Distance between struct members

[lovecreatesbea...@gmail.com]

1. The following code snippet uses minus operation on two pointers to
calculate the distance between struct members. This is illegal, right?

2. s1 and s2 are type of the same struct S. Can the distance of s1.i4
between i3 be used to deduce the distance between s2.i4 and s2.i3?

Thank you for your time.


#include <stdio.h>
#include <stddef.h>

struct S {
/*...*/
int i3;
/*...*/
int i4;
};

int main(void)
{
struct S s1, s2;
ptrdiff_t distance;

distance = &s1.i4 - &s1.i3;
s1.i3 = 11;
s1.i4 = 12;
s2.i3 = 13;
s2.i4 = 14;
printf("%d, %d\n", s2.i3, *(&s2.i3 + distance));
return 0;
}

[Eric Sosman]

Re: Distance between struct members

lovecreatesbea. ..@gmail.com wrote On 10/18/07 12:47,:

1. The following code snippet uses minus operation on two pointers to
calculate the distance between struct members. This is illegal, right?

Yes. To see why (or one reason why, anyhow), remember
that pointer arithmetic operates in units of the pointed-to
type. Now consider what might lie in the /*...*/ between
members i3 and i4. If the size of what's there is not an
exact multiple of the size of an int, i3 and i4 are separated
by something-and-a-fraction units. Pointer arithmetic can't
handle the -and-a-fraction part.

2. s1 and s2 are type of the same struct S. Can the distance of s1.i4
between i3 be used to deduce the distance between s2.i4 and s2.i3?

Yes, but let's tighten up what "distance" means. If
you express everything in units of bytes (rather than ints
or whatever), all will be well. C guarantees that

(char*)&s1.i4 - (char*)&s1.i3
== (char*)&s2.i4 - (char*)&s2.i3

However, there are no guarantees about

(char*)&s1.i3 - (char*)s2.i3

Thank you for your time.
>
>
#include <stdio.h>
#include <stddef.h>
>
struct S {
/*...*/
int i3;
/*...*/
int i4;
};
>
int main(void)
{
struct S s1, s2;
ptrdiff_t distance;
>
distance = &s1.i4 - &s1.i3;
s1.i3 = 11;
s1.i4 = 12;
s2.i3 = 13;
s2.i4 = 14;
printf("%d, %d\n", s2.i3, *(&s2.i3 + distance));
return 0;
}
>

[Keith Thompson]

Re: Distance between struct members

Eric Sosman <Eric.Sosman@su n.comwrites:

lovecreatesbea. ..@gmail.com wrote On 10/18/07 12:47,:

>1. The following code snippet uses minus operation on two pointers to
>calculate the distance between struct members. This is illegal, right?

>
Yes. To see why (or one reason why, anyhow), remember
that pointer arithmetic operates in units of the pointed-to
type. Now consider what might lie in the /*...*/ between
members i3 and i4. If the size of what's there is not an
exact multiple of the size of an int, i3 and i4 are separated
by something-and-a-fraction units. Pointer arithmetic can't
handle the -and-a-fraction part.

[...]

Yes, but that's just one reason, and it depends on what you mean by
"illegal".

The real reason is that pointer subtraction invokes undefined behavior
if the two pointers point to distinct objects. See C99 6.5.6p9. This
applies even to subtraction of char* pointers, which are not affected
by alignment.

(In a typical implementation, the subtraction is likely to give you a
somewhat meaningful result. If the the difference is not a multiple
of the size of the pointed-to object, the remainder is likely to be
quitely ignored. But there are, of course, absolutely no guarantees.)

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Jack Klein]

Re: Distance between struct members

On Thu, 18 Oct 2007 15:21:21 -0700, Keith Thompson <kst-u@mib.org>
wrote in comp.lang.c:

Eric Sosman <Eric.Sosman@su n.comwrites:

lovecreatesbea. ..@gmail.com wrote On 10/18/07 12:47,:

1. The following code snippet uses minus operation on two pointers to
calculate the distance between struct members. This is illegal, right?

Yes. To see why (or one reason why, anyhow), remember
that pointer arithmetic operates in units of the pointed-to
type. Now consider what might lie in the /*...*/ between
members i3 and i4. If the size of what's there is not an
exact multiple of the size of an int, i3 and i4 are separated
by something-and-a-fraction units. Pointer arithmetic can't
handle the -and-a-fraction part.

