const void * const x

[codeslinger]

What is the point of the construct in the subject line? Why the use of
two const labels? Does that do something different than const void *x?
And what would void * const x indicate? Thanks in advance :)
--
Tobias DiPasquale

[Artie Gold]

codeslinger wrote:

What is the point of the construct in the subject line? Why the use of
two const labels? Does that do something different than const void *x?
And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.

HTH,
--ag

--
Artie Gold -- Austin, Texas

"What they accuse you of -- is what they have planned."

[E. Robert Tisdale]

Tobias DiPasquale wrote:

What is the point of the construct in the subject line?
Why the use of two const labels?
Both x and *x are constant.
Does that do something different than const void *x?
Yes.
And what would void * const x indicate?
It indicates that x is a constant.
cat main.c #include <stdio.h>
#include <stdlib.h>

int* ramp(size_t n) {
int* p = (int*)malloc(n* sizeof(int));
for (size_t j = 0; j < n; ++j)
p[j] = j;
return p;
}

int main(int argc, char* argv[]) {
const
size_t n = 32;
const
int* const p = ramp(n);
//p = NULL; // error! p is a constant
//p[0] = 13; // error! *p is a constant
for (size_t j = 0; j < n; ++j)
fprintf(stdout, "%2d = p[%2d]\n", p[j], j);
free((void*)p);
return 0;
}
gcc -Wall -std=c99 -pedantic -o main main.c
./main

0 = p[ 0]
1 = p[ 1]

[Keith Thompson]

Artie Gold <ar*******@aust in.rr.com> writes:

codeslinger wrote:

What is the point of the construct in the subject line? Why the use of
two const labels? Does that do something different than const void *x?
And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.

Right, but since it points to void, what it points to can't be changed
anyway.

--
Keith Thompson (The_Other_Keit h) ks***@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.

[Artie Gold]

Keith Thompson wrote:

Artie Gold <ar*******@aust in.rr.com> writes:

codeslinger wrote:

What is the point of the construct in the subject line? Why the use of
two const labels? Does that do something different than const void *x?
And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.

Right, but since it points to void, what it points to can't be changed
anyway.

Point taken. :-)

--ag

--
Artie Gold -- Austin, Texas

"What they accuse you of -- is what they have planned."

[E. Robert Tisdale]

Keith Thompson wrote:

Artie Gold writes:

codeslinger wrote:

What is the point of the construct in the subject line? Why the use of
two const labels? Does that do something different than const void *x?
And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.

Right, but since it points to void,
what it points to can't be changed anyway.

#include <string.h>

void *memcpy(void *dest, const void *src, size_t n);

Are you saying that memcpy cannot change what dest points to
just because dest is of type void*?

[Emmanuel Delahaye]

In 'comp.lang.c', Keith Thompson <ks***@mib.or g> wrote:

Artie Gold <ar*******@aust in.rr.com> writes:

codeslinger wrote:

> What is the point of the construct in the subject line? Why the use of
> two const labels? Does that do something different than const void *x?
> And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.

Right, but since it points to void, what it points to can't be changed
anyway.

Nonsense. A pointer can be aliased by a typed one. The point is that the
alias must be const too.

char s[] = "Hello"; /* R/W data */
void const *p = s; /* not dereferencable. */
char *pa = p; /* Diagnostic */
char const *pb = p; /* Correct. Read only acces */

For a more real approach, think in terms of parameters.

--
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[Dan Pop]

In <cc**********@n ntp1.jpl.nasa.g ov> "E. Robert Tisdale" <E.************ **@jpl.nasa.gov > writes:

Keith Thompson wrote:

Artie Gold writes:

codeslinge r wrote:

What is the point of the construct in the subject line? Why the use of
two const labels? Does that do something different than const void *x?
And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.

Right, but since it points to void,
what it points to can't be changed anyway.

#include <string.h>

void *memcpy(void *dest, const void *src, size_t n);

Are you saying that memcpy cannot change what dest points to
just because dest is of type void*?

Nope, he's *obviously* saying that dest itself cannot be used for this
purpose: it has to be converted to an object pointer type first.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Dan Pop]

In <Xn************ *************** @212.27.42.70> Emmanuel Delahaye <em**********@n oos.fr> writes:

In 'comp.lang.c', Keith Thompson <ks***@mib.or g> wrote:

Artie Gold <ar*******@aust in.rr.com> writes:

codeslinger wrote:
> What is the point of the construct in the subject line? Why the use of
> two const labels? Does that do something different than const void *x?
> And what would void * const x indicate? Thanks in advance :)

It means that neither the pointer nor what it points to may be changed.
Right, but since it points to void, what it points to can't be changed
anyway.

Nonsense. A pointer can be aliased by a typed one.

But you're still not using the original pointer for this purpose. So,
where is the nonsense?!?
The point is that the alias must be const too.
If there is such a point, your example doesn't reflect it.
char s[] = "Hello"; /* R/W data */
void const *p = s; /* not dereferencable. */
char *pa = p; /* Diagnostic */
char const *pb = p; /* Correct. Read only acces */

I can't see any pointer aliasing another pointer in your example. Are
you sure you know what you're talking about? And what's incorrect with

char *pa = (char *)p;

? You can even use pa in write mode, because the pointed-to data is
modifiable. It didn't become read-only simply because you pointed a
pointer to const at it.

Far too often, using the type pointer to const means: "I promise not to
use this pointer to modify the data" rather than "this pointer points to
non-modifiable data". Just as in your example.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Emmanuel Delahaye]

In 'comp.lang.c', Da*****@cern.ch (Dan Pop) wrote:

In 'comp.lang.c', Keith Thompson <ks***@mib.or g> wrote:

Artie Gold <ar*******@aust in.rr.com> writes:
codeslinger wrote:
> What is the point of the construct in the subject line? Why the use
> of two const labels? Does that do something different than const
> void *x? And what would void * const x indicate? Thanks in advance
> :)

It means that neither the pointer nor what it points to may be
changed.

Right, but since it points to void, what it points to can't be changed
anyway.
Nonsense. A pointer can be aliased by a typed one.

But you're still not using the original pointer for this purpose. So,
where is the nonsense?!?

<quote>
"but since it points to void, what it points to can't be changed anyway."
</>

This is wrong. It's not because a pointer is void that the pointed data is
not accessible, at least indirectly.

The point is that the alias must be const too.

If there is such a point, your example doesn't reflect it.

char s[] = "Hello"; /* R/W data */
void const *p = s; /* not dereferencable. */
char *pa = p; /* Diagnostic */
char const *pb = p; /* Correct. Read only acces */

I can't see any pointer aliasing another pointer in your example. Are
you sure you know what you're talking about?

I came with the idea that 'aliasing' was the result of more than one pointer
pointing to the same location. If you have a better definition, I'd be glad
to ear it.

Hence, in my example, 'pa' and 'pb' are aliasing 'p'. My wording might be
incorrect; correctness welcome.
char *pa = (char *)p;
Evil!
? You can even use pa in write mode, because the pointed-to data is
modifiable. It didn't become read-only simply because you pointed a
pointer to const at it.

Far too often, using the type pointer to const means: "I promise not to
use this pointer to modify the data" rather than "this pointer points to
non-modifiable data". Just as in your example.

Hehe! Abusive typecast users will burn in hell ;-)

--
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The C-language FAQ: http://www.eskimo.com/~scs/C-faq/top.html
C-reference: http://www.dinkumware.com/manuals/reader.aspx?lib=c99
FAQ de f.c.l.c : http://www.isty-info.uvsq.fr/~rumeau/fclc/