void vs void* (philosophical question)

[Giannis Papadopoulos]

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship? I could think of only one - a pointer
to an incomplete type. However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?
--
one's freedom stops where others' begin

Giannis Papadopoulos
Computer and Communications Engineering dept. (CCED)
University of Thessaly
http://dop.freegr.net/

[Mark F. Haigh]

Giannis Papadopoulos wrote:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship? I could think of only one - a pointer
to an incomplete type. However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?

Here's my best explanation:

A void * is a pointer to (just about) anything. If you dereference a
void *, you might be referring to a 32 bit integer or a 128 bit
floating point value. You might be referring to a structure or a
union. You might be referring to damn near __anything__. In C, you do
the bookkeeping, not the compiler (usually).

Since a void * may point to just about anything, it's safest for the
compiler to treat it as if it points to __nothing in particular__. You
must convert a void * to another pointer type before using it:

#include <stdio.h>

int main(void)
{
int a = 42;
void *p = &a;
printf("%d\n", *(int *) p);
return 0;
}

[mark@icepick]$ gcc -ansi -pedantic -Wall -O2 -o foo foo.c
[mark@icepick]$ ./foo
42
Mark F. Haigh
mf*****@sbcglob al.net

[Giannis Papadopoulos]

Mark F. Haigh wrote:

Giannis Papadopoulos wrote:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship? I could think of only one - a pointer
to an incomplete type. However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?

Here's my best explanation:

A void * is a pointer to (just about) anything. If you dereference a
void *, you might be referring to a 32 bit integer or a 128 bit
floating point value. You might be referring to a structure or a
union. You might be referring to damn near __anything__. In C, you do
the bookkeeping, not the compiler (usually).

Since a void * may point to just about anything, it's safest for the
compiler to treat it as if it points to __nothing in particular__. You
must convert a void * to another pointer type before using it:

#include <stdio.h>

int main(void)
{
int a = 42;
void *p = &a;
printf("%d\n", *(int *) p);
return 0;
}

[mark@icepick]$ gcc -ansi -pedantic -Wall -O2 -o foo foo.c
[mark@icepick]$ ./foo
42
Mark F. Haigh
mf*****@sbcglob al.net

I do not want to know about the usage of void and void*. I want to know
if they have some common meaning, apart from the first 4 letters.

Thanks anyway however.

--
one's freedom stops where others' begin

Giannis Papadopoulos
Computer and Communications Engineering dept. (CCED)
University of Thessaly
http://dop.freegr.net/

[Richard Heathfield]

Giannis Papadopoulos said:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void)
Yes.
or they have a closer relationship? I could think of only one - a pointer
to an incomplete type.
Apart from that, they pretty much mean *opposite* things. void means
"nothing", and void * means "pointer to anything" (in the "whaddya mean,
what am I pointing at? How should I know?" sense).
However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?

If void means "absence of value", surely void * means "a pointer which you
can't dereference because to do so would yield the absence of a value".

But yes, it's sticky-icky. I'd have bitten the bullet and introduced a new
keyword, e.g. object, which I'd have used to replace void in pointer
contexts. Thus: object *malloc(size_t) , etc.

You can do this easily enough in your own code with:

#define object void /* possible; just not very wise */

But I am NOT advocating this! :-) If the language supported it, that would
be different, but it doesn't.
--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[Ian Malone]

Giannis Papadopoulos wrote:

Mark F. Haigh wrote:

Giannis Papadopoulos wrote:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship? I could think of only one - a pointer
to an incomplete type. However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?

I do not want to know about the usage of void and void*. I want to know
if they have some common meaning, apart from the first 4 letters.

My 2 pence: void is an incomplete type, a pointer to the incomplete
type must be able to point to any possible object, therefore void *
must be the generic pointer.

Consider analogously the pointer to an incomplete struct.

