sizeof a relative pointer in a struct

**Noob** · Mar 20 '08, 03:55 PM

Re: sizeof a relative pointer in a struct

ManicQin wrote:

#define offsetof(s,m) (size_t)&(((s *)0)->m)

AFAIU, however offsetof is defined, it is defined in stddef.h

$ cat foo.c
#include <stdio.h /* printf */
#include <stddef.h /* offsetof */
struct ThePimp { char rings[10]; char blings[20]; };
int main(void)
{
printf("%d\n", (int)offsetof(s truct ThePimp, blings));
return 0;
}

$ gcc -std=c89 -pedantic -Wall -Wextra foo.c

$ ./a.out
10

**REH** · Mar 20 '08, 04:45 PM

Re: sizeof a relative pointer in a struct

On Mar 20, 11:39 am, ManicQin <Manic...@gmail .comwrote:

Hi all.
I'm trying to get the size of a variable in a struct by his relative
postion i.e.
>
///
#define offsetof(s,m) (size_t)&(((s *)0)->m)
>
struct ThePimp{
char rings[10];
char blings[20];
>
};
>
int sizeOfBling = sizeof( (*char)&ThePimp +
(char)offsetof( ThePimp,blings) );
>
///
(If I have any syntax mistakes please ignore)
>
You can easily predict that sizeOfBling will be 4... (sizeof(int*))
Is there any way to acomplish it?
>
Thanks

1. offsetof is already defined for you.
2. why not just use sizeof?
3. Where does the "easily predicted" 4 come from?
4. How does your method adjust for alignment?

REH

**rpgfan3233** · Mar 20 '08, 04:55 PM

Re: sizeof a relative pointer in a struct

On Mar 20, 11:41 am, REH <spamj...@stny. rr.comwrote:

On Mar 20, 11:39 am, ManicQin <Manic...@gmail .comwrote:
>
>
>

Hi all.
I'm trying to get the size of a variable in a struct by his relative
postion i.e.

>

///
#define offsetof(s,m) (size_t)&(((s *)0)->m)

>

struct ThePimp{
char rings[10];
char blings[20];

>

};

>

int sizeOfBling = sizeof( (*char)&ThePimp +
(char)offsetof( ThePimp,blings) );

>

///
(If I have any syntax mistakes please ignore)

>

You can easily predict that sizeOfBling will be 4... (sizeof(int*))
Is there any way to acomplish it?

>

Thanks

>
1. offsetof is already defined for you.
2. why not just use sizeof?
3. Where does the "easily predicted" 4 come from?
4. How does your method adjust for alignment?
>
REH

The "easily predicted" 4 comes from a machine where sizeof(TYPE*) = 4
because it is the native size for that machine (most likely a 32-bit
machine).

**REH** · Mar 20 '08, 09:05 PM

Re: sizeof a relative pointer in a struct

On Mar 20, 12:46 pm, rpgfan3233 <rpgfan3...@gma il.comwrote:

The "easily predicted" 4 comes from a machine where sizeof(TYPE*) = 4
because it is the native size for that machine (most likely a 32-bit
machine).

So? Where is the pointer he is trying to determine the size of. I
see two arrays, neither of which would be 4 bytes in size, regardless
of the machine.

REH

**REH** · Mar 22 '08, 04:15 PM

Re: sizeof a relative pointer in a struct

On Mar 21, 9:23 am, ManicQin <Manic...@gmail .comwrote:

I would ignore most of the comments
but I do want to reply to somethings
1) sizeof a pointer is 4 (platform specific)

But you are not getting the size of a pointer, both of your fields are
arrays. The size of "rings" is 10. The size of "blings" is 20. This
will always be true, regardless of the target system, unless you
change the type of your array elements, or the number of elements in
your array.

REH

**REH** · Mar 22 '08, 04:15 PM

Re: sizeof a relative pointer in a struct

On Mar 21, 9:23 am, ManicQin <Manic...@gmail .comwrote:

I cannot rely on people not messing with my struct so i cannot know
what is after or before the requested var thanks NOOB
and by the way the alignment is dealed thanks REH
>

How is the alignment problem dealt with? If there is padding before a
field, it will get included in your total when your calculate
offsetof. How do you remove it?

