You're correct in practical terms, but I'm making a very pedantic point about what the standard requires happen, mainly because this pedantry has important implications for e.g. safety critical C. Note 1 to the definition in 3.4.3 provides some clarification about the extent of UB and states that UB can manifest at translation time. It also gives says that the translator should behave in a documented manner when encountering UB, but does not require that it do so.

C has both translation-time UB and runtime UB. (C++ explicitly separates the two concepts into "ill-defined, no diagnostic required" and "undefined behavior".) You can tell them apart from the condition for UB to occur: if it's a translation-time condition, then it's translation-time UB, and if it's a runtime condition, then it's runtime UB. (Same with implicit UB: is it a translation-time or a runtime assumption being violated?)

Usually when we talk about UB, we're implicitly talking about runtime UB, since translation-time UB is generally far less subtle. If a program contains only conditional runtime UB, the compiler is not permitted to break the entire program from the very beginning, since all possible executions that do not trigger runtime UB must execute correctly as per 5.1.2.3.

5.1.2.3 only binds conforming programs. Programs containing UB are by definition non-conforming.

I hadn't considered the C++ standard here, but 1.9 is much more clear than corresponding C verbiage. 1.9.5 is exactly what's described upthread, where any "execution [that] contains an undefined operation" has no prescribed behavior. But the note to the requirement immediately before that (1.9.4) doesn't use that language and instead "imposes no requirements on programs that contain UB". If they had intended only to avoid specifying semantics for programs that hit UB during some possible execution, they would have used the same language as 1.9.5.

Your claim is actually false. C differentiates between a conforming program and a strictly conforming program. 5.1.2.3 binds to conforming programs which is permitted to produce output dependent on undefined behavior.

Only strictly conforming programs may not produce output dependent on undefined behavior.

No? Conformance allows unspecified and implementation defined. Strict conformance is the absence of that (i.e. same output in every conforming environment). Neither includes UB, as UB is "outside the standard" in some sense and doesn't have defined semantics.

It's a common misconception that a conforming program may not engender undefined behavior. In fact this very article touches on how realloc has introduced new (and backwards incompatible) undefined behavior precisely to accommodate the POSIX standard (so that POSIX compliant implementations of C can redefine the otherwise undefined behavior however they please).

Can you cite that? It runs against a plain reading of the standards (both C and C++) and would be insane for the standard to allow "correct" programs to include those with undefined behavior. There was even an unadopted proposal (n853 [1]) attempting to clarify this.

While I was making sure I wasn't missing something obvious, I took a look through the rest of the WG14 proposals to see if I was somehow off in my understanding regarding translators being allowed to barf over UB anywhere in the program. There was a proposal clarifying the situation to the possible-execution understanding from upthread submitted by Victor Yodaiken (n2278 [2]), but unfortunately it was also never adopted.

[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n853.htm

[2] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2278.pdf

You are now mixing correct and conforming programs. Neither the C or C++ Standard mention anything involving correct programs. C uses the term conforming program and C++ uses the term well-formed program.

In C a conforming program is any program that satisfies any single conforming implementation, even if said implementation includes extensions or non-portable constructs. A strictly conforming program is a program that satisfies every conforming implementation, which implies that said program does not produce an output that depends on undefined, unspecified, or implementation defined behavior.

In C++ a well-formed program is any program that satisfies the syntax rules, diagnosable semantic rules and the one-definition rule.

Going off of the C17 numbering,

4/7: "A conforming program is one that is acceptable to a conforming implementation."

This definition has no restrictions regarding runtime requirements, unlike for strictly conforming programs.

5/1: "An implementation translates C source files and executes C programs in two data-processing-system environments, which will be called the translation environment and the execution environment in this International Standard. Their characteristics define and constrain the results of executing conforming C programs constructed according to the syntactic and semantic rules for conforming implementations."

So clause 5 binds all "conforming C programs constructed according to the syntactic and semantic rules for conforming implementations", not just strictly conforming programs.

Now, 4/3: "A program that is correct in all other aspects, operating on correct data, containing unspecified behavior shall be a correct program and act in accordance with 5.1.2.3."

We can interpret this as saying that "a program that is correct in all other aspects... containing unspecified behavior" is "constructed according to the syntactic and semantic rules for conforming implementations" even if it only works when "operating on correct data".

From there, it does not seem very difficult to conclude that in general, a conforming program which contains fully specified behavior, assuming it operates on correct data, is also "constructed according to the syntactic and semantic rules for conforming implementations", and is therefore bound by clause 5. If we were to instead take the negation of this conclusion, that a program is not bound by clause 5 if any possible input data causes it to violate a runtime requirement, then the wording of 4/3 would not make any sense.

