pointer to void property

[aegis]

Given the following:

int a = 10;

int *p;
void *p1;
unsigned char *p2;

p = &a;

p1 = p;

p2 = p1;

is a guarantee made that I can access the object representation
via 'p2'?

Will the value assigned between different pointer to object types
through use of a generic pointer preserve this value?

I know it is guaranteed to be preserved if it is assigned from a
pointer to an object type to a void pointer and back again to said
pointer to object type. But will the value assigned between different
pointer to object types through use of a generic pointer, preserve this
value?
--
aegis

[Nerox]

The pointer value will be the same, but you must dereference it in the
correct way to get the expected result.
If you perform:
*p2, you will get the first byte of the a variable read as a uchar.
That's usefull when you want to see the internal representation of
individuals bytes.

Also remember to cast from void to a type in order to avoid warnings:
p2 = (unsigned char *) pl;

[Irrwahn Grausewitz]

"aegis" <ae***@mad.scie ntist.com> wrote:

Given the following:

int a = 10;

int *p;
void *p1;
unsigned char *p2;

p = &a;

p1 = p;

p2 = p1;

is a guarantee made that I can access the object representation
via 'p2'?

Will the value assigned between different pointer to object types
through use of a generic pointer preserve this value?

I know it is guaranteed to be preserved if it is assigned from a
pointer to an object type to a void pointer and back again to said
pointer to object type. But will the value assigned between different
pointer to object types through use of a generic pointer, preserve this
value?

As long as you stick with a character pointer, yes. Consider:

ISO/IEC 9899:1999 (E)

6.2.5p26
A pointer to void shall have the same representation and alignment
requirements as a pointer to a character type.

6.3.2.3p1
A pointer to void may be converted to or from a pointer to any
incomplete or object type. [...]

6.3.2.3p7
A pointer to an object or incomplete type may be converted to a
pointer to a different object or incomplete type. If the resulting
pointer is not correctly aligned for the pointed-to type, the
behavior is undefined. Otherwise, when converted back again, the
result shall compare equal to the original pointer. When a pointer
to an object is converted to a pointer to a character type, the
result points to the lowest addressed byte of the object. Successive
increments of the result, up to the size of the object, yield
pointers to the remaining bytes of the object.

Note that (AFAICT) it's not guaranteed to work with, e.g., a
pointer-to-int and a pointer-to-double. But casting from any
pointer-to-sometype to pointer-to-void is safe, as is casting from any
pointer-to-void to a pointer-to-charactertype.

Best regards.
pointer.
--
Irrwahn Grausewitz (ir*******@free net.de)
welcome to clc : http://www.ungerhu.com/jxh/clc.welcome.txt
clc faq-list : http://www.faqs.org/faqs/C-faq/faq/
clc frequent answers: http://benpfaff.org/writings/clc

[Irrwahn Grausewitz]

"Nerox" <ne****@gmail.c om> wrote:

Please preserve some context when posting replies. Context restored:

"aegis" <ae***@mad.scie ntist.com> wrote:

Given the following:

int a = 10;

int *p;void *p1;
unsigned char *p2;

p = &a;

p1 = p;

p2 = p1;

is a guarantee made that I can access the object representation
via 'p2'?

The pointer value will be the same, but you must dereference it in the
correct way to get the expected result.
If you perform:
*p2, you will get the first byte of the a variable read as a uchar.
That's usefull when you want to see the internal representation of
individuals bytes.

Also remember to cast from void to a type in order to avoid warnings:
p2 = (unsigned char *) pl;

Uck, no, don't. Implicit conversion to and from void is perfectly
valid, as opposed to conversions between pointers to different types
other than void. After all, that's one of the reasons why generic
pointers were introduced in C, IIRC.

Best regards.
--
Irrwahn Grausewitz (ir*******@free net.de)
welcome to clc : http://www.ungerhu.com/jxh/clc.welcome.txt
clc faq-list : http://www.faqs.org/faqs/C-faq/faq/
clc frequent answers: http://benpfaff.org/writings/clc

[Emmanuel Delahaye]

Nerox wrote on 11/09/05 :

Also remember to cast from void to a type in order to avoid warnings:
p2 = (unsigned char *) pl;

Assuming p2 has the type 'void *' and as long as you are using a
C-compiler, there is no need for that.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

I once asked an expert COBOL programmer, how to
declare local variables in COBOL, the reply was:
"what is a local variable?"

[Jack Klein]

On Sun, 11 Sep 2005 12:11:00 +0200, Irrwahn Grausewitz
<ir*******@free net.de> wrote in comp.lang.c:

"aegis" <ae***@mad.scie ntist.com> wrote:

Given the following:

int a = 10;

int *p;
void *p1;
unsigned char *p2;

p = &a;

p1 = p;

p2 = p1;

is a guarantee made that I can access the object representation
via 'p2'?

Will the value assigned between different pointer to object types
through use of a generic pointer preserve this value?

I know it is guaranteed to be preserved if it is assigned from a
pointer to an object type to a void pointer and back again to said
pointer to object type. But will the value assigned between different
pointer to object types through use of a generic pointer, preserve this
value?

As long as you stick with a character pointer, yes. Consider:

ISO/IEC 9899:1999 (E)

6.2.5p26
A pointer to void shall have the same representation and alignment
requirements as a pointer to a character type.

