OK, so to paraphrase your position: C has clear and complete semantics because anything written in C that doesn't have clear and complete semantics is, by definition, not actually written in C. Yes?

To me, that's like saying the tax code doesn't contain any loopholes, because any loopholes that it contains are, by definition, not actually part of the tax code.

While amusing, I don't think this is a fruitful way to look at the problem. :)

I think this answers your earlier question. We don't have tools that implement "C". We only have tools that implement supersets of C. In particular, these tools take code like the above and do "something" with them; the exact thing they do varies with optimization level and phase of the moon. What you want is a tool that takes such code and says "Oops, undefined behavior on line 9, here's a stack trace."

I think you're confusing syntax and semantics. The claim is that C is not really semantically well defined, but your toplevel posts talk about the fact that C has well defined syntax, which is true. However, that's besides the point. Syntax is roughly the surface form, whereas semantics refers to meaning. The problem is that a well formed program (ie a syntactically correct program; this is actually the precise definition of well-formed) in C yields undefined behavior on a semantic level, exactly what we don't want to happen. You need to maintain a clear separation between the syntax and meaning. When parent said that its legal C, he meant syntactically. The point is by definition you cannot tell from the surface/lexical form of C what is semantically valid and what is not semantically valid, ie what is well defined and what is not. So by saying its undefined behavior you are proving parent's point, that a syntactically valid piece of C yields a semantically not well defined chunk of code.

> It's undefined behavior meaning, by definition, it's not C. It's not defined in C.

The C99 standard defines undefined behaviour as

"behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements"

Undefined constructs are not necessarily erroneous (and therefore, in a way, "not C"). Many of them are perfectly legal constructs for which the standard you're targeting simply imposes no specific requirements. In fact, C standard documents list several (as in, I think, hundreds of) examples, many of them perfectly correct in terms of syntax. The standard just doesn't dictate what the compiler should do with them.

If you really like playing the words game, you could, at best, claim that undefined behaviour isn't C99 or C89 or whatever your compiler is targeting at a specific time, but it's very much C.

> It's not defined in C. If I had an list of all definitions, would an undefined item be in my list? No.

And yet Annex J of the C99 standard has a list of undefined behaviours.

And that list itself is just a reference, because the C standard is otherwise full of cases where it does not dictate requirements -- effectively, you know, defining a bunch of undefined items. E.g. 6.2.4, 2 in C99, where object lifetimes are defined, states that "If an object is referred to outside of its lifetime, the behaviour is undefined".

Besides, come on, look at code like this:

int foo = INT_MAX;

foo++;

How exactly is this "not C"? Can you think of any perfectly standards-abiding compiler that will not compile this? Does it use any keywords, identifiers or grammar that a C compiler does not understand?

"When you are using Undefined Behavior, you are not using C."

Yes you are using C when you're using undefined behavior. It's one of the cornerstones of C's "design." The standard is poorly defined enough that different people building C compilers will come up with their own interpretation of what specific, legal, C statements mean. These are statements used in real projects that also compile with real compilers. The massive problems that resulted led to static analyzers and compiler checks for undefined behavior. Plus, people recommending you avoid it wherever possible for predictability, safety, and security. It is legal C, though.

Whereas, in languages like Pascal or Ada, you could express similar operations with their semantics making it totally clear what you could expect. They'll either not compile or include runtime checks to avoid similar problems wherever possible. C's designers just didn't give a shit: they were using what modifications of unsafe BCPL they could to squeeze max performance & memory utilization out of a PDP-11. Wirth's Modula-2 at least gave a clean language with many checks & sanity by default that you could selectively turn off if absolutely necessary. Not C, though.

So, it's legal, real C designed without safety, clear semantics, and so on. Different projects implement "real C" differently due to unclear semantics. Different, broken executions result.