Keith Thompson <[email protected]> writes:

Tim Rentsch <[email protected]> writes:

Keith Thompson <[email protected]> writes:

The code we're discussing was snipped at some point, so here it
is again:

int main(void) {
int obj = 42;
typedef void func(void);
if (0) {
func *fptr = (func*)&obj;
fptr();
}
}

It illustrates the issue that the standard does not define the behavior
of a conversion from an object pointer type to a function pointer type
(other than the special case of a null pointer constant), but does not
make such conversion, expressed as a cast, a constraint violation.

I asked: "On what basis do you think a conforming implementation may
reject it?

The code is not strictly conforming.

Do you see a syntax rule or constraint that it violates?

No.

Do you see some other basis for rejecting it?"

Yes. The conversion expression can be untranslatable on some
platforms. That means any such expression cannot be part of
a strictly conforming program.

I disagree. A conforming implementation could implement a
conversion of an object pointer to a function pointer that always
yields a null pointer, or that yields a pointer whose representation
is derived from the representation of the operand. The latter is
typically done for pointer-to-integer or integer-to-pointer
conversions with mismatched sizes (6.3.2.3 explicitly says that
integers may be converted to pointers and vice versa).

If, hypothetically, the standard said that an otherwise valid
program containing a conversion of an object pointer to a function
pointer type must be successfully translated, then compiler
implementers could easily find a way to implement such conversions,
even if the result is not useful. It is incorrect to claim that
such a conversion expression may be "untranslatable".

What you mean by translatable is different from what I mean. Of
course it is possible to produce a sequence of bits that conforms
to the bit-level representation of a function pointer. The
problem is that it might not be possible to do that in a way that
is sensible, meaningful, or useful.

The contrast with converting between pointers and integers is
helpful here. Converting a pointer to an integer, even if the
integer type isn't as big as the pointer type, is always
potentially useful, because for example the integer could be used
as a hash function. Integers also have the nice property that
they are dense (not counting padding bits), so producing any old
integer value will never be problematic. (Yes I know about the
rule that if the value cannot be represented the behavior is
undefined, but I'm not talking about that.) Going the other
direction, an integer type whose size is large enough (and almost
always there are such types) can guarantee that converting a
pointer to an integer is invertible. There is a deeper principle
here: the addressing structures used in actual hardware use
integers as the basis for addresses. Converting between integers
and pointers always makes sense at some low level. An exception
to that rule is elaborate function pointers, which can use very
large structured values to represent a pointer to function. This
exception is the primary motivation for the C standard saying
that "The result [of converting a pointer to an integer] need not
be in the range of values of any integer type"; in such cases
the behavior is undefined but it is still always allowed to write
an expression asking for such conversions - it is only trying to
execute these expressions that causes a problem.

Note that the C standard reflects the distinction I'm making
here. The standard specifically specifies that integers may
be converted to pointers, and vice versa. The standard also
specifically specifies that object pointers may be converted to
other object pointers, and that function pointers may be
converted to other function pointers. The standard does NOT
specify that function pointers may be converted to object
pointers, or that object pointers may be converted to function
pointers. This difference corresponds exactly to what I mean
by translatable and untranslatable. This distinction is not
incidental, accidental, or meaningless. On the contrary, it is
certainly deliberate, and goes to the heart of the question here.

I have no interest in treating you as a student. I'd be much
happier if you would do more thinking for yourself.

I would be happier if you would not assume that I'm not already
doing that.

It's not assumption but observation. To give an example, not too
long ago you asked a question in comp.std.c asking about undefined
behavior and indeterminate answers. I wrote a long posting in
response, basically going through a thorough and systematic review
of the history in different versions of the C standard. I'm sure
you could have done that yourself if you had tried to do so.

Please note that I'm not saying that you *should* have done that,
only that you *could* have done that.

Be less arrogant.

I don't think it's arrogant to think or to say that you are
more capable than your comments would otherwise indicate. If
anything it seems just the opposite, that I have a higher
impression of your abilities than might seem to be the case
to a casual reader.

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On Fri, 19 Jan 2024 12:49:57 -0800, Tim Rentsch wrote:

An exception to that rule is elaborate
function pointers, which can use very large structured values to
represent a pointer to function.

Real-world example: PowerPC/POWER, where a function reference is two
addresses, one for the code and the other for, I think it’s called the GOT (“Global Object Table”). Every piece of code assumes its GOT register has been set up with the right value.

