hardware, like a Java for people who enjoy the casino.
Fixing C without breaking existing C code is pretty much impossible. You can start by defining warnings for UB, but then you will break any of the more trivial examples in the article. You can also start by simply killing off weird platforms (force a specific amount of bits for instance, screw the weird 16 bit char chips). Making casts explicit would probably fix a lot of problems too, though you'd need better syntax for that.
There is no fixing C without changing what C really is.
I'm not an expert in either language but my anecdotal experience disagrees with this - writing Zig has been far simpler and less error-prone than writing C.
Good open source ones:
Frama-C
IKOS (from NASA)
(I hope casting fear is not UB)
I'm sure that's UB in C
In C++ just use <reinterpret_cast>
I say this as an experienced C developer.
The other obvious issue with the overall perspective is that C and C++ are being thrown together directly as if somehow they’re nearly the same language, but they are really very far apart nowadays.
Most C++ today will be immediately obvious and not accidentally mixed up with C.
It’s not. All that matters is what C compilers actually do and what real C programs expect.
This is a good thing. It creates a culture where the two sides meet each other where they’re at
But why I’m saying has always been true. What has changed is that the effective portability of C and C++ code has increased due to the reduction in number of compilers and arches
"Explicit casts only" worked fine in Modula-2, which doesn't have as many scalar types.
The part about hardware is wrong BTW. In all the cases about null pointers and out-of-bounds access and integer overflow and whatnot, the hardware semantics are clearly defined, and the assembler code does exactly what is written. The way modern compilers act on your code makes C less safe than assembler in that sense.
> The part about hardware is wrong BTW
Could you be more specific? I think by "wrong" you may mean "not actually relevant to UB", and you're right about that. If that's what you mean then that part is not for you. It's for the "but it's demonstrably fine" crowd.
> the hardware semantics are clearly defined
Yup. The article means to dive from the C abstract machine to illustrate how your defined intentions (in your head), written as UB C, get translated into defined hardware behavior that you did not intend.
I'm not saying the CPU has UB, and I wonder what part made you think I did.
That's what I mean game of telephone. The UB parts get interpreted as real instructions by the hardware, and it will definitely do those things. But what are those things? It's not the things you intended, and any "common sense" reading of the C code is irrelevant, because the C representation of your intentions were UB.
When comparing signed and unsigned integers of same size the signed one will be converted to unsigned. In a reasonably configured project compiler will warn about it.
In case of integers smaller than int, promotion to int happens first.
In case of signed and unsigned integers of different size, the smaller one will be converted to bigger one.
Additionally, some (most?) UB is intentionally UB so that optimisers are free to do fancy tricks assuming that certain cases will never happen. Indeed, this is required for high performance. If they do happen, again, it can lead to unexpected behaviour.
PS: Most languages that don't have a specification declare their primary implementation to be specification-as-code. Rust is an example of that, and it does still have UB: the cases that the compiler assumes will not happen.
edit: for example I'm typing this into Safari which means probably every key press and event is going through JSC JIT compiled functions—which have, structurally and necessarily and intentionally, COMPLETELY undefined behavior according to the spec—and yet it miraculously works, perfectly, because the spec doesn't really matter
(e.g. just compiling with address sanitizer and using static analyzers catch pretty much all of the 'trivial' memory corruption issues).
Especially compared with modern languages with lambdas/exceptions/virtual functions and so on.
The one thing I see can make it harder is function pointers.
The problem lies with compilers, not with the language and its specification, or with the creators of the C programming language.
Anyone can write a compiler that transforms all undefined behaviors (UB) into defined behaviors (DB). And your compiler will be used by people, including me.
OTOH one could argue that creating truly portable programs is not possible since a programming language is a leaky abstraction - different machines have different endianness, different alignment requirements, different amounts of memory, etc. One could argue therefore that the language should not make any assumptions about the alignment restrictions, or lack of them, on the machine you are compiling for. Just document that "manually created" pointers may be unaligned and have machine-dependent behavior. A nice compiler could still generate a warning or error if you create a pointer that doesn't meet the alignment requirements of the target you are compiling for.
C/C++'s provision of type casts reflects that the language has made the design decision to not restrict the user, and let them step outside the bounds of any guarantees the language provides if they want to. Unions are also a form of type cast.
completely agree!
