Structure size

[Bruce.]

I would like to allocate a structure size of 1024 bytes but I want the
compiler to do the calculation for me.

typedef struct
{
int var1;
int var2;
int var3;
char var4[ ?????? ];
} MYSTRUCT;

What I want to do is replace the ?????? something that will automattically
make the total structure 1024 bytes without having to manually count the
bytes of the other members myself.

Is that possible in c++?

Thanks.

Bruce.

[James Kanze]

On Jul 19, 7:15 pm, "Bruce." <no...@nowhere. comwrote:

"LR" <lr...@superlin k.netwrote in message
news:46******** *************** @news.uslec.net ...

Bruce. wrote:

I would like to allocate a structure size of 1024 bytes but I want the
compiler to do the calculation for me.

typedef struct
{
int var1;
int var2;
int var3;
char var4[ ?????? ];
} MYSTRUCT;

What I want to do is replace the ?????? something that will
automattically make the total structure 1024 bytes without having to
manually count the bytes of the other members myself.

Is that possible in c++?

May I ask why you want to do that?

I'm creating a new message type, and the message buffers are
hard coded at 1024 bytes. So I need var4 to be as big as
possible but the entire structure can not exceed 1024 bytes.

If you're streaming the data, you probably can't use int
directly anyway. So just use char buffer[whatever].

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

On 2007-07-20 16:45, LR wrote:

Victor Bazarov wrote:

>LR wrote:

>>(I'm assuming that as you said in another post, you don't have a
portability problem and you can control your compiler to take care of
packing issues.)

struct x1 {
int a,b,c;
};

struct x2 {
int a; double e,f,g,h,i;
};

template<type name T>
struct Message {
T t;
char v[1024 - sizeof(T)];
};

then, else where you can...

Message<x1m 1;
Message<x2m 2;

>>>
[..]

I am not sure here, but I believe there is no guarantee that the size
of the 'Message<x1>' or 'Message<x2>' is actually going to be 1024
even with the mechanism you give here.

Is that because of templates? Nested structs? Are you referring to the
OP's particular compiler and it's control of packing/alignment?

The structs I would guess, there's no guarantee that there wont be any
padding between the t and v in Message.

--
Erik Wikström

[James Kanze]

On Jul 20, 4:27 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

LR wrote:

[...]

(I'm assuming that as you said in another post, you don't have a
portability problem and you can control your compiler to take care of
packing issues.)

struct x1 {
int a,b,c;
};

struct x2 {
int a; double e,f,g,h,i;
};

template<typena me T>
struct Message {
T t;
char v[1024 - sizeof(T)];
};

then, else where you can...

Message<x1m1;
Message<x2m2;

there will be the added annoyance of having to reference the
individual members of x1 and x2 by doing something like m1.t.a = 43;
It might be wise to pick a better name for t.

[..]

I am not sure here, but I believe there is no guarantee that the size
of the 'Message<x1>' or 'Message<x2>' is actually going to be 1024
even with the mechanism you give here.

It's certainly not guaranteed by the standard. A compiler could
insert padding between the t and v, and still be conform. As a
quality of implementation issue... I would consider it a poor
choice on a byte addressed machine (and none of the compilers
I've ever used did insert padding here).

I wonder, however, if there
is something in the Standard related to the sizes (and alignments)
that I missed, and it might actually impose certain limitations thus
giving programmers more leeway when it comes to determining type sizes.

I don't think so.

--
James Kanze (Gabi Software) email: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Francis Glassborow]

Bruce. wrote:

"John Harrison" <jo************ *@hotmail.comwr ote in message
news:ve******** ******@newsfe4-gui.ntli.net...

>Since you aren't worried about padding bytes the obvious answer is

typedef struct
{
int var1;
int var2;
int var3;
char var4[ 1024 - 3*sizeof(int) ];
} MYSTRUCT;

Thanks but my example was oversimplified to keep it brief. The actual
structures are much more complex, too complex for a solution like that.

I guess what I was looking for doesn't exist.

But thanks anyway.

Bruce.

Try:

struct mystruct {
struct {
int var1;
int var2;
int var3;
} header;
char data[1024 - sizeof(header)];
}

we still have a slight padding issue because in theory the compiler
could add padding between header and data but in general it won't though
there maybe alignment padding at the end of header but that will be part
of the header object and so included in the sizeof(header).
The above method also has the advantage that it is more maintainable
(easier to change type of a field in the header or add new fields to the
header specification.
--
Please do NOT use robinton.demon. co.uk addresses
They cease to be valid on July 14
replace with Francis.Glassbo row at btinternet.com

[Victor Bazarov]

Francis Glassborow wrote:

Bruce. wrote:

>"John Harrison" <jo************ *@hotmail.comwr ote in message
news:ve******* *******@newsfe4-gui.ntli.net...