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Lawrence D'Oliveiro <[email protected]d> writes:

On Fri, 19 Jan 2024 12:49:57 -0800, Tim Rentsch wrote:

An exception to that rule is elaborate
function pointers, which can use very large structured values to
represent a pointer to function.

Real-world example: PowerPC/POWER, where a function reference is two >addresses, one for the code and the other for, I think it’s called the GOT >(“Global Object Table”). Every piece of code assumes its GOT register has >been set up with the right value.

That's called dynamic linking and is only used for inter-library function calls. Intralibrary and intraapplication function calls don't need the
GOT (Global Offset Table), which is used by all modern Unix and Linux distributions to support dynamic linking.

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On Mon, 22 Jan 2024 16:36:46 GMT, Scott Lurndal wrote:

.. GOT (Global Offset Table) ...

There had to be actual addresses in there somewhere though, did there not? Otherwise you would have to ask “offsets to what?”.

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I am responding here to two different messages from James Kuyper,
combined for convenience in a single posting. Each segment
starts with an attribution line naming James and giving his email
address, as would appear in a normal followup, and then gives
a Message-ID for that posting.

Incidentally, both of these message seems to be a response to a
posting (or postings) of mine, but they seem not to be linked
in the usual way that newgroup postings. I'm not sure how or
why that happened, but I thought I should mention it.

[..first message..]

James Kuyper <[email protected]> writes:
[Message-ID: <uje6vv$gei$[email protected]>]

[This message is missing an attribution line that might have
been something like the following line]

Tim Rentsch <[email protected]> writes:

Keith Thompson <[email protected]> writes:

The code we're discussing was snipped at some point, so here it
is again:

int main(void) {
int obj = 42;
typedef void func(void);
if (0) {
func *fptr = (func*)&obj;
fptr();
}
}

It illustrates the issue that the standard does not define the
behavior of a conversion from an object pointer type to a function
pointer type (other than the special case of a null pointer
constant), but does not make such conversion, expressed as a cast,
a constraint violation.

I asked: "On what basis do you think a conforming implementation
may reject it?

The code is not strictly conforming.

Do you see a syntax rule or constraint that it violates?

No.

Do you see some other basis for rejecting it?"

Yes. The conversion expression can be untranslatable on some
platforms. That means any such expression cannot be part of
a strictly conforming program.

The committee has officially ruled in DR 109 that code with undefined behavior renders the program not strictly conforming, only if execution
of that code is an inevitable consequence of running the program. Not
only is execution of this code not inevitable, it is in fact impossible
for it to be executed.

The remarks in DR 109 are irrelevant to what I'm saying. The
program given above fails to be strictly conforming for reasons
that have nothing to do with undefined behavior.

I'm curious - on what platform is it impossible, or at least
difficult, to translate an if(0) block as a no-op? I'd think that
simply failing to generate any corresponding machine code would be
sufficient on most, if not all, platforms.

I hope you realize that this question is quite irrelevant to the
matter being addressed here.

The code that appeared in DR 109 was not a no-op, but the behavior
of any program that called the function would be undefined, which
means that the standard imposes no requirements on its behavior.
On what platform is it difficult to generate code that has no
requirements on how it behaves? I'd think it would be trivial
translate it as, for instance, the equivalent of

fprintf(stderr, "ISO C does not define the behavior of converting \n"
"a pointer to an object into a pointer to a function.\n"); exit(EXIT_FAILURE);

You seem to think that the problem has to do with how to compile
program text that has undefined behavior. It doesn't. Please
see also my response to Keith Thompson's message regarding what
I mean by "translatable".


[..second message..]

James Kuyper <[email protected]> writes:
[in a posting with Message-ID: <ujdgc9$3t0ni$[email protected]>]

On 2023-11-19 at 02:28 EST, Tim Rentsch wrote:

James Kuyper <[email protected]> writes:

Most fundamentally, what I an hoping to accomplish is to convince
you to answer Keith's question.

Consider a C compiler for a machine where function pointers are
much bigger than any object pointer type. Functions might even
live in a completely different address space than data (a
so-called Harvard architecture). On such a machine there is no
sensible way to change an object pointer into a function pointer,
or vice versa. Naturally the compiler would prefer to choose not
to translate any source file that contains such a conversion.

But the standard does not allow such behavior -

Yes, it does. There's a flaw in your logic in trying to decide
whether the program is strictly conforming.

if they wish to
claim conformance to the C standard, they'll have to do something
other than what they'd prefer. Since the code with undefined
behavior is protected by an if(0), an implementation is not
required to generate the impossible conversion code, and is
required to accept the program.