I mean, you have to go out of your way and use a cast to get the UB in the first example.
For the `isxdigit` implementation, using a parameter to index into an array without a length check is pretty suspect already. I don't think any of my code actually indexes an array without checking the length in some way.
For the float -> int conversion, converting a float to an int without picking a conversion does not make sense in the first place - math.h has rounding and ceiling functions.
> For all you know the compiler has no internal way to even express your intention here.
I'm human, not a compiler, and even I cannot tell what the intention is behind trying to call NULL as a function. What exactly is expected to happen?
> Because the argument needs to be a pointer, and the NULL macro may be misinterpreted as an integer zero.
I don't think this is true for C. The NULL macro is defined to be a pointer in the C standard, AFAIK. Just because comparisons with zero are allowed, does not imply that the standard implicitly promotes NULL to `int`.
I think only the final one is of note (the 24-bit shift assigned to a uint64_t).
Probably confusion with C++ where NULL is 0 which is a special case that can be implicitly cast to both integers and pointers, unlike non-zero constants. C doesn't need this because it doesn't require explicit casts from void pointers to others.
First let me state the case for C. It’s meant to be used as a systems language that’s as close to assembly as possible while remaining portable (compared to assembly). As such it’s the first high-level language developed for any new processor.
Given the above predicate: Isn’t everything described in the article as it should be?
Add too much to the language and it becomes less possible to implement on new architectures, right? Because the undefined behavior lets implementors stand up new compilers fairly quickly.
For less undefined behavior isn’t it better to use languages that have that in their DNA? D, Zig, Go, Java, etc?
I think the real trick question is "as it should be for whom?".
Reading the comments I think people underestimate the complex interaction between:
- engineers that design hardware (they don't care much about the compiler, except when it has to fix their mistakes)
- engineers that do the compiler (they have to struggle with all quirks of the new architecture and all of the complaints of the users)
- users of the new system (hardware + compiler) that just want to take their 100k lines of code (libraries) and just use it on the new system with better performance (as that's what the hardware people promissed!)
- users working on one architecture all their lives
For the compiler people, yes, probably most what is described is as it should be. For the users (that care about performance and not making porting efforts), probably no.
Now, even when I was doing compiler work we had a hard time explaining our users why we couldn't do some things they wanted (while also improving performance and not changing code that was writting), so explaining that on the internet seems to me a lost battle.
I am sure there are things that can be improved, and standards evolve. But the problem is very complex given the sheer amount of code written and the strange architectures out there.
the only people complaining about being able to do awful things are people that do awful things
- unless you are trying to sink it in mercury. then it floats
- unless it is an uranium bar
- go sink uranium bars in mercury yourself
Doesn't matter though because you aren't writing standards conforming C. You're writing whatever dialect your compilers support, and that's probably (module bugs) much better behaved than the spec suggests.
Or you're writing C++ and way more exposed to the adversarial-and-benevolent compiler experience.
The type aliasing rules are the only ones that routinely cause me much annoyance in C and there's always a workaround, whether if it's the launder intrinsic used to implement C++, the may_alias attribute or in extremis dropping into asm. So they're a nuisance not a blocker.
> When programming in C, to avoid unexpected pitfalls, one must be acutely aware of a whole slew of implicit behaviors (some of which are implementation-defined or even undefined).
Part of the reason for all the UB in OpenBSD is that UBSan doesn't run on that platform. When I ported OpenBSD's httpd to Linux, I found that UBSan tripped before the server even came up because the config flag parsing shifts into the MSB of a signed integer.
I tried to contribute back a patch (just make the flag bitfield unsigned), but it was ignored. I think if UBSan ran natively on OpenBSD, then there would be a lot more of these patches, and the maintainers would have to take an official stance on whether they think these bugs matter.
As for UB, the compiler has the final say. Nobody should write nontrivial c without understanding their compiler, the same as nobody should write c without understanding their text editor.
Code in other languages breaks between versions, in c there are projects with code from every version at once!
Looking at it another way, work put into a c compiler enables you to write nontrivial code.
- Making a Turing machine have deterministic and predictable results is hard.
- Modern hardware is complex and getting all hardware to behave the same way requires a strong mathematical abstraction.
