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On 08/09/2021 18:35, Roberto Ierusalimschy wrote:

I completely agree with Roberto that the sentence is confusing. And
(I guess) especially for us non-native English speakers (Roberto
and I, I mean), because, as you pointed out, it is grammatically
incorrect. At least the first comma should be omitted.

[...]


A more plausible explanation is that the grammar is fine, it says
what it wants to say, and you are undestanding it correctly, but
nevertheless you want to understand something else. :-)

Scrutinizing once again the standard, I found this:

4. Conformance [...] 2 If a ''shall'' or ''shall not'' requirement
that appears outside of a constraint is violated, the behavior is
undefined. Undefined behavior is otherwise indicated in this
International Standard by the words ‘‘undefined behavior’’ or by the
omission of any explicit definition of behavior. There is no
difference in emphasis among these three; they all describe
‘‘behavior that is undefined’’.

It says with all words that the omission of any explicit definition
of behavior is as "splorky" as the words "undefined behavior".



Urgh! I definitely missed that! So I stand corrected. Back to square
one. :-(

Now I have to reconcile in my head this fact and the fact that people
much more expert than me keep on focusing on enumerating all the source
of UB as there were no other ones.

Even the standard plays this game with annex J (I know it's informative
and not normative, however...). But I admit that, reading that carefully now, at the beginning it says (J.2): 

"The behavior is undefined in the following circumstances:"

and not

"The behavior is undefined ONLY in the following circumstances:"

but I always interpreted it in this latter way. Uff!


<...*browses his PDF collection*...>
Maybe an explanation is what I found in the book by Derek M. Jones (a former C committee member and compiler expert) "The New C Standard - An Economic and Cultural Commentary" (freely available online here http://www.knosof.co.uk/cbook/cbook.html). 

It addresses the C99 standard (the language part only), commenting every
sentence in the perspective of gaining insight in how to choose
programming guidelines.

Commenting that sentence in the "conformance" section it states the
following (clause n.85 of the book at page 205 and 206 of the PDF):

---------------------------------------------
"Undefined behavior is otherwise indicated in this International
Standard by the words “undefined behavior” or undefined behavior
indicated by the omission of any explicit definition of behavior."

Commentary

Failure to find an explicit definition of behavior could, of course, be
because the reader did not look hard enough. Or it could be because there was nothing to find, implicitly undefined behavior. On the whole the Committee does not seem to have made any obvious omissions of definitions of behavior. Those DRs that have been submitted to WG14, which have later turned out to be implicitly undefined behavior, have involved rather convoluted constructions. This specification for the omissions of an explicit definition is more of a catch-all rather than an intent to minimize wording in the standard (although your author has heard some Committee members express the view that it was never the intent to specify every detail). 
---------------------------------------------
So this confirms the words of the standard in that "implicit UB" by omission is a thing, but states that the intent of the committee was to really enumerate explicitly every source of UB and left the "implicit UB" there just as a fallback for something they might have not caught, although Mr.Jones is careful not to state his interpretation of the committee's intent as 100% sure.
As I said in a previous message, I keep on getting puzzled by C and its standard. Oh well! At least I learned something new today, even if it is another "small-print paragraph" in the C standard! :-) 



-- Roberto



Cheers!

-- Lorenzo