pointer q
 

A recent post of mine showed a sufficiently large gaffe on pointers as to
need to return to K&R 5.1-6 appendix A8.6.1 . So we have type specifiers:
int
long
.. One dreams himself variable names: qwe, qwr, writes

int qwe;
long qwr;

and thinks he knows what types he's declared. Had you asked me about:

int *qwe;
long *qwr;

I would have said the pointers declared were--while pointng to different
types--of the same type. Why am I wrong? Joe

Re: pointer q
 

"Joe Smith" <grumpy196884@netzero.net> writes:

>Had you asked me about:
>
> int *qwe;
> long *qwr;
>
> I would have said the pointers declared were--while pointng to different
> types--of the same type. Why am I wrong?

"qwe" is of type "pointer to int".
"qwr" is of type "pointer to long".
Those are different types.
--
"If I've told you once, I've told you LLONG_MAX times not to
exaggerate."
--Jack Klein

Re: pointer q
 

"Joe Smith" <grumpy196884@netzero.net> writes:
> A recent post of mine showed a sufficiently large gaffe on pointers as to
> need to return to K&R 5.1-6 appendix A8.6.1 . So we have type specifiers:
> int
> long
> . One dreams himself variable names: qwe, qwr, writes
>
> int qwe;
> long qwr;
>
> and thinks he knows what types he's declared. Had you asked me about:
>
> int *qwe;
> long *qwr;
>
> I would have said the pointers declared were--while pointng to different
> types--of the same type. Why am I wrong? Joe

int* and long* are two different types (both pointer types).

I'm curious, what led you to think that they're the same type?

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Re: pointer q
 

"Keith Thompson" <kst-u@mib.org> wrote in message
news:lnlkt5bqv3.fsf@nuthaus.mib.org...
> "Joe Smith" <grumpy196884@netzero.net> writes:
>> A recent post of mine showed a sufficiently large gaffe on pointers as to
>> need to return to K&R 5.1-6 appendix A8.6.1 . So we have type
>> specifiers:
>> int
>> long
>> . One dreams himself variable names: qwe, qwr, writes
>>
>> int qwe;
>> long qwr;
>>
>> and thinks he knows what types he's declared. Had you asked me about:
>>
>> int *qwe;
>> long *qwr;
>>
>> I would have said the pointers declared were--while pointng to different
>> types--of the same type. Why am I wrong? Joe
>
> int* and long* are two different types (both pointer types).
>
> I'm curious, what led you to think that they're the same type?

What's to stop me from swapping an int * and a long *? Joe

Re: pointer q
 

"Joe Smith" <grumpy196884@netzero.net> writes:
> "Keith Thompson" <kst-u@mib.org> wrote in message
> news:lnlkt5bqv3.fsf@nuthaus.mib.org...
>> "Joe Smith" <grumpy196884@netzero.net> writes:
>>> A recent post of mine showed a sufficiently large gaffe on pointers as to
>>> need to return to K&R 5.1-6 appendix A8.6.1 . So we have type
>>> specifiers:
>>> int
>>> long
>>> . One dreams himself variable names: qwe, qwr, writes
>>>
>>> int qwe;
>>> long qwr;
>>>
>>> and thinks he knows what types he's declared. Had you asked me about:
>>>
>>> int *qwe;
>>> long *qwr;
>>>
>>> I would have said the pointers declared were--while pointng to different
>>> types--of the same type. Why am I wrong? Joe
>>
>> int* and long* are two different types (both pointer types).
>>
>> I'm curious, what led you to think that they're the same type?
>
> What's to stop me from swapping an int * and a long *? Joe

Um, they're different types. Given:
int *ip;
long *lp;
these assignments:
ip = lp;
lp = ip;
are both illegal (constraint violations requiring diagnostics).

You can do the assignments with explicit conversion (casts), but
there's no guarantee that the results will be meaningful.

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Re: pointer q
 

Joe Smith wrote:
>
> What's to stop me from swapping an int * and a long *? Joe

The type system. Of course, it's not there only to annoy you, but has
very real, objective reasons to stop you from doing so.

Probably the most obvious reason is the fact that the compiler needs to
know what the pointer points in order to dereference it (in other words,
access the value of the object it points to). Different types have
different sizes and since pointer values are actually addresses of
objects, the compiler needs to know how many bits/bytes it has to read
from that address to retrieve the whole object, and only the object.