6.3.2.3p1
A pointer to void may be converted to or from a pointer to any
incomplete or object type. [...]

6.3.2.3p7
A pointer to an object or incomplete type may be converted to a
pointer to a different object or incomplete type. If the resulting
pointer is not correctly aligned for the pointed-to type, the
behavior is undefined. Otherwise, when converted back again, the
result shall compare equal to the original pointer. When a pointer
to an object is converted to a pointer to a character type, the
result points to the lowest addressed byte of the object. Successive
increments of the result, up to the size of the object, yield
pointers to the remaining bytes of the object.

Note that (AFAICT) it's not guaranteed to work with, e.g., a
pointer-to-int and a pointer-to-double. But casting from any
pointer-to-sometype to pointer-to-void is safe, as is casting from any
pointer-to-void to a pointer-to-charactertype.

Or even better, just assigning and allowing the automatic conversion
to happen. No casting required.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
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alt.comp.lang.l earn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[aegis]

Irrwahn Grausewitz wrote:

"aegis" <ae***@mad.scie ntist.com> wrote:

Given the following:

int a = 10;

int *p;
void *p1;
unsigned char *p2;

p = &a;

p1 = p;

p2 = p1;

is a guarantee made that I can access the object representation
via 'p2'?

Will the value assigned between different pointer to object types
through use of a generic pointer preserve this value?

I know it is guaranteed to be preserved if it is assigned from a
pointer to an object type to a void pointer and back again to said
pointer to object type. But will the value assigned between different
pointer to object types through use of a generic pointer, preserve this
value?

As long as you stick with a character pointer, yes. Consider:

ISO/IEC 9899:1999 (E)

6.2.5p26
A pointer to void shall have the same representation and alignment
requirements as a pointer to a character type.

6.3.2.3p1
A pointer to void may be converted to or from a pointer to any
incomplete or object type. [...]

6.3.2.3p7
A pointer to an object or incomplete type may be converted to a
pointer to a different object or incomplete type. If the resulting
pointer is not correctly aligned for the pointed-to type, the
behavior is undefined. Otherwise, when converted back again, the
result shall compare equal to the original pointer. When a pointer
to an object is converted to a pointer to a character type, the
result points to the lowest addressed byte of the object. Successive
increments of the result, up to the size of the object, yield
pointers to the remaining bytes of the object.

Note that (AFAICT) it's not guaranteed to work with, e.g., a
pointer-to-int and a pointer-to-double. But casting from any
pointer-to-sometype to pointer-to-void is safe, as is casting from any
pointer-to-void to a pointer-to-charactertype.

I wasn't asking about pointer-to-int to pointer-to-double
nor about pointer-to-sometype to pointer-to-void but specifically about
pointer-to-sometype to pointer-to-void to
pointer-to-[signed|unsigned]char
With something involving pointer-to-int to pointer-to-void to
pointer-to-double, the result from what I can see, may not be correctly
aligned.

--
aegis

[pete]

aegis wrote:

Irrwahn Grausewitz wrote:

When a pointer
to an object is converted to a pointer to a character type, the
result points to the lowest addressed byte of the object.

I wasn't asking about pointer-to-int to pointer-to-double
nor about pointer-to-sometype to pointer-to-void
but specifically about
pointer-to-sometype to pointer-to-void to
pointer-to-[signed|unsigned]char

Pointer to an object to pointer-to-[signed|unsigned]char,
is defined.

--
pete

[Irrwahn Grausewitz]

Jack Klein <ja*******@spam cop.net> wrote:

On Sun, 11 Sep 2005 12:11:00 +0200, Irrwahn Grausewitz

<snip>

Note that (AFAICT) it's not guaranteed to work with, e.g., a
pointer-to-int and a pointer-to-double. But casting from any
pointer-to-sometype to pointer-to-void is safe, as is casting from any
pointer-to-void to a pointer-to-charactertype.

Or even better, just assigning and allowing the automatic conversion
to happen. No casting required.

I meant to say conversion, not casting. You're of course right.
--
Irrwahn Grausewitz (ir*******@free net.de)
welcome to clc : http://www.ungerhu.com/jxh/clc.welcome.txt
clc faq-list : http://www.faqs.org/faqs/C-faq/faq/
clc frequent answers: http://benpfaff.org/writings/clc

[Irrwahn Grausewitz]

"aegis" <ae***@mad.scie ntist.com> wrote:

Irrwahn Grausewitz wrote: <snip>

But casting from any ^^^^^^^^^^^^^^^ ^^^^^^^^ pointer-to-sometype to pointer-to-void is safe, as is casting from any ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^ pointer-to-void to a pointer-to-charactertype.

^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^

s/casting/conversion/
I wasn't asking about pointer-to-int to pointer-to-double
nor about pointer-to-sometype to pointer-to-void but specifically about
pointer-to-sometype to pointer-to-void to
pointer-to-[signed|unsigned]char

If it's safe to convert a p-to-sometype to p-to-void, AND it's safe
to convert a p-to-void to p-to-char, THEN it's safe to convert a
p-to-int to p-to-void to p-to-char.

What a typing mess...

Best regards.
--
Irrwahn Grausewitz (ir*******@free net.de)
welcome to clc : http://www.ungerhu.com/jxh/clc.welcome.txt
clc faq-list : http://www.faqs.org/faqs/C-faq/faq/
clc frequent answers: http://benpfaff.org/writings/clc