[...]
>
Yes, but that's just one reason, and it depends on what you mean by
"illegal".
>
The real reason is that pointer subtraction invokes undefined behavior
if the two pointers point to distinct objects. See C99 6.5.6p9. This
applies even to subtraction of char* pointers, which are not affected
by alignment.

I disagree about using pointer to char, specifically pointer to
unsigned.

Any object, including the structure in the OP's post, can be accessed
as a suitably sized array of unsigned char. It is legal, therefore,
to subtract the addresses of two members of the same structure,
provided of course they are cast to pointers to unsigned char.

The result will be a ptrdiff_t representing the number of bytes
between the first byte in the representation of the first member and
the first byte in the representation of the second member.

I do agree about using pointers to int, regardless of alignment
issues, because clearly two different int members of a structure are
not elements of the same array of ints.

(In a typical implementation, the subtraction is likely to give you a
somewhat meaningful result. If the the difference is not a multiple
of the size of the pointed-to object, the remainder is likely to be
quitely ignored. But there are, of course, absolutely no guarantees.)

Now the question is, can anybody find wording in the standard
(probably scattered abound in multiple places) that definitively makes
doing this with pointer to char or pointer to signed char well-defined
because it is well-defined for pointer to unsigned char?

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html

[Keith Thompson]

Re: Distance between struct members

Jack Klein <jackklein@spam cop.netwrites:

On Thu, 18 Oct 2007 15:21:21 -0700, Keith Thompson <kst-u@mib.org>
wrote in comp.lang.c:

>Eric Sosman <Eric.Sosman@su n.comwrites:

lovecreatesbea. ..@gmail.com wrote On 10/18/07 12:47,:
>1. The following code snippet uses minus operation on two pointers to
>calculate the distance between struct members. This is illegal, right?
>
Yes. To see why (or one reason why, anyhow), remember
that pointer arithmetic operates in units of the pointed-to
type. Now consider what might lie in the /*...*/ between
members i3 and i4. If the size of what's there is not an
exact multiple of the size of an int, i3 and i4 are separated
by something-and-a-fraction units. Pointer arithmetic can't
handle the -and-a-fraction part.

>[...]
>>
>Yes, but that's just one reason, and it depends on what you mean by
>"illegal".
>>
>The real reason is that pointer subtraction invokes undefined behavior
>if the two pointers point to distinct objects. See C99 6.5.6p9. This
>applies even to subtraction of char* pointers, which are not affected
>by alignment.

>
I disagree about using pointer to char, specifically pointer to
unsigned.
>
Any object, including the structure in the OP's post, can be accessed
as a suitably sized array of unsigned char. It is legal, therefore,
to subtract the addresses of two members of the same structure,
provided of course they are cast to pointers to unsigned char.

Certainly.

I may have misread your comments above. I thought you were talking
about subtracting pointers to members of distinct objects, rather than
pointers to members of the same object.

Subtracting two char* pointers, if they both point into the same
object (or just past its end) is valid. Subtracting two pointers of
any type that point to distinct objects invokes undefined behavior.
Subtracting two pointers to non-char types, both of which point into
the same structure, probably invokes undefined behavior because of the
alignment issues you mentioned above.

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Eric Sosman]

Re: Distance between struct members

Keith Thompson wrote:

Eric Sosman <Eric.Sosman@su n.comwrites:

>lovecreatesbea ...@gmail.com wrote On 10/18/07 12:47,:

>>1. The following code snippet uses minus operation on two pointers to
>>calculate the distance between struct members. This is illegal, right?

> Yes. To see why (or one reason why, anyhow), remember
>that pointer arithmetic operates in units of the pointed-to
>type. Now consider what might lie in the /*...*/ between
>members i3 and i4. If the size of what's there is not an
>exact multiple of the size of an int, i3 and i4 are separated
>by something-and-a-fraction units. Pointer arithmetic can't
>handle the -and-a-fraction part.

[...]
>
Yes, but that's just one reason, and it depends on what you mean by
"illegal".
>
The real reason is that pointer subtraction invokes undefined behavior
if the two pointers point to distinct objects. See C99 6.5.6p9. This
applies even to subtraction of char* pointers, which are not affected
by alignment.

Well, that's no "reason" at all: It just states the Law
and offers no argument for why the Law should be as it is.
The most famous example of that particular argument is surely
"I am that I am," which few mortals can bring off believably.

In the example you snipped, the subtraction of int* pointers
was not well-defined but the subtraction of char* pointers was.

--
Eric Sosman
esosman@ieee-dot-org.invalid

[Keith Thompson]

Re: Distance between struct members

Eric Sosman <esosman@ieee-dot-org.invalidwrit es:
[...]