Let me say again that the question of undefined behavior is
not relevant here. The given program fails to be strictly
conforming for reasons that have nothing to do with undefined
behavior.

The C standard doesn't say function pointers may be converted to
object pointers, or the other way around. The obvious thing to
do is simply give an error message and forego the translation
effort. It doesn't make sense that the standard would insist
that an implementation translate a construct that is nonsensical,
not just in a particular case but in every possible case.

I appreciate that you feel that way - but the committee resolved
DR 109 in conflict with your feeling.

That is your misunderstanding. The remarks in DR 109 have no
bearing on my conclusions.

The code referred to by DR
109 divided an integer by an integer constant of 0, which wasn't
even protected by an if(0). It was the only line in the only
function defined in that translation unit, which was absolutely
guaranteed, if function were called, to execute that line. The
only reason the committee gave for it being strictly conforming
was that there was no guarantee that the rest of the program ever
actually called the function. But the committee decided that that
fact was sufficient to make the code, in itself, strictly
conforming, and the implementation was therefore required to
accept it. If the program as a whole did call that function, that
fact would be sufficient to justify rejecting the whole program,
but it was not sufficient to justify rejecting that particular
translation unit.

Your phrasing here is slightly off. What was actually said is
that

A conforming implementation must not fail to translate
a strictly conforming program simply because some
possible execution of that program would result in
undefined behavior.

The undefined behavior not being evaluated doesn't guarantee the
program is strictly conforming. If a program /is/ otherwise
strictly conforming then undefined behavior that is potentially
unevaluated doesn't interfere with that. Do you see the
difference? The problem here is that the code in Keith's program
is not strictly conforming, regardless of whether there is
evaluated undefined behavior.

Basically, code with undefined behavior prevents a program from
being strictly conforming only if execution of such code is an
inevitable consequence of starting the program. If executing that
code can be avoided, then only a failure to avoid it renders the
the behavior undefined. If that failure is not realized at
compile time, then the implementation must still accept it.

If undefined behavior is unavoidable, then the program is not
strictly conforming, and it need not be accepted.

However, if all undefined behavior is potentially not evalauted,
that does not by itself guarantee that the program is strictly
conforming. Furthermore you yourself quoted, in another posting,
the relevant sentence from the C standard that bears on the
question of strict conformance.

If the committee ruled that way on that code, how could you
possibly expect it to support rejection of this code?

You think the only things that matter in the two programs are
analogous. They aren't.

Converting between function pointers and object pointers isn't
like dividing by zero.

True - there's implementations which can meaningfully do such a
conversion - there's no meaningful way to divide an integer by 0
(a floating point calculation could reasonably produce infinity,
if infinity can be represented on that hardware). Which makes the committee's decision on DR 109 even stronger.

Again you miss the point. The question of undefined behavior
has no bearing on the conclusion.

[.. some remarks unrelated to the primary topic ..]

[...]
However, if you're unable to provide a convincing argument that
your point of view is correct, then it's a point of view that is,
quite frankly, of no interest.

I have very little interest in trying to offer an argument
that convinces you. My conclusions are correct, whether
my comments convince you or not.

[... incidental commentary left out ...]

Try compiling Keith's program with gcc -pedantic-errors. I expect
you will find, as I did, that gcc flags the conversion with an
error and doesn't produce an output file. The C standard allows
such behavior only if the source file being compiled is not
strictly conforming. I have more confidence in gcc's understanding
of the C standard than I do in yours.

I have considerable trust in gcc, but more trust in the actual
words of the standard. Where there's a disagreement, I favor the
actual words of the standard.

The problem is not the words of the C standard but what you think
they mean.

And, having seen how you interpret the standard, I certainly
trust my interpretation more than yours.

Of course. You're living in your own language bubble. Unlike you,
I calibrate my understanding of what text in the C standard means
by comparing it with other sources, including especially remarks
written or spoken by C standard committee members in other contexts.

Yes, it does. And I frequently despair of ever figuring out a way
to say what I mean that will be understood as intended by you.
[...]

The problem is not me understanding you, but you not understanding
me. Your mental model for how to use and understand natural
language is peculiar. Also rigid and inflexible. Probably the
most troublesome property is that you are focused on argument more
than on understanding. You read the words but don't understand the
meaning. Instead you insist on playing idiosyncratic word games,
apparently believing that how you interpret the words is universal
and absolute. It isn't. Assigning meaning to natural language is
inherently a subjective process, not an objective one. You're a
lot better at arguing than you are at listening. You might want to
think about that.