REH

**Ben Bacarisse** · Mar 23 '08, 02:05 PM

Re: sizeof a relative pointer in a struct

ManicQin <ManicQin@gmail .comwrites:

On Mar 22, 6:08 pm, REH <spamj...@stny. rr.comwrote:

>On Mar 21, 9:23 am, ManicQin <Manic...@gmail .comwrote:
>>

I would ignore most of the comments
but I do want to reply to somethings
1) sizeof a pointer is 4 (platform specific)

>>
>But you are not getting the size of a pointer, both of your fields are
>arrays. The size of "rings" is 10. The size of "blings" is 20. This
>will always be true, regardless of the target system, unless you
>change the type of your array elements, or the number of elements in
>your array.
>>
>REH

REH:
Sorry I rechecked it... The way I use it, it returns 4 (or
size_of(pointer ) for the pedants)
(See the code I posted and try him - read this post all through to see
why I coded it that way)

Check what REH is saying. He (or she) is saying that you seem to be
trying to get the size of something that is not a pointer. Of course
you get 4 (on your system) if you apply sizeof to a pointer, but your
code:

struct ThePimp{
char rings[10];
char blings[20];

};

int sizeOfBling = sizeof( (*char)&ThePimp +
(char)offsetof( ThePimp,blings) );

suggested you wanted the size of the part of struct (just from the
name of the variable). I think all REH is saying is that you should
not take the size of a pointer if you want the size of something else.

Ha Ha Ha Keith:
In normal systems you were right... But in this hideous system I only
know the offset of the variable...
(That's why I added the offsetof in my code above...)
Are you starting to catch my drift...

Nope. I, for one, am now lost about what your question is. Keith
Thompson told you how to get the size of struct member (which is what
I though was the question) but it seems it is not. You know how to
find the offset of a member (though you should not need to define your
own offsetof macro) so that can't be it.

Are you maybe confused as to why sizeOfBling is 4? It is 4 simply
because the type of the expression sizeof is applied to is a pointer.
Are you confused as to why adding a char to a char * gives a pointer?

There's a huge switch case goes like:
switch (param)
{
case offsetof(rings) ... return 10
case offsetof(blings )... return 20
};

That does not help us follow your problem. Are you trying to write
something like this? Are you trying to modify it in some way?

--
Ben.

**ManicQin** · Mar 24 '08, 06:45 AM

Re: sizeof a relative pointer in a struct

Ok Please dont ask me why am I writing this code the way it is, It's
an abbreviation of a much bigger complexed system
and I cant explain why it was chosen to work this way. I'm trying to
fix a particular point in the system.

I'm trying to achieve the line with the comment (and the
returnSizeofPar am function) in a smaller, nicer and more DYNAMIC way.
As you can see whenever I add a new Parameter to Parameters struct I
need to update returnSizeofPar amByOffest ... for me it looks a bit
crooked...
Is there a way to know the sizeof a variable in a struct only by his
offset. is it possible?

struct Parameters
{
char strIDNum[10];
char strFirstName[20];
char strFamilyName[30];
};

int returnSizeofPar amByOffest(int lParamOffset)
{
Parameters cTemp;

switch (lParamOffset) {
case offsetof(Parame ters,strIDNum): return sizeof(cTemp.st rIDNum);
break;
case offsetof(Parame ters,strFirstNa me): return
sizeof(cTemp.st rFirstName); break;
case offsetof(Parame ters,strFamilyN ame): return
sizeof(cTemp.st rFamilyName); break;
};

return 0;
}

int main() {
Parameters cTemp= {"AAA" , "BBB" , "CCC"};
char strTempForPrint ing[100] = {0};
int lParamToPrint = offsetof(Parame ters , strFirstName);
int sizeOfParam =
returnSizeofPar amByOffest(lPar amToPrint ); //
*************** *****

memcpy(strTempF orPrinting,(cha r*)&cTemp + (char)lParamToP rint ,
sizeOfParam );
printf("The parameter is: %s \n",strTempForP rinting);
return 0;
}

p.s. Please tell me why I wasn't understood in the above posts. (It's
important for me to understand why there was a mis-connection)

**Ben Bacarisse** · Mar 24 '08, 12:35 PM

Re: sizeof a relative pointer in a struct

ManicQin <ManicQin@gmail .comwrites:

Ok Please dont ask me why am I writing this code the way it is,

OK.