Pointer arithmetic also depends on what the pointer object points to.
Say you want to iterate over an array of int, with an int *:

int foo[10], *bar;

bar = &foo[0];
for(i = 0; i < 10; i++)
bar++;

How does the compiler know how much it needs to increment the value of
bar so it points to the next element of foo? It is obvious that using a
long * would advance it too much if sizeof (long) != sizeof (int).

Imagine a function expecting an int * as its argument and doing
something with the object it points to. Also imagine what would happen
if you passed to it the wrong type of pointer.

Also note that on some implementations the size of different pointer
types are not the same.

In short, it's not a logical thing to do. I think your confusion stems
from the (incorrect) assumption that the term 'pointer' denotes a single
kind of abstract type that 'points' regardless of what it points /to/.
So, to summarize, the type of the object the pointers points to *is* the
part of the pointer type. You cannot, therefore, exchange int * and long
* freely without casts.

-- Denis

Re: pointer q
 

Joe Smith wrote:

> "Keith Thompson" <kst-u@mib.org> wrote in message
> news:lnlkt5bqv3.fsf@nuthaus.mib.org...
>
>>"Joe Smith" <grumpy196884@netzero.net> writes:
>>
>>>A recent post of mine showed a sufficiently large gaffe on pointers as to
>>>need to return to K&R 5.1-6 appendix A8.6.1 . So we have type
>>>specifiers:
>>>int
>>>long
>>>. One dreams himself variable names: qwe, qwr, writes
>>>
>>>int qwe;
>>>long qwr;
>>>
>>>and thinks he knows what types he's declared. Had you asked me about:
>>>
>>>int *qwe;
>>>long *qwr;
>>>
>>>I would have said the pointers declared were--while pointng to different
>>>types--of the same type. Why am I wrong? Joe
>>
>>int* and long* are two different types (both pointer types).
>>
>>I'm curious, what led you to think that they're the same type?
>
>
> What's to stop me from swapping an int * and a long *? Joe

Assume sizeof(int) < sizeof(long), and consider

long l = 42;
int *ip = &l; /* illegal, but Let's Pretend */
*ip = 76;

What value is now stored in `l'?

That's why int* and long* aren't interchangeable.

--
Eric Sosman
esosman@acm-dot-org.invalid

Re: pointer q
 

"Eric Sosman" <esosman@acm-dot-org.invalid> wrote in message
news:QpSdnVM7F9qwuvrZRVn-iw@comcast.com...
> Joe Smith wrote:
>
>> "Keith Thompson" <kst-u@mib.org> wrote in message
>> news:lnlkt5bqv3.fsf@nuthaus.mib.org...
>>
>>>"Joe Smith" <grumpy196884@netzero.net> writes:
>>>
>>>>A recent post of mine showed a sufficiently large gaffe on pointers as
>>>>to
>>>>need to return to K&R 5.1-6 appendix A8.6.1 . So we have type
>>>>specifiers:
>>>>int
>>>>long
>>>>. One dreams himself variable names: qwe, qwr, writes
>>>>
>>>>int qwe;
>>>>long qwr;
>>>>
>>>>and thinks he knows what types he's declared. Had you asked me about:
>>>>
>>>>int *qwe;
>>>>long *qwr;
>>>>
>>>>I would have said the pointers declared were--while pointng to different
>>>>types--of the same type. Why am I wrong? Joe
>>>
>>>int* and long* are two different types (both pointer types).
>>>
>>>I'm curious, what led you to think that they're the same type?
>>
>>
>> What's to stop me from swapping an int * and a long *? Joe
>
> Assume sizeof(int) < sizeof(long), and consider
>
> long l = 42;
> int *ip = &l; /* illegal, but Let's Pretend */
> *ip = 76;
>
> What value is now stored in `l'?
>
> That's why int* and long* aren't interchangeable.