--
imalone

[Ben C]

On 2006-06-30, Giannis Papadopoulos <ip******@inf.u th.gr> wrote:

Mark F. Haigh wrote:

Giannis Papadopoulos wrote:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship? I could think of only one - a pointer
to an incomplete type. However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?

Here's my best explanation:

A void * is a pointer to (just about) anything. If you dereference a
void *, you might be referring to a 32 bit integer or a 128 bit
floating point value. You might be referring to a structure or a
union. You might be referring to damn near __anything__. In C, you do
the bookkeeping, not the compiler (usually).

Since a void * may point to just about anything, it's safest for the
compiler to treat it as if it points to __nothing in particular__. You
must convert a void * to another pointer type before using it:

[snip]
I do not want to know about the usage of void and void*. I want to know
if they have some common meaning, apart from the first 4 letters.

Thanks anyway however.

I think Mark F. Haigh has elucidated this a bit: "void" means "nothing",
"void *" means "pointer to anything".

Anything != nothing, although anything ~= "nothing in particular". So
they're not the same void. But closely related ideas, so we use the same
keyword.

void * is not quite consistent with void as follows:

int *p;

can be read as "p is a pointer and *p is an int". But:

void *p;

"p is a pointer [true] and *p is a void [false]"

[pete]

Giannis Papadopoulos wrote:

According to the standard (ISO C99 draft WG14/N1124),
void is the incomplete type that cannot be completed
and comprises the empty set of values.
Since it declares the absense of a value can it be considered a
data type?

No.

sizeof(void) is undefined.
You can't declare an object of type void.
You can't have an array of type void elements.
An expression of type void can neither be
the left nor right operand of the assignment operator.

--
pete

[Harald van Dijk]

Ben C wrote:

void * is not quite consistent with void as follows:

int *p;

can be read as "p is a pointer and *p is an int". But:

void *p;

"p is a pointer [true] and *p is a void [false]"

[true] and [true], actually. *p is a valid expression of type void, and
&*p evaluates to p. (Of course, it's pointless to do this except for
very special cases.)

[Thomas J. Gritzan]

Giannis Papadopoulos schrieb:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?

Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship? I could think of only one - a pointer
to an incomplete type. However, if one could say that void is the
absense of a value, how can we have a pointer to something that does not
exist?

Its logical if you think about it in terms of set theorie.

Think of two structs:

struct person
{
char* name;
};
struct worker
{
char* name;
char* job;
};

A pointer person* can point to a struct person and to a struct worker,
so it is a pointer to "at least a person".

A void* is a pointer to "at least nothing", so it can't point to
effectively everything.

Thomas

[Chris Torek]

In article <e8**********@v olcano1.grnet.g r>
Giannis Papadopoulos <ip******@inf.u th.gr> wrote:

According to the standard (ISO C99 draft WG14/N1124), void is the
incomplete type that cannot be completed and comprises the empty set of
values. Since it declares the absense of a value can it be considered a
data type?
In a larger, "not Standard C" sense, yes. In Standard C, however,
types are partitioned into "object types", "function types", and
"incomplete types". (The Standard does not define "data type" but
logically it would appear to map to "object type". Interestingly,
my C99 draft *uses* the phrase "data types" in the description of
<wchar.h>, in paragraph 1 of section 7.19.1. It is clear enough
that it does not mean "incomplete types" here. Perhaps the phrasing
was changed in the final standard, though.)

(If I were in charge of things, I would probably make "void" an
ordinary object type whose size is zero, so that sizeof(void) ==
0; its value, upon conversion to any other scalar type, would be
zero. Thus:

void v, *pv = &v;
printf("%d %d\n", v, *pv);

would print "0 0\n". :-) Of course, I would also add zero-sized
arrays:

int a[0], b;

with the allowance -- but not requirement -- that &a[0] == &b. Then
I might also remove the "flexible array member" and simply bless the
Struct Hack.)
Regarding void*, is it just a simple reuse of the same keyword (void) or
they have a closer relationship?

As most others have said, it is just reuse of the keyword.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.