In the example you snipped, the subtraction of int* pointers
was not well-defined but the subtraction of char* pointers was.

Yes, because I misread it.

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Kenneth Brody]

Re: Distance between struct members

Keith Thompson wrote:

>
Eric Sosman <Eric.Sosman@su n.comwrites:

lovecreatesbea. ..@gmail.com wrote On 10/18/07 12:47,:

1. The following code snippet uses minus operation on two pointers to
calculate the distance between struct members. This is illegal, right?

Yes. To see why (or one reason why, anyhow), remember
that pointer arithmetic operates in units of the pointed-to
type. Now consider what might lie in the /*...*/ between
members i3 and i4. If the size of what's there is not an
exact multiple of the size of an int, i3 and i4 are separated
by something-and-a-fraction units. Pointer arithmetic can't
handle the -and-a-fraction part.

[...]
>
Yes, but that's just one reason, and it depends on what you mean by
"illegal".
>
The real reason is that pointer subtraction invokes undefined behavior
if the two pointers point to distinct objects. See C99 6.5.6p9. This
applies even to subtraction of char* pointers, which are not affected
by alignment.

But &s1.i4 and &s1.i3 are both pointers within s1, and therefore are
not "distinct objects". (I suppose the typical "IMO" disclaimer may
apply?)

Plus, as I understand it, it is perfectly legal to overlay an array
of unsigned chars on any object, and access any and all bytes within
that object through this array. How is casting &s1.i4 and &s1.i3 to
"unsigned char *" any different than overlaying an unsigned char
array?

On second thought, however, I can see that taking the addresses of
the two as their native "int *", you can say that the two ints are
not part of the same object, as they are not part of an array of
ints. (Which is why they may not be a multiple-of-sizeof-int bytes
apart.) It is the casting to "unsigned char *" which means that the
addresses can be treated "as-if" they were part of an array of
unsigned chars the size of the struct.

Perhaps we're both right?

(In a typical implementation, the subtraction is likely to give you a
somewhat meaningful result. If the the difference is not a multiple
of the size of the pointed-to object, the remainder is likely to be
quitely ignored. But there are, of course, absolutely no guarantees.)

I think offsetof() is the way to go here. The offset of s1.i3 is
guaranteed to be the same as the offset of s2.i3, assuming that s1
and s2 are the same type, and any arithmetic which arrives at that
offset is guaranteed to be properly aligned.

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:ThisIsA SpamTrap@gmail. com>

[lovecreatesbea...@gmail.com]

Re: Distance between struct members

On Oct 19, 1:46 am, Eric Sosman <Eric.Sos...@su n.comwrote:

lovecreatesbea. ..@gmail.com wrote On 10/18/07 12:47,:
>

1. The following code snippet uses minus operation on two pointers to
calculate the distance between struct members. This is illegal, right?

>
Yes. To see why (or one reason why, anyhow), remember
that pointer arithmetic operates in units of the pointed-to
type. Now consider what might lie in the /*...*/ between
members i3 and i4. If the size of what's there is not an
exact multiple of the size of an int, i3 and i4 are separated
by something-and-a-fraction units. Pointer arithmetic can't
handle the -and-a-fraction part.
>

2. s1 and s2 are type of the same struct S. Can the distance of s1.i4
between i3 be used to deduce the distance between s2.i4 and s2.i3?

>
Yes, but let's tighten up what "distance" means. If
you express everything in units of bytes (rather than ints
or whatever), all will be well. C guarantees that
>
(char*)&s1.i4 - (char*)&s1.i3
== (char*)&s2.i4 - (char*)&s2.i3
>
However, there are no guarantees about
>
(char*)&s1.i3 - (char*)s2.i3
>

Thank you.

So, the extra casts make the code in the original post legal and
portable, doesn't it?

#include <stdio.h>
#include <stddef.h>

struct S {
/*...*/
int i3;
/*...*/
int i7;
};

int main(void)
{
struct S s1 = {11, 12}, s2 = {13, 14};
ptrdiff_t distance;

distance = (char *)&s1.i7 - (char *)&s1.i3;
printf("%d, %d\n", s2.i3, (int)*((char *)&s2.i3 + distance));
return 0;
}

Thank you for your time.