<snip>

Is there a way to know the sizeof a variable in a struct only by his
offset. is it possible?

No.

struct Parameters
{
char strIDNum[10];
char strFirstName[20];
char strFamilyName[30];
};
>
int returnSizeofPar amByOffest(int lParamOffset)
{
Parameters cTemp;
>
switch (lParamOffset) {
case offsetof(Parame ters,strIDNum): return sizeof(cTemp.st rIDNum);
break;
case offsetof(Parame ters,strFirstNa me): return
sizeof(cTemp.st rFirstName); break;
case offsetof(Parame ters,strFamilyN ame): return
sizeof(cTemp.st rFamilyName); break;
};
>
return 0;
}
>
int main() {
Parameters cTemp= {"AAA" , "BBB" , "CCC"};
char strTempForPrint ing[100] = {0};
int lParamToPrint = offsetof(Parame ters , strFirstName);
int sizeOfParam =
returnSizeofPar amByOffest(lPar amToPrint ); //
*************** *****
>
memcpy(strTempF orPrinting,(cha r*)&cTemp + (char)lParamToP rint ,
sizeOfParam );
printf("The parameter is: %s \n",strTempForP rinting);
return 0;
}

This is not C (I am sure you know that -- the compiler will have told
you so) but since your objective is impossible, there is no point in
trying to make it C.

p.s. Please tell me why I wasn't understood in the above posts. (It's
important for me to understand why there was a mis-connection)

I think this because you question was not clear. You may have hinted
at the above question before, but since the answer is such a clear and
obvious "no" (can you tell how many pages are in a book just from its
position on the shelf?) people may have tried to re-interpret the
hints as some other question.

--
Ben.

**ManicQin** · Mar 24 '08, 12:45 PM

Re: sizeof a relative pointer in a struct

On Mar 24, 2:25 pm, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

ManicQin <Manic...@gmail .comwrites:

Ok Please dont ask me why am I writing this code the way it is,

>
OK.
>
<snip>
>

Is there a way to know the sizeof a variable in a struct only by his
offset. is it possible?

>
No.
>
>
>

struct Parameters
{
char strIDNum[10];
char strFirstName[20];
char strFamilyName[30];
};

>

int returnSizeofPar amByOffest(int lParamOffset)
{
Parameters cTemp;

>

switch (lParamOffset) {
case offsetof(Parame ters,strIDNum): return sizeof(cTemp.st rIDNum);
break;
case offsetof(Parame ters,strFirstNa me): return
sizeof(cTemp.st rFirstName); break;
case offsetof(Parame ters,strFamilyN ame): return
sizeof(cTemp.st rFamilyName); break;
};

>

return 0;
}

>

int main() {
Parameters cTemp= {"AAA" , "BBB" , "CCC"};
char strTempForPrint ing[100] = {0};
int lParamToPrint = offsetof(Parame ters , strFirstName);
int sizeOfParam =
returnSizeofPar amByOffest(lPar amToPrint ); //
*************** *****

>

memcpy(strTempF orPrinting,(cha r*)&cTemp + (char)lParamToP rint ,
sizeOfParam );
printf("The parameter is: %s \n",strTempForP rinting);
return 0;
}

>
This is not C (I am sure you know that -- the compiler will have told
you so) but since your objective is impossible, there is no point in
trying to make it C.
>

p.s. Please tell me why I wasn't understood in the above posts. (It's
important for me to understand why there was a mis-connection)

>
I think this because you question was not clear. You may have hinted
at the above question before, but since the answer is such a clear and
obvious "no" (can you tell how many pages are in a book just from its
position on the shelf?) people may have tried to re-interpret the
hints as some other question.
>
--
Ben.

"I'm trying to get the size of a variable in a struct by his relative
postion"
.... I thought that would be self explanatory - Never mind that now.
Thanks you just shattered my dreams of creating a better world... ;)
BTW why isnt it C?

**Richard Heathfield** · Mar 24 '08, 01:05 PM

Re: sizeof a relative pointer in a struct

ManicQin said:

<snip>

"I'm trying to get the size of a variable in a struct by his relative
postion"
... I thought that would be self explanatory

It is. So is "this can't be done".

