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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Compatibility question |
| Date | 2012-10-24 14:38 -0700 |
| Organization | None to speak of |
| Message-ID | <ln625zv9cf.fsf@nuthaus.mib.org> (permalink) |
| References | (6 earlier) <ap54B1.hSk.bor8X@gmail.com> <lnr4opdm7m.fsf@nuthaus.mib.org> <VTQKnL.Au1.NMBnf@gmail.com> <lnzk3cvlzy.fsf@nuthaus.mib.org> <LAWkIU.3db.huHtX@gmail.com> |
Steve Thompson <stevet810@gmail.com> writes:
> On Tue, Oct 23, 2012 at 03:53:21PM -0700, Keith Thompson wrote:
>> Steve Thompson <stevet810@gmail.com> writes:
>> > On Tue, Oct 23, 2012 at 12:26:05PM -0700, Keith Thompson wrote:
>> >> Steve Thompson <stevet810@gmail.com> writes:
>> >> > On Mon, Oct 22, 2012 at 09:06:39PM +0100, Ben Bacarisse wrote:
>> >> [...]
>> >> >> Whatever goes on under the hood (and I'd dispute that 'a' is "really a
>> >> >> pointer") it's much easier to learn a language in its own terms, at
>> >> >> least as far as that's possible.
>> >> >
>> >> > Well, no. 'a' is never a pointer as such in the terms of the language
>> >> > syntax, but its treatment by the compiler internally is nominally
>> >> > equivalent to they way it handles a pointer, with the exception that
>> >> > the language grammar forbids its use in the program text as a pointer.
>> >>
>> >> I don't believe that's true. Do you have some evidence that it is?
>> >
>> > The distinction is largely moot. In a practical sense, all variables
>> > have a value which is stored somewhere. The compiler is making an
>> > explicit use of a pointer when it does something with the value. The
>> > problem is that "use" in this context may as well be referring to ether
>> > or memory and there's no easy way of knowing which it is.
>>
>> When you say "explicit use of a pointer", do you mean that there's
>> a pointer *object* somewhere, or are you talking about a pointer
>> *value*, also known as an "address"?
>
> I suppose address is the better term, but that does not work for
> registers.
Neither "address" nor "pointer" works for register objects -- or for bit
fields.
>> How much do you know about the internal workings of compilers?
>
> Precious little. My brain is only starting to think about
> interpreters; I plan to hold off polluting my brain with compiler
> internals until I find a pressing need to write one.
Then I'm mystified by your decision to try to understand C
semantics in terms of compiler internals. There's a perfectly good
definition of C semantics: the ISO C standard; the latest draft
is <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf>.
>> >> > In a simplistic sense, all variables are pointers to a segment of
>> >> > program data, modifiable or otherwise.
>>
>> No, a variable is not a pointer to a chunk of memory, it *is* a chunk of
>> memory (which can contain a value at execution time).
>>
>> More precisely, the C standard doesn't define the word "variable", but I
>> presume that a "variable" is a kind of "object", which is defined as a
>> "region of data storage in the execution environment, the contents of
>> which can represent values". Not a pointer in sight.
>
> My bad, I have assumed that a variable must be distinguished from
> other variables by an address, which implies a pointer of some sort,
> but you might choose to call that an identifier instead.
An identifier is a character string that exists in source code. An
object is a "region of data storage in the execution environment, the
contents of which can represent values". The standard doesn't define
the term "variable", and uses it in that sense only in a few footnotes,
but it consistently uses the word "variable" to refer to objects.
Given:
int obj;
register int reg;
struct {
int member;
int bit_field:8;
}s;
obj, reg, s, s.member, and s.bit_field are all objects (or, perhaps more
precisely, are all names by which we can refer to objects). Not all of
them have addresses. Which of them are "variables" might be an
interesting question, but I don't think it would illumate what we're
talking about. I suggest we stick to the term "object" from now on,
since it has an unambiguous meaning.
> More generally, a value must be distinguished from other values with a
> unique identifier. In a program, we call this identifier a 'named
> variable', which in a sense may be thought of as is its 'address' by
> virtue of its entry in the program's 'dictionary' of identifiers.
> Anything else implies magic of some unspecified/unknowable nature, and
> I'm not all that keen on the idea of magic.
Now you introduce the term "value", which is very different from an
"object". The expression `42`, when evaluated, yields a "value" which
is not associated with any object. And not all objects have names; some
of them are anonymous:
int *ptr = malloc(sizeof *ptr);
// *ptr is an int object with no name of its own
An object defined inside a recursive function will have a single
entry in the compiler's symbol table, but can have multiple
instances, with distinct addresses, during execution. An object
defined in a function that's never called likewise has a single
symbol table entry, but has *no* address during execution.
These are just some of the reasons why using the term "address" to
refer to a compiler symbol table entry (which is what I think you
mean by "its entry in the program's 'dictionary' of identifiers")
is just wrong and confusing.
> This 'address' has distinct qualities in the compiler's representation
> of the program and in the assembled object code, which I suppose are
> representative of distinct, but related ontologies. The standard, of
> course, is not concerned with the philosophy of computer software,
> only its behavior in the execution environment.
The standard is not concerned either with the "philosophy of computer
software" or with the details of compiler implementation.
>> >> > values may move around in and from RAM to CPU registers and back while
>> >> > being identified as a single consistent entity. This helps to
>> >> > illustrate the difference between a language and its implementation.
>> >>
>> >> So a char object is really a pointer to char, an int object is
>> >> really a pointer to int, and a char** object is really a char**?
>>
>> Sorry, what I meant to write is that a char** object is really (in your
>> model) a char***, i.e., a pointer to a char**.
>>
>> > No, a char ** object is a pointer to a char ** object.