(In other words, every conforming program has a corresponding set of "correct input data", and it is correct and bound by clause 5 if it does not violate any runtime requirements when given any input data within that set. A program is only incorrect if that set is empty, i.e., the UB is unconditional.)

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Meanwhile, I suppose you're looking at C++17. The note in [intro.execution]/4 is non-normative, and all of the normative language (e.g., on the very next paragraph) attaches runtime UB to the execution as a function of the input data, not the pure program.

[intro.compliance]/(2.1) and its (non-normative) footnote further clarify the distinction, stating, "If a program contains no violations of the rules in this International Standard, a conforming implementation shall, within its resource limits, accept and correctly execute that program.... 'Correct execution' can include undefined behavior, depending on the data being processed; see 1.3 and 1.9." This suggests that a program that executes undefined operations does not necessarily contain any rule violations.

I think your confusion here is coming from the fact that "unspecified behavior" is a specific thing in the standard's terminology (looking specifically at n3088 right now), distinct from the concept of "undefined behavior". So when it says "constructed according to the semantic rules", that inherently excludes UB, which definitionally has no semantics prescribed for it (unlike unspecified behavior). For brevity's sake, I'm ignoring the allowance that an implementation can give any particular UB defined semantics.

To reduce my point to a list of options:

* No UB in the program -> Specified by 5.*

* No UB for certain inputs -> Specified for those inputs, not specified otherwise

* UB present, but not on any possible execution path -> Not specified (this is the argument)

* UB present on every possible execution path -> Not specified (definitionally)

I linked this in a sibling comment, but there was a proposal to amend C2x's wording here to specifically exclude this type of insanity (n2278), but it wasn't adopted because it could potentially prohibit optimizations and the working group doesn't really want to address the issue of undefined behavior with more definitions.

My claim is, "If a program violates the semantic rules at runtime given input A, but does not violate the semantic rules at runtime given input B, then the execution of the program given input B will be defined by clause 5." This is because the wording of 4/3 implies that "being constructed according to the semantic rules" is a function of both the program and the data it is given, such that the behavior can be defined on some inputs by clause 5, but undefined on other inputs.

As I understand it, your claim is, "If a program violates the semantic rules at runtime given input A, then the behavior of the program is undefined given input B, even if the program would not have violated the semantic rules at runtime given input B." Am I misunderstanding your claim?

> I linked this in a sibling comment, but there was a proposal to amend C2x's wording here to specifically exclude this type of insanity (n2278), but it wasn't adopted because it could potentially prohibit optimizations and the working group doesn't really want to address the issue of undefined behavior with more definitions.

N2278 seems entirely irrelevant to this question, of whether potential UB given one input can cause unexpected behavior given another input. Instead, it seems to say, "If the program violates the semantic rules at runtime given input A, causing UB, then the implementation is forbidden from making that behavior identical to the program's hypothetical behavior if it had been given another input B." That looks pretty unworkable in the general case. (E.g., must compilers operate as if an out-of-bounds write on one object can modify the value of a totally different object?)

I realized on thinking a bit more that two of the cases I mentioned are actually identical, so you're correct. Good to know going forward!

Fine. HN is, after all, a place where you can be pedantic.

But those of us who are actually writing programs mostly care about "in practical terms", and in practical terms, this doesn't happen, so we don't care. We've got enough trouble worrying about what does happen; we don't have time and energy to worry about what doesn't and won't happen.

To provide some more context/motivation for why you might care, I write safety-critical code. I'm often advising people what they need to do for certification, etc. If all you need to do is ensure that you never execute undefined operations and knock out the list of specified UB, that's totally, 100% manageable. Throw some sanitizers on, provide realistic input, and test the hell out of it. Normal stuff.

If the reality is that any UB can invalidate the entire program (as is the interpretation taken by other standards re: C), then that's not remotely sufficient. You have to ensure the complete absence of UB.

That's like saying: "I don't care what the standard says!"

Sure, this is perfectly fine.

Only that you're not writing any C/C++ than, but something in the "gcc 12 language with some switches", or maybe the "LLVM 15 language with some switches", or something like that.

Well, if Visual Studio (or whatever Microsoft calls their compiler these days), and all known versions of gcc, and all known versions of LLVM all do something sane, then I'm not sure I care all that much about the theoretical possibility that some compiler someday might do something insane.