offsetof

[Alejo]

Hello,

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Could I get some pointers on what I am doing wrong (apart from being coding
so late at night).

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
struct new
{
int a;
char b;
char c;
} x;

size_t num;

num = (size_t)((struc t new *)&x.c - &x);

printf( "%u\n", num );

return EXIT_SUCCESS;
}

It always returns 0.

Thanks.

--

Alejo

[Tom St Denis]

Alejo wrote:

Hello,

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Could I get some pointers on what I am doing wrong (apart from being coding
so late at night).

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
struct new
{
int a;
char b;
char c;
} x;

size_t num;

num = (size_t)((struc t new *)&x.c - &x);

printf( "%u\n", num );

return EXIT_SUCCESS;
}

It always returns 0.

Leave that aside. Why are you doing this anyways? The offset of
members is not a terribly portable quantity. So if this has todo with
loading/saving to a file [or memory buffer] you ought to rethink your
design.

As to the general question, maybe .c is the first element as packed by
the compiler. Try doing

x.c = 4;

and get the assembler code the compiler produces. My compiler [gcc
3.3.1 pre-release] produces

main:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
andl $-16, %esp
movb $0, x+5
xorl %eax, %eax
leave
ret

Which seems to be at the end [not start].

Tom

[Alejo]

Sorry, I already realised where the mistake is. I should hve done a cast
(char *) instead of (struct new *). Like:

num = (size_t)((char *)&x.c - (char *)&x);

Well, thanks anyway.

Alejo wrote:

Hello,

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Could I get some pointers on what I am doing wrong (apart from being
coding so late at night).

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
struct new
{
int a;
char b;
char c;
} x;

size_t num;

num = (size_t)((struc t new *)&x.c - &x);

printf( "%u\n", num );

return EXIT_SUCCESS;
}

It always returns 0.

Thanks.

--

Alejo

[Marco de Boer]

"Alejo" <al***@chello.n o> wrote in message
news:fKGQa.1661 6$KF1.303303@am stwist00...

Hello,

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Could I get some pointers on what I am doing wrong (apart from being coding so late at night).

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
struct new
{
int a;
char b;
char c;
} x;

size_t num;

num = (size_t)((struc t new *)&x.c - &x);

printf( "%u\n", num );

return EXIT_SUCCESS;
}

It always returns 0.

Thanks.

--

Alejo

Hi,

Both(x.c and x) are interpreted as struct new *.
And the difference is less than one struct new: so 0.
This one is working:
num=(size_t)(&x .c-(char*)&x);

Marco

[Christian Bau]

In article <fKGQa.16616$KF 1.303303@amstwi st00>, Alejo <al***@chello.n o>
wrote:

Hello,
First, I will _not_ send emails, no matter how much you request them. I
will send emails to you if I intend you to read something that I don't
want the whole world to read, but if I post to the newsgroup then there
is no point in sending an email. You post here, you read here.
My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Could I get some pointers on what I am doing wrong (apart from being coding
so late at night).

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
struct new
{
int a;
char b;
char c;
} x;

size_t num;

num = (size_t)((struc t new *)&x.c - &x);

printf( "%u\n", num );

return EXIT_SUCCESS;
}

It always returns 0.

Not suprisingly. If you had an array

struct new array [100];

how many bytes are &array [0] and &array [1] apart? What is the
difference (&array [1]) - (&array [0])? If two pointers p and q are x
byte apart, how does the compiler calculate p - q?

[Jack Klein]

On Tue, 15 Jul 2003 01:09:28 +0200, Alejo <al***@chello.n o> wrote in
comp.lang.c:

Hello,

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Then you shouldn't be posting to comp.lang.c because whatever you
have, it is not a C compiler.

Or perhaps you have neglected to include <stddef.h>?

Understand that C provides for free-standing implementations , for
embedded systems and such, which are not required to provide even one
single function from the standard library, but they are still required
to provide the macros defined in <stddef.h>.

If what you have does not provide a working offsetof() macro in
<stddef.h>, it is literally not a C compiler.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++ ftp://snurse-l.org/pub/acllc-c++/faq

[Jim]

On Mon, 14 Jul 2003 23:16:15 GMT, Tom St Denis <to********@iah u.ca>
wrote:

Alejo wrote:

Hello,

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Could I get some pointers on what I am doing wrong (apart from being coding
so late at night).

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
struct new
{
int a;
char b;
char c;
} x;

size_t num;

num = (size_t)((struc t new *)&x.c - &x);

printf( "%u\n", num );

return EXIT_SUCCESS;
}

It always returns 0.

Leave that aside. Why are you doing this anyways? The offset of
members is not a terribly portable quantity. So if this has todo with
loading/saving to a file [or memory buffer] you ought to rethink your
design.

As to the general question, maybe .c is the first element as packed by
the compiler. Try doing

x.c = 4;

and get the assembler code the compiler produces. My compiler [gcc
3.3.1 pre-release] produces

main:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
andl $-16, %esp
movb $0, x+5
xorl %eax, %eax
leave
ret

Which seems to be at the end [not start].

If your compiler generates that for x.c = 4, then it is wrong.

Jim

[Richard Bos]

Tom St Denis <to********@iah u.ca> wrote:

Alejo wrote:

struct new
{
int a;
char b;
char c;
} x;

As to the general question, maybe .c is the first element as packed by
the compiler.

Or maybe not, since it isn't allowed to shuffle elements this way. The
first element of a struct new in memory _must_ be a. There _cannot_ be
any padding before a. Hence, offsetof(struct new, a) must be 0, and
offsetof(struct new, c) is not allowed to be. Moreover, offsetof(struct
new, a) < offsetof(struct new, b) < offsetof(struct new, c).

Richard

[Arthur J. O'Dwyer]

On Tue, 15 Jul 2003, Jack Klein wrote:

On Tue, 15 Jul 2003 01:09:28 +0200, Alejo wrote in comp.lang.c:

My implementation does not define offsetof, so I have designed a little
program that 'attempts' to find the relative position of a member in its
structure. It just does not work.

Then you shouldn't be posting to comp.lang.c because whatever you
have, it is not a C compiler.

I thought offsetof() was new in C99. Am I mistaken?

-Arthur

[Hallvard B Furuseth]

Arthur J. O'Dwyer wrote:

I thought offsetof() was new in C99. Am I mistaken?

Yes. It's in ANSI C <stddef.h>.

--
Hallvard