>>
>> Was that really what you meant to type? I messed up the number of *s,
>> so I can hardly criticize you for doing the same thing, but I'd like to
>> know what you meant.
>
> I must suggest that the fundamental nature of objects is that they
> have an identifier and existence. Existence implies location,
> especially in a computer program. Proving the proposition that
> identifiers have existence independent of their object I leave as an
> exercise for the student.
You didn't answer my question. And not all objects have identifiers.
>> In your model, as I understand it, an int object is really a pointer to
>> int, or an int*. It would follow that a char** object is really a
>> pointer to a char**, or a char***. Or am I misunderstanding you?
>
> Perhaps you can revisit that question after considering the
> philosophical statements above.
No thanks. Perhaps you can *answer* that question.
>> In fact, an int object is an int object. I find that so obvious
>> that it seems odd to have to state it. For an operation on such
>> an object, a compiler might have to generate code that determines
>> that object's address. That does not in any sense imply that an
>> int object is "really" a pointer. You can construct a pointer to
>> an int object, but the pointer and the object it points to are two
>> different things.
>
> We're using pointer in two different senses: in the sense defined by
> the C standard, and in the sense that an identifier points to
> an object, by definition. The concept of 'pointer' is implicit to the
> idea of 'object'. You seem to be confused by the use of pointer to
> refer to anything other than a C pointer object.
Of course I'm confused by your use of the word "pointer" to refer to
things other than actual pointers. This is comp.lang.c, where we
discuss the C programming language, which is defined by the ISO C
standard, which has an unambiguous meaning for the word "pointer".
And I still don't understand what your other meaning of "pointer" is.
Can you (a) explain exactly what you mean by "pointer", and (b) use a
different word to refer to that concept, to avoid further confusion?
The concept of "pointer" is not implicit in the idea of "object".
> Religions seem to rely on that same kind of confusion to obscure the
> origins and nature of some kinds of religious 'knowledge'.
I'm not interested in discussing religion, at least not here.
> Moreover,
> thinking about philosophy and ontology is all but outlawed today.
Nonsense. But philosophy and ontology are not what this newsgroup is
about. Perhaps you'd be happer in a forum that does discuss philosophy.
Perhaps what you're saying would make sense to someone who has studied
more philosphy than I have.
[snip]
[...]
>> Perhaps. But an object of pointer-to-struct type is a region
>> of memory holding the address of the struct object. Why would a
>> compiler allocate and initialize such a pointer object, or generate
>> code to do so, in the absence of C code to take the addrss of the
>> struct object?
>
> Because it is natural to do so. The fact that your compiler copies
> such structures in assignment contexts (seemingly) one field at a time
> is an artifact of its design. Another compiler might use a form of
> memcpy(), in which case there would have to be a pointer -- just not
> one that you defined explicitly in your source code.
The generated code I posted upthread was not copying the structure a
field at a time, it was copying it a word at a time. But the CPU
instructions a compiler chooses to implement an assignment operation
have nothing to do with the conceptual nature of an object.
>> >> Struct objects are very often manipulated via pointers (simulating
>> >> by-reference argument passing, for example), but that's a coding
>> >> convention, not anything inherent to the language.
>> >
>> > Agreed.
>>
>> So where do these "pointers" you keep talking about come from?
>> In what sense do they exist?
And you haven't answer that question either.
[...]
> I think the philosophical perspective better illustrates what I was
> thinking. The phase space is referential to a program's execution
> environment, which we generally assume to exist, and hence any object
> in that phase space has an address which is, by definition, a pointer.
>
> I hope this makes things clearer.
Not even a little bit.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-20 03:08 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-20 16:20 +1300
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-20 11:50 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-20 11:41 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 23:04 +0000
Re: Compatibility question Barry Schwarz <schwarzb@dqel.com> - 2012-10-21 20:27 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-20 04:01 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-21 09:26 +1300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 22:54 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-22 12:29 +1300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 23:50 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 02:27 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 02:28 +0000
Re: Compatibility question Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-22 11:18 -0400
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 16:19 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:37 +0100
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:40 +0100
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:30 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 17:23 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 10:52 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 18:42 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 21:06 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 22:52 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-23 03:33 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 12:28 +0000
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 16:45 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 17:53 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 12:41 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 21:28 +0100
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-23 20:57 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 12:26 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 20:39 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 15:53 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 00:57 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 20:06 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 11:18 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 12:04 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 23:10 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:58 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-25 01:46 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 19:40 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-24 20:20 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 14:38 -0700
Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-24 17:55 -0400
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:29 -0700
Re: Compatibility question Greg Martin <greg@softsprocket.com> - 2012-10-24 15:21 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-26 03:30 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-26 13:02 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-27 00:04 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-27 20:15 +0100
Re: Compatibility question Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-10-27 16:36 -0600
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-28 17:36 +0000
Re: Compatibility question Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-10-27 22:42 +0300
Re: Compatibility question Greg Martin <greg@softsprocket.com> - 2012-10-27 14:30 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-27 22:57 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 17:04 -0700
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 17:14 -0700
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 18:04 -0700
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:46 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-25 01:18 +0000
Re: Compatibility question Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-10-27 21:08 +0300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-28 17:38 +0000
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:38 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 16:50 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 12:33 +0000
Re: Compatibility question Öö Tiib <ootiib@hot.ee> - 2012-10-22 14:23 -0700
Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-22 17:26 -0400
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-23 10:55 +1300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 23:27 +0000
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-22 22:44 +0100
Re: Compatibility question Les Cargill <lcargill99@comcast.com> - 2012-10-22 17:15 -0500
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:13 +0100
Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-22 18:35 -0400
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:14 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 23:42 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-23 13:35 +1300
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 17:01 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 01:47 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-21 19:27 -0700
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