>

#include <stdio.h>
#include <stddef.h>

>

struct S {
/*...*/
int i3;
/*...*/
int i4;
};

>

int main(void)
{
struct S s1, s2;
ptrdiff_t distance;

>

distance = &s1.i4 - &s1.i3;
s1.i3 = 11;
s1.i4 = 12;
s2.i3 = 13;
s2.i4 = 14;
printf("%d, %d\n", s2.i3, *(&s2.i3 + distance));
return 0;
}- Hide quoted text -

>
- Show quoted text -

[lovecreatesbea...@gmail.com]

Re: Distance between struct members

On Oct 19, 10:28 pm, Kenneth Brody <kenbr...@spamc op.netwrote:

I think offsetof() is the way to go here. The offset of s1.i3 is
guaranteed to be the same as the offset of s2.i3, assuming that s1
and s2 are the same type, and any arithmetic which arrives at that
offset is guaranteed to be properly aligned.

But the offsetof() uses size_t other than "char *" or "unsigned char
*" to designate the type of the addresses, why?

[Keith Thompson]

Re: Distance between struct members

Kenneth Brody <kenbrody@spamc op.netwrites:

Keith Thompson wrote:

[...]

>The real reason is that pointer subtraction invokes undefined behavior
>if the two pointers point to distinct objects. See C99 6.5.6p9. This
>applies even to subtraction of char* pointers, which are not affected
>by alignment.

>
But &s1.i4 and &s1.i3 are both pointers within s1, and therefore are
not "distinct objects". (I suppose the typical "IMO" disclaimer may
apply?)

Yes; as I've acknowledged, my statement above was the result of my
misreading the previous material.

Plus, as I understand it, it is perfectly legal to overlay an array
of unsigned chars on any object, and access any and all bytes within
that object through this array. How is casting &s1.i4 and &s1.i3 to
"unsigned char *" any different than overlaying an unsigned char
array?
>
On second thought, however, I can see that taking the addresses of
the two as their native "int *", you can say that the two ints are
not part of the same object, as they are not part of an array of
ints. (Which is why they may not be a multiple-of-sizeof-int bytes
apart.) It is the casting to "unsigned char *" which means that the
addresses can be treated "as-if" they were part of an array of
unsigned chars the size of the struct.

The standard's requirement isn't really that they point to "the same
object". The actual wording, in C99 6.5.6p9, is:

When two pointers are subtracted, both shall point to elements of
the same array object, or one past the last element of the array
object; the result is the difference of the subscripts of the two
array elements.

It's stated elsewhere that any object of type T can be treated as an
array of type T[1], and that any object can be treated as an array of
unsigned char. The latter lets you get away with converting the
pointers &s1.i4 and &s1.i3 to ``unsigned char*'' before subtracting
them. Other rules, which I'm too lazy to look up, allow you to do the
same thing with ``char*'' or ``signed char*''. But if i4 and i3 are
both of type int, there's no rule that lets you treat them as elements
of the same array. (If the required alignment for type int is the
same as its size, you're very likely to get away with it unless the
implementation goes out of its way to stop you, but it's still
undefined behavior.)

[...]

I think offsetof() is the way to go here. The offset of s1.i3 is
guaranteed to be the same as the offset of s2.i3, assuming that s1
and s2 are the same type, and any arithmetic which arrives at that
offset is guaranteed to be properly aligned.

Agreed. If you care about the distince in bytes between the members
i3 and i4 of some struct type, then
offsetof(struct foo, i4) - offsetof(struct foo, i3)
is a clearer way to express it.

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

Re: Distance between struct members

"lovecreatesbea ...@gmail.com" <lovecreatesbea uty@gmail.comwr ites:

On Oct 19, 10:28 pm, Kenneth Brody <kenbr...@spamc op.netwrote:

>I think offsetof() is the way to go here. The offset of s1.i3 is
>guaranteed to be the same as the offset of s2.i3, assuming that s1
>and s2 are the same type, and any arithmetic which arrives at that
>offset is guaranteed to be properly aligned.

>
But the offsetof() uses size_t other than "char *" or "unsigned char
*" to designate the type of the addresses, why?

No, offsetof() uses size_t for the offset; it doesn't express any
address as a size_t.

Here's the standard's definition (C99 7.17p3):

offsetof(type, member-designator)

which expands to an integer constant expression that has type
size_t, the value of which is the offset in bytes, to the
structure member (designated by member-designator), from the
beginning of its structure (designated by type). The type and
member designator shall be such that given

static type t;

then the expression &(t.member-designator) evaluates to an address
constant. (If the specified member is a bit-field, the behavior is
undefined.)

--
Keith Thompson (The_Other_Keit h) kst-u@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."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Eric Sosman]

Re: Distance between struct members

lovecreatesbea. ..@gmail.com wrote On 10/19/07 14:43,:

>
So, the extra casts make the code in the original post legal and
portable, doesn't it?

