Casting a pointer to an int and back

[pinkfloydhomer]

Is it well-defined to make a cast from a pointer to an int and back?
Like:

typedef struct
{
int whatever;
} S;

int main(void)
{
S* s = malloc(sizeof(S ));
int i = (int) s;
S* t = (S*) i;
t.whatever = 42;
etc.
}

/David

[Vladimir S. Oka]

pi************@ gmail.com wrote:

Is it well-defined to make a cast from a pointer to an int and back?
No, it is not.

You may be lucky that it works on your implementation, but don't count
on it (i.e. it's not portable).

Some concerns:

- how do pointers map to integers?
- how do integers map to pointers (not necessarily same as above)?
- is an integer wide enough to hold an address value (even if above is
fine)?

Even if all of the above are fine for one implementation/architecture,
the may not be on others.

Just don't do it. Also, have a look at other things you got wrong,
below:
Like:
#include <stdlib.h>

Otherwise `malloc` is unknown.
typedef struct
{
int whatever;
} S;

int main(void)
{
S* s = malloc(sizeof(S ));
int i = (int) s;
S* t = (S*) i;
t.whatever = 42;
You certainly mean:

t->whatever = 42;

or

(*t).whatever = 42;
etc.
This is a syntax error.

Please post full, compileable, minimal examples.
}

--
BR, Vladimir

[Richard G. Riley]

On 2006-02-22, Vladimir S. Oka <no****@btopenw orld.com> wrote:

etc.

This is a syntax error.

Well spotted. Go to top of the pedants class.

[Grumble]

pi************@ gmail.com wrote:

Is it well-defined to make a cast from a pointer to an
int and back?

I don't think so.

For example, on some platforms,
sizeof(int) = 4 and sizeof(void *) = 8

[Vladimir S. Oka]

Richard G. Riley wrote:

On 2006-02-22, Vladimir S. Oka <no****@btopenw orld.com> wrote:

etc.

This is a syntax error.

Well spotted. Go to top of the pedants class.

Yes! I finally made it!

Snipping relevant parts of the post is bad etiquette. I also said:

Please post full, compileable, minimal examples.

Which was the whole point. It's in the OP's interest, really.

--
BR, Vladimir

[SM Ryan]

"pi************ @gmail.com" <pi************ @gmail.com> wrote:
# Is it well-defined to make a cast from a pointer to an int and back?
# Like:

You can cast to some integer, but not necessarily an int. However
because so much legacy code depends on this working, compilers
will generaly do whatever is necessary to make it work.

You shouldn't need to cast ints to/from pointers anymore unless
you need to enter absolute address or similar issues. (void*)
can be assigned any data pointer the same way int used to be
a universal pointer type.

--
SM Ryan http://www.rawbw.com/~wyrmwif/
Mention something out of a Charleton Heston movie, and suddenly
everybody's a theology scholar.

[Richard Bos]

"pi************ @gmail.com" <pi************ @gmail.com> wrote:

Is it well-defined to make a cast from a pointer to an int and back?
Like:

typedef struct
{
int whatever;
} S;

int main(void)
{
S* s = malloc(sizeof(S ));
int i = (int) s;
S* t = (S*) i;
t.whatever = 42;
etc.
}

On two conditions:
- that the cast from pointer to int doesn't lose any information, i.e.,
that an int is wide enough to hold all pointer values;
- that the pointer is cast to int and then back to the _same_ pointer
type;
AFAICT this is well-defined.

The second condition, only you yourself have any influence over. Just
don't convert a pointer to an integer and then to a different pointer
type. In this case, you're OK.
The first depends on the platform. If you have C99, you may be able to
use (u)intptr_t, which are integer types (not necessarily ints) capable
of holding any void * value safely. Declare i as intptr_t, and cast s to
void * before assigning it to i, and back the other way 'round, too:

intptr i = (int)(void *)s;.
S *t = (void *)i;

You don't need a cast to (S *) in the second line, since a void * will
be automatically and correctly converted to any object pointer type.
Unfortunately, these types are optional, and they don't exist in C89 at
all. If it's unavailable to you, you may just have to resort to using
unsigned long (or unsigned long long) and hoping that that's large and
well-aligned enough. It probably is, but it's not guaranteed. If you
have intptr_t, use it.

Richard

[ais523]

SM Ryan wrote:

"pi************ @gmail.com" <pi************ @gmail.com> wrote:
# Is it well-defined to make a cast from a pointer to an int and back?
# Like:

You can cast to some integer, but not necessarily an int. However
because so much legacy code depends on this working, compilers
will generaly do whatever is necessary to make it work. I believe C99 provides a typedef that's guaranteed to be some sort of
int large enough to hold any sort of pointer.

You shouldn't need to cast ints to/from pointers anymore unless
you need to enter absolute address or similar issues. (void*)
can be assigned any data pointer the same way int used to be
a universal pointer type.

[Richard G. Riley]

On 2006-02-22, Vladimir S. Oka <no****@btopenw orld.com> wrote:

Richard G. Riley wrote:

On 2006-02-22, Vladimir S. Oka <no****@btopenw orld.com> wrote:

>> etc.
>
> This is a syntax error.

Well spotted. Go to top of the pedants class.

Yes! I finally made it!

Snipping relevant parts of the post is bad etiquette. I also said:

> Please post full, compileable, minimal examples.

Which was the whole point. It's in the OP's interest, really.

The snipped part had no relevance on my pithy reply to your pedant
prize winning reply. The guy was asking a question about casting : not for a
code review or smart assed comments on "etc." being a syntx error.

The fact that he didnt put in the includes and did include the ".etc"
would, to me, indicate that he was pseudo coding a bit to get an
answer to the pertient question which was "can one cast between
pointers and ints". I'm sure many others would have made this leap in
logic too : it is, after all, a programming language group :)

Or do you insist on working, compilable code for all questions on
casts?

Come on. Admit it. You were over the top

[Vladimir S. Oka]

Richard G. Riley wrote:

On 2006-02-22, Vladimir S. Oka <no****@btopenw orld.com> wrote:

Richard G. Riley wrote:

On 2006-02-22, Vladimir S. Oka <no****@btopenw orld.com> wrote:
>> etc.
>
> This is a syntax error.

Well spotted. Go to top of the pedants class.
Yes! I finally made it!

Snipping relevant parts of the post is bad etiquette. I also said:

> Please post full, compileable, minimal examples.

Which was the whole point. It's in the OP's interest, really.

The snipped part had no relevance on my pithy reply to your pedant
prize winning reply. The guy was asking a question about casting : not for a
code review or smart assed comments on "etc." being a syntx error.

Which question I answered /first/, and than added a few comments as
well. What is your problem with this?
The fact that he didnt put in the includes and did include the ".etc"
would, to me, indicate that he was pseudo coding a bit to get an
answer to the pertient question which was "can one cast between
pointers and ints". I'm sure many others would have made this leap in
logic too : it is, after all, a programming language group :)
He obviously wasn't "pseudocodi ng". His "pseudocode " looked very much
like an attempt at a C program. It actually wasn't even required to
support his question, especially not with the struct.
Or do you insist on working, compilable code for all questions on
casts?
Not necessarily working (otherwise there'll be no question, right? ;-)
), but certainly compilable (and not just for the cast questions). Or,
do you always re-type the posted code to try it out?
Come on. Admit it. You were over the top

Continuing to indulge you? I admit to that.

--
BR, Vladimir