Subjectivity is fine for poetry - but we're talking about a
standard. To the extent that the meaning of words is subjective,
they are unsuitable for use in a standard. The whole point of the
standard is to standardize the requirements - that is, to make
them exactly the same for everyone. If you have to use subjective interpretations to determine what the requirements are, the
standard has failed the purpose for which it was intended. An
implementor could use it's subjective opinion to justify
implementing C in a way that would be inconsistent with the
subjective opinion of a user.

I'm sorry you missed the point of what I was saying.

Just one more item. Here is a quote from the C Rationale document:

Consequences of the treatment of pointer types in the
Standard include:

* [...]

* Even with an explicit cast, it is invalid to convert
a function pointer to an object pointer or a pointer
to void, or vice versa.

Note in particular the word /invalid/. Not undefined, but
invalid. What do you suppose is the basis for that statement?
What bearing does it have on the question being discussed?
What do you think the implications are for the various assertions
made during the discussion?

--- SoupGate-Win32 v1.05
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I have just posted a response to this posting in combination
with some remarks on another posting.

--- SoupGate-Win32 v1.05
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On 2024-01-22 00:08, Tim Rentsch wrote:
...

Incidentally, both of these message seems to be a response to a
posting (or postings) of mine, but they seem not to be linked
in the usual way that newgroup postings. I'm not sure how or
why that happened, but I thought I should mention it.

I currently have Thunderbird configured to delete any usenet messages
from you. This is not intended to punish you, nor does it incur any
obligation on my part to ignore your questions. It's intended solely to
reduce the amount of aggravation I feel as a result of reading your
messages. As a result, if I end up learning about one of your messages
by other means, and decide that I do want to respond, it's inconvenient
to do so. This is both an advantage and a disadvantage - it discourages
me from responding unless strongly motivated, but it also makes it
inconvenient to respond if I am sufficiently motivated.
I've been using Google Groups to post such messages, but when they cut
that off, I've been forced to switched to a couple of other
work-arounds. I'm not sure which one I used in that message, and the
details aren't important, but I'm not surprised if either method messed
up the message links.

[..first message..]

James Kuyper <[email protected]> writes:
[Message-ID: <uje6vv$gei$[email protected]>]

[This message is missing an attribution line that might have
been something like the following line]
The remarks in DR 109 are irrelevant to what I'm saying. The
program given above fails to be strictly conforming for reasons
that have nothing to do with undefined behavior.

And once again, typical Tim Rentsch behavior. This would have been a
perfectly obvious and natural opportunity to insert a statement of what
you think the actual reason is, but you refuse to do so.

I'm curious - on what platform is it impossible, or at least
difficult, to translate an if(0) block as a no-op? I'd think that
simply failing to generate any corresponding machine code would be
sufficient on most, if not all, platforms.

I hope you realize that this question is quite irrelevant to the
matter being addressed here.

You said that implementing such a conversion would be difficult. I don't
see what's difficult about a conversion that does not in fact need to be implemented.

The code that appeared in DR 109 was not a no-op, but the behavior
of any program that called the function would be undefined, which
means that the standard imposes no requirements on its behavior.
On what platform is it difficult to generate code that has no
requirements on how it behaves? I'd think it would be trivial
translate it as, for instance, the equivalent of

fprintf(stderr, "ISO C does not define the behavior of converting \n"
"a pointer to an object into a pointer to a function.\n");
exit(EXIT_FAILURE);

You seem to think that the problem has to do with how to compile
program text that has undefined behavior. It doesn't. Please
see also my response to Keith Thompson's message regarding what
I mean by "translatable".

The relevant code does not need to be translated, and therefore does not
need to be translatable, since it is literally impossible for it to be executed.

...

Yes, it does. There's a flaw in your logic in trying to decide
whether the program is strictly conforming.

Once again, you passed upon on a perfectly obvious and natural
opportunity to insert a statement explaining what actually renders the
program not strictly conforming.

You could have easily, at any time, short-circuited months of circuitous arguing by identifying the feature of the program that renders it not
strictly conforming. The only suspect I can find for such a feature is
the fact that the code which never gets executed would have undefined
behavior if it were executed. Since you say that's not the problem, I
have no idea what you think the problem actually is.

...