The code in the original post was illegal and non-
portable. The revised code in this post is almost all
right: It will print 13 and something else (because the
third argument to printf should be `*(int*)((char* )...)'
instead of what you wrote).

#include <stdio.h>
#include <stddef.h>
>
struct S {
/*...*/
int i3;
/*...*/
int i7;
};
>
int main(void)
{
struct S s1 = {11, 12}, s2 = {13, 14};
ptrdiff_t distance;
>
distance = (char *)&s1.i7 - (char *)&s1.i3;
printf("%d, %d\n", s2.i3, (int)*((char *)&s2.i3 + distance));
return 0;
}

... but I must ask: WHY do you want to do this?
If you want to print the value of s2.i7, just do it:
don't fool around with all this pointer-bashing. Even
if it is *possible* to perform an appendectomy with two
teaspoons and an eggbeater, that doesn't make it a
good idea.

--
Eric.Sosman@sun .com

[lovecreatesbea...@gmail.com]

Re: Distance between struct members

On Oct 20, 3:53 am, Keith Thompson <ks...@mib.orgw rote:

"lovecreatesbea ...@gmail.com" <lovecreatesbea ...@gmail.comwr ites:

On Oct 19, 10:28 pm, Kenneth Brody <kenbr...@spamc op.netwrote:

I think offsetof() is the way to go here. The offset of s1.i3 is
guaranteed to be the same as the offset of s2.i3, assuming that s1
and s2 are the same type, and any arithmetic which arrives at that
offset is guaranteed to be properly aligned.

>

But the offsetof() uses size_t other than "char *" or "unsigned char
*" to designate the type of the addresses, why?

>
No, offsetof() uses size_t for the offset; it doesn't express any
address as a size_t.
>
Here's the standard's definition (C99 7.17p3):
>
offsetof(type, member-designator)
>
which expands to an integer constant expression that has type
size_t, the value of which is the offset in bytes, to the
structure member (designated by member-designator), from the
beginning of its structure (designated by type). The type and
member designator shall be such that given
>
static type t;
>
then the expression &(t.member-designator) evaluates to an address

Thank you.

Why it's not in this form

(char *) &(t.member-designator)

I read it from other posts, some peopoe said that the standard
definition

#define offsetof(type, memb) ((size_t) &((type *) 0)-memb)

implies

#define offsetof(type, memb) ((size_t) &((type *) 0)-memb -
&((type *) 0))


Isn't the following one better?

#define offsetof(type, memb) \
((size_t) ((char *) &((type *) 0)-memb - (char *) &((type
*) 0)))

constant. (If the specified member is a bit-field, the behavior is
undefined.)

[lovecreatesbea...@gmail.com]

Re: Distance between struct members

On Oct 20, 4:23 am, Eric Sosman <Eric.Sos...@su n.comwrote:

lovecreatesbea. ..@gmail.com wrote On 10/19/07 14:43,:
>
>
>

So, the extra casts make the code in the original post legal and
portable, doesn't it?

>
The code in the original post was illegal and non-
portable. The revised code in this post is almost all
right: It will print 13 and something else (because the
third argument to printf should be `*(int*)((char* )...)'
instead of what you wrote).
>

Thank you.

I wrote the third argument wrongly, thanks for the correction.

>
>
>
>

#include <stdio.h>
#include <stddef.h>

>

struct S {
/*...*/
int i3;
/*...*/
int i7;
};

>

int main(void)
{
struct S s1 = {11, 12}, s2 = {13, 14};
ptrdiff_t distance;

>

distance = (char *)&s1.i7 - (char *)&s1.i3;
printf("%d, %d\n", s2.i3, (int)*((char *)&s2.i3 + distance));
return 0;
}

>
... but I must ask: WHY do you want to do this?
If you want to print the value of s2.i7, just do it:
don't fool around with all this pointer-bashing. Even
if it is *possible* to perform an appendectomy with two
teaspoons and an eggbeater, that doesn't make it a
good idea.

I didn't know the knowledge of these details about structs before. and
locating of struct members by offset.

Some people said the offsetof macro in this way

#define offsetof(type, memb) ((size_t) &((type *) 0)-memb)

dereferences NULL /* 0 */ pointer and it's undefined behavior. I'm
even more anxious on this. And it's not put in this form

#define offsetof(type, memb) \
((size_t) ((char *) &((type *) 0)-memb - (char *) &((type *)
0)))

Could you please talk about this more?