Thank you for replies. Mr. Thompson establishes that the reason not to swap
an int * and long * is that, as usual, assigning one type to another is
illegal, and that you can kludge away with castes if you so desire. Mr
Kasak and Mr Sosman show examples of bad things that would happen if you
did. What I'm trying to get my head around is how to view the memory with
C. If it is true that memory is a bunch of boxes with numbers on the side
and that different implementations may have differing sizes of pointers,
then the numbers on the sides of those boxes are less homogenous than I
thought. Joe

Re: pointer q
 

Joe Smith wrote:

<snip>

> Thank you for replies. Mr. Thompson establishes that the reason not to swap
> an int * and long * is that, as usual, assigning one type to another is
> illegal, and that you can kludge away with castes if you so desire. Mr
> Kasak and Mr Sosman show examples of bad things that would happen if you
> did. What I'm trying to get my head around is how to view the memory with
> C. If it is true that memory is a bunch of boxes with numbers on the side
> and that different implementations may have differing sizes of pointers,
> then the numbers on the sides of those boxes are less homogenous than I
> thought. Joe

Don't think of memory as a bunch of boxes with numbers on the side.
Don't think of memory at all. Think instead of arrays, ints and other
objects just floating randomly in space 42 dimensional space (OK, you
don't need to use that many dimensions). This will get rid of lots of
misconceptions about pointers. For example, when objects can be left,
right, above or below each other you start to see why you can't compare
two pointer to see which is larger if they don't point in to the same
object although you can always compare them for equality. Some of the
boxes have names (the ones for variables you declare) and others are
created by calling malloc.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

Re: pointer q
 

Joe Smith wrote:
> "Eric Sosman" <esosman@acm-dot-org.invalid> wrote in message
> news:QpSdnVM7F9qwuvrZRVn-iw@comcast.com...
>
>>Joe Smith wrote:
>>
>>
>>>"Keith Thompson" <kst-u@mib.org> wrote in message
>>>news:lnlkt5bqv3.fsf@nuthaus.mib.org...
>>>
>>>
>>>>"Joe Smith" <grumpy196884@netzero.net> writes:
>>>>
>>>>
>>>>>A recent post of mine showed a sufficiently large gaffe on pointers as
>>>>>to
>>>>>need to return to K&R 5.1-6 appendix A8.6.1 . So we have type
>>>>>specifiers:
>>>>>int
>>>>>long
>>>>>. One dreams himself variable names: qwe, qwr, writes
>>>>>
>>>>>int qwe;
>>>>>long qwr;
>>>>>
>>>>>and thinks he knows what types he's declared. Had you asked me about:
>>>>>
>>>>>int *qwe;
>>>>>long *qwr;
>>>>>
>>>>>I would have said the pointers declared were--while pointng to different
>>>>>types--of the same type. Why am I wrong? Joe
>>>>
>>>>int* and long* are two different types (both pointer types).
>>>>
>>>>I'm curious, what led you to think that they're the same type?
>>>
>>>
>>>What's to stop me from swapping an int * and a long *? Joe
>>
>> Assume sizeof(int) < sizeof(long), and consider
>>
>>long l = 42;
>>int *ip = &l; /* illegal, but Let's Pretend */
>>*ip = 76;
>>
>>What value is now stored in `l'?
>>
>> That's why int* and long* aren't interchangeable.
>
>
> Thank you for replies. Mr. Thompson establishes that the reason not to swap
> an int * and long * is that, as usual, assigning one type to another is
> illegal, and that you can kludge away with castes if you so desire. Mr
> Kasak and Mr Sosman show examples of bad things that would happen if you
> did. What I'm trying to get my head around is how to view the memory with
> C. If it is true that memory is a bunch of boxes with numbers on the side
> and that different implementations may have differing sizes of pointers,
> then the numbers on the sides of those boxes are less homogenous than I
> thought. Joe

What you're missing, I think, is that a pointer carries
more information than just the "number on the side" of the
box it points to. Specifically, it carries information about
how many adjacent boxes are involved, and how their contents
are to be understood. On many machines, for example, an int*
holds the "number" of a box along with the information that
three additional boxes are involved and that the four boxes
taken together represent an int. A double* might point to the
same box number, but could imply that seven more boxes are to
be considered along with the first, and that the eight taken as
a whole contain a double value. 42 and 42.0 probably don't
look the same.

The "number on the side of a box" is its address. A pointer
denotes not only an address but also a type; a pointer is a
"decorated" address.

--
Eric Sosman
esosman@acm-dot-org.invalid