Think about it. If it were possible to do that, then we could control the
size of an object simply by placing it at a certain position in the
struct. If, say, the relative position "first" yields "2 bytes", then we
could fit a double into two bytes just by making it the first member of a
structure! That simply isn't how the world works. Sorry.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**Ben Bacarisse** · Mar 24 '08, 04:25 PM

Re: sizeof a relative pointer in a struct

ManicQin <ManicQin@gmail .comwrites:

On Mar 24, 2:25 pm, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

>ManicQin <Manic...@gmail .comwrites:

Ok Please dont ask me why am I writing this code the way it is,

>>
>OK.
>>
><snip>
>>

Is there a way to know the sizeof a variable in a struct only by his
offset. is it possible?

>>
>No.
>>
>>
>>

struct Parameters
{
char strIDNum[10];
char strFirstName[20];
char strFamilyName[30];
};

>>

int returnSizeofPar amByOffest(int lParamOffset)
{
Parameters cTemp;

>>

switch (lParamOffset) {
case offsetof(Parame ters,strIDNum): return sizeof(cTemp.st rIDNum);
break;
case offsetof(Parame ters,strFirstNa me): return
sizeof(cTemp.st rFirstName); break;
case offsetof(Parame ters,strFamilyN ame): return
sizeof(cTemp.st rFamilyName); break;
};

>>

return 0;
}

>>

int main() {
Parameters cTemp= {"AAA" , "BBB" , "CCC"};
char strTempForPrint ing[100] = {0};
int lParamToPrint = offsetof(Parame ters , strFirstName);
int sizeOfParam =
returnSizeofPar amByOffest(lPar amToPrint ); //
*************** *****

>>

memcpy(strTempF orPrinting,(cha r*)&cTemp + (char)lParamToP rint ,
sizeOfParam );
printf("The parameter is: %s \n",strTempForP rinting);
return 0;
}

>>
>This is not C (I am sure you know that -- the compiler will have told
>you so) but since your objective is impossible, there is no point in
>trying to make it C.
>>

p.s. Please tell me why I wasn't understood in the above posts. (It's
important for me to understand why there was a mis-connection)

>>
>I think this because you question was not clear. You may have hinted
>at the above question before, but since the answer is such a clear and
>obvious "no" (can you tell how many pages are in a book just from its
>position on the shelf?) people may have tried to re-interpret the
>hints as some other question.
>>
>--
>Ben.

Best not to quote sigs.

"I'm trying to get the size of a variable in a struct by his relative
postion"
... I thought that would be self explanatory - Never mind that now.

Well it was and I answered it. However, this:

BTW why isnt it C?

made me look in detail more. First the answer: it is not C because it
has syntax errors and does not comply with C's other rules. In
particular, offsetof does produce a constant suitable for use in a
switch statement. However, any switch can be replaced by if-then-else
so that would work.

Programmers look for general solutions, so your general question got
answered (at least by me) in the most general way: can't be done. The
solution for one structure, written out by hand, is so obvious that I
did not think you were asking for it:

struct S temp;
if (arg == offsetof(struct S, member1))
return sizeof temp.member1;
if (arg == offsetof(struct S, member2))
return sizeof temp.member2;

and so on. I am sorry if this sounds rude -- I really did not think
you could be asking for this. If you want something more automatic
and general, then the original answer still stands: you can't.

You can use C99 features to make it less of a maintenance problem:

-------------------
#include <stddef.h>

struct S {
int member1;
int member2;
};

size_t get_size(size_t arg)
{
struct S temp;
size_t sizes[] = {
[offsetof(struct S, member1)] = sizeof temp.member1,
[offsetof(struct S, member2)] = sizeof temp.member2,
};
return sizes[arg];
}
-------------------

but that is about it.

--
Ben.

**ManicQin** · Mar 24 '08, 08:05 PM

Re: sizeof a relative pointer in a struct

Lets say that the answer did not shock me, I thought maybe there's
something I missed so it was worth asking.
Thank You all.

**pete** · Mar 29 '08, 03:35 AM

Re: sizeof a relative pointer in a struct

ManicQin wrote:

"I'm trying to get the size of a variable in a struct by his relative
postion"
... I thought that would be self explanatory

It's bizarre.
It's like "I'm trying to shoot a target by stabbing at it with a knife."

You use the sizeof operator to get the size of a variable.

--
pete

sizeof a relative pointer in a struct

sizeof a relative pointer in a struct

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