C was never intended to be a fully defined mathematical abstraction. It was a language which was easy to write a compiler for. That's its original strength. Trying to make it something it isn't is the problem. Either choose a language which does have such abstractions or understand the drawbacks of the tool you are using.
Right tool for the right job.
Is "nontrivial" defined
How would one identify "nontrivial" C code
Is there an objective measure (defined)
Or is it a matter of personal opinion that could vary from person to person (undefined)
``` int isxdigit(int c) { if (c == EOF) { return false; } return some_array[c]; } ```
If you write code like this, then everything in programming is UB.
Unaligned pointer accesses are UB because different systems handle it differently. This 'should' be to allow the program to be portable by doing what the system normally does.
Instead it's been highjacked by compiler writers, with the logic that "X is UB, therefore can't happen, therefore can be optimised away."
Int c = abs(a) + abs(b); If (a > c) //overflow
Is UB because some system might do overflow differently. In practice every system wraps around.
That should be a valid check, instead it gets optimised away because it 'can't' happen.
C gives you enough rope to hang yourself. The compiler writers don't trust you to use the rope properly.
C is horrible for trying to write a portable user-mode program in 2026. There are lots of better options.
C is great for writing low-level system code where you need to optimize performance down to the last cycle. It not abstracting away the hardware is super important for some use cases. A classic example is all of the platform-specific flavors of memcpy in the Linux kernel that are C/assembly hybrids hand-optimized for the SIMD pipelines of some CPUs.
C is a tool, Rust is a tool, Java is a tool, Python is a tool. Use the right tool for the job ¯\_(ツ)_/¯.
https://nickyreinert.de/2023/2023-05-16-nerd-enzyklop%C3%A4d...
Shrug.
Sigh. s/sizeof(int)/_Alignof(int)/.
There are good reasons for an implementation to have sizeof(int) = _Alignof(int) and not a mere multiple of it, but if you are going to discuss subtle points and UB, just stick to the language guarantees.
> But let’s say you have a modern machine, where NULL is a pointer to address zero, and you actually have an object there.
You don't program in C on such a machine. Or maybe memory is virtualized, and it does not matter that your object lives at physical address zero, as long as you can map a non-zero virtual address to it.
> So how do you print an uid_t?
if ((uid_t)-1 < (uid_t)0) {
// uid_t is signed
printf("%" PRIdMAX, (intmax_t)id);
} else {
// uid_t is unsigned
printf("%" PRIuMAX, (uintmax_t)id);
}
It's not rare for the array index to come from untrusted input.
It's not rare for the supposedly valid UTF-8 string to come from untrusted input.
...
Why single out division? This problem affects every partially defined operation. In the case of division at least, everyone learned in school that thou shalt not divide by zero. Adding two untrusted integers and forgetting that signed overflow is UB, not defined as a modulo? Your average programmer is much less likely to see that coming.
> unsigned char a = 0xff;
> unsigned char b = 1;
> unsigned char zero = 0;
> bool overflowed = (a + b) == zero;
>
> unsigned char a = 0x80;
> uint64_t b = a << 24;
Here:
unsigned char a = 0xff;
unsigned char b = 1;
unsigned char zero = 0;
bool overflowed = (unsigned char)(a + b) == zero;
unsigned char a = 0x80;
uint64_t b = (uint32_t)a << 24;I stopped reading there. If you have decades of experience in C/C++ and don't know what that means (and that it's arch specific), I'll assume those decades were mostly the same year over and over.
C/C++ are horrible languages, but they deserve better opponents than that.
What a contradiction. Strong evidence that standard-driven programming language development is much worse than implementation-driven development. Standards should be used for data types and external interfaces/protocols, not programming languages.
Write compiler which will define all this behavior. Usually people forget that UB exists only in standard. In practice it is always defined.
P.S. of course, while your hardware + firmware staying unchanged
P.S. not always defined in documentation - I mean defined in e.g. code
So Linus was right? But for a second reason too:
C++ is a horrible language. It’s made more horrible by the fact that a lot of substandard programmers use it, to the point where it’s much, much easier to generate total and utter crap with it. Quite frankly, even if the choice of C were to do _nothing_ but keep the C++ programmers out, that in itself would be a huge reason to use C.
That is, accepting C++ code from programmers who use C++ could be a SOX violation ;-)
Everything else is a waste of time!