Let me say again that the question of undefined behavior is
not relevant here. The given program fails to be strictly
conforming for reasons that have nothing to do with undefined
behavior.

Again, you passed up on a perfectly obvious and natural opportunity to
insert a statement identifying what you think the reason actually is.

...

The undefined behavior not being evaluated doesn't guarantee the
program is strictly conforming. If a program /is/ otherwise
strictly conforming then undefined behavior that is potentially
unevaluated doesn't interfere with that. Do you see the
difference?

Not in any way that is applicable. I see no other way in which it fails
to be strictly conforming. I may have missed something, and you might
have noticed it, which is why we've repeatedly asked you to identify it,
but you have repeatedly and steadfastly refused to explain what that
other way is.
Again, you missed a perfectly obvious and natural opportunity to insert
a statement identifying what it is that you think makes the program not strictly conforming.

...

If the committee ruled that way on that code, how could you
possibly expect it to support rejection of this code?

You think the only things that matter in the two programs are
analogous. They aren't.

Because you refuse to identify what thing it is you think matters.
Once again, you missed a perfectly obvious and natural opportunity to
insert a statement identifying what it is that you think matters.

...

Again you miss the point. The question of undefined behavior
has no bearing on the conclusion.

Again, you missed a perfectly natural and obvious opportunity to
identify what it is that you think does have a bearing on the conclusion.

I have very little interest in trying to offer an argument

that convinces you. My conclusions are correct, whether
my comments convince you or not.

Yes, I've noticed your lack of interest in convince me. But do you have
any idea how frustrating it is to shadow box with someone who refuses to
put his arguments out there so we can decide whether or not they make
any sense. There's a perfectly natural conclusion that can be reached
when someone behaves that way, and that is they are afraid to explain
their arguments, because they know that those arguments are not good
enough to survive public exposure. That might not be your actual reason,
but your behavior entitles us to assume that it is - not because that
would be the most reasonable guess, but as a built-in punishment for
your refusal to communicate.

...

The problem is not the words of the C standard but what you think
they mean.

And, again, you missed a perfectly obvious and natural opportunity to
explain what it is you think they actually mean.

...

I calibrate my understanding of what text in the C standard means
by comparing it with other sources, including especially remarks
written or spoken by C standard committee members in other contexts.

And again, you missed a perfectly obvious and natural opportunity to
insert a citation of the relevant remarks that informed you interpretation.

...

I'm sorry you missed the point of what I was saying.

That's because you refuse to explain it. You just criticize what I say
without bothering to explain your criticism. How could I possibly have
any idea what your point is when you won't explain it?
Again, you missed a perfectly obvious and natural opportunity to insert
an explanation of what your point is.

Just one more item. Here is a quote from the C Rationale document:

Consequences of the treatment of pointer types in the
Standard include:

* [...]

* Even with an explicit cast, it is invalid to convert
a function pointer to an object pointer or a pointer
to void, or vice versa.

Note in particular the word /invalid/. Not undefined, but
invalid. What do you suppose is the basis for that statement?

Since the standard doesn't use the term "invalid" in any applicable text
that I could find, I would assume that the basis for this statement is
the fact that the behavior of such a conversion is undefined. The terms
"syntax error", "constraint violation", "unspecified behavior", "implementation-defined behavior", "undefined behavior", and
"locale-specific", among others, all have requirements (or the lack
thereof) attached to them by the C standard. "invalid" does not. The
behavior of such a conversion is undefined "by omission of any explicit definition of the behavior", and as far as I know that is the only
relevant problem with it. It doesn't violate any applicable syntax rule
or constraint that I'm aware of. And you've repeatedly refused to
identify one.

What bearing does it have on the question being discussed?
What do you think the implications are for the various assertions
made during the discussion?

I don't think it has a bearing, because I think it is using "invalid" as
an informal reference to the fact that the behavior is undefined. We
appear to both be in agreement that the undefined behavior of that
statement, if it were executed, is irrelevant to the strict conformance
of this code, since it cannot be executed, so I would expect that to
have no implications for the various assertions.

Again, you passed upon on a perfectly obvious and natural opportunity
for inserting statements explaining what bearing you think it has on the discussion and what the implications are for the various statements made
during the discussion.

If you've not willing to explain, why do you even bother posting a
message? A message that merely asserts that there is a problem, while
failing to identify it, does nothing except raise the suspicion that you
don't actually have a valid reason for thinking that there's a problem.

--- SoupGate-Win32 v1.05
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