>>Since you aren't worried about padding bytes the obvious answer is

typedef struct
{
int var1;
int var2;
int var3;
char var4[ 1024 - 3*sizeof(int) ];
} MYSTRUCT;

Thanks but my example was oversimplified to keep it brief. The
actual structures are much more complex, too complex for a solution
like that. I guess what I was looking for doesn't exist.

But thanks anyway.

Bruce.

Try:

struct mystruct {
struct {
int var1;
int var2;
int var3;
} header;
char data[1024 - sizeof(header)];
}

we still have a slight padding issue because in theory the compiler
could add padding between header and data but in general it won't
though there maybe alignment padding at the end of header but that
will be part of the header object and so included in the
sizeof(header).

The above method also has the advantage that it is more maintainable
(easier to change type of a field in the header or add new fields to
the header specification.

I just thought of a slightly different way:

struct mystruct {
int var1, var2, var3;
char data[1024 - offsetof(data)];
};

Should that alleviate any trouble with possible padding?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Victor Bazarov]

Victor Bazarov wrote:

Francis Glassborow wrote:

>Bruce. wrote:

>>"John Harrison" <jo************ *@hotmail.comwr ote in message
news:ve****** ********@newsfe 4-gui.ntli.net...
Since you aren't worried about padding bytes the obvious answer is

typedef struct
{
int var1;
int var2;
int var3;
char var4[ 1024 - 3*sizeof(int) ];
} MYSTRUCT;

Thanks but my example was oversimplified to keep it brief. The
actual structures are much more complex, too complex for a solution
like that. I guess what I was looking for doesn't exist.

But thanks anyway.

Bruce.

Try:

struct mystruct {
struct {
int var1;
int var2;
int var3;
} header;
char data[1024 - sizeof(header)];
}

we still have a slight padding issue because in theory the compiler
could add padding between header and data but in general it won't
though there maybe alignment padding at the end of header but that
will be part of the header object and so included in the
sizeof(header) .

The above method also has the advantage that it is more maintainable
(easier to change type of a field in the header or add new fields to
the header specification.

I just thought of a slightly different way:

struct mystruct {
int var1, var2, var3;
char data[1024 - offsetof(data)];

Of course this ought to be

char data[1024 - offsetof(mystru ct, data)];

};

Should that alleviate any trouble with possible padding?

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[James Kanze]

On Jul 22, 9:03 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

struct mystruct {
int var1, var2, var3;
char data[1024 - offsetof(data)];

That's "offsetof( mystruct, data )", of course (as you've
already corrected).

};

Should that alleviate any trouble with possible padding?

I don't think you can use offsetof on an incomplete type.

--
James Kanze (Gabi Software) email: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Bruce.]

"Victor Bazarov" <v.********@com Acast.netwrote in message
news:Xt******** *************** *******@comcast .com...

I just thought of a slightly different way:

struct mystruct {
int var1, var2, var3;
char data[1024 - offsetof(mystru ct, data)];
};

Unfortunately that won't compile:

error C2027: use of undefined type 'mystruct'

Bruce.

[James Dennett]

Victor Bazarov wrote:

Victor Bazarov wrote:

>Francis Glassborow wrote:

>>Bruce. wrote:
"John Harrison" <jo************ *@hotmail.comwr ote in message
news:ve***** *********@newsf e4-gui.ntli.net...
Since you aren't worried about padding bytes the obvious answer is
>
typedef struct
{
int var1;
int var2;
int var3;
char var4[ 1024 - 3*sizeof(int) ];
} MYSTRUCT;
Thanks but my example was oversimplified to keep it brief. The
actual structures are much more complex, too complex for a solution
like that. I guess what I was looking for doesn't exist.

But thanks anyway.

Bruce.
Try:

struct mystruct {
struct {
int var1;
int var2;
int var3;
} header;
char data[1024 - sizeof(header)];
}

we still have a slight padding issue because in theory the compiler
could add padding between header and data but in general it won't
though there maybe alignment padding at the end of header but that
will be part of the header object and so included in the
sizeof(header ).

The above method also has the advantage that it is more maintainable
(easier to change type of a field in the header or add new fields to
the header specification.

I just thought of a slightly different way:

struct mystruct {
int var1, var2, var3;
char data[1024 - offsetof(data)];

Of course this ought to be

char data[1024 - offsetof(mystru ct, data)];

>};

Should that alleviate any trouble with possible padding?

It shouldn't even compile, as offsetof requires mystruct
to be complete, and hence can't be used in the definition
of mystruct.

-- James

[BobR]

James Kanze <ja*********@gm ail.comwrote in message...
On Jul 22, 9:03 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

>struct mystruct {
int var1, var2, var3;
char data[1024 - offsetof(data)];

>That's "offsetof( mystruct, data )", of course (as you've
already corrected).

>};
Should that alleviate any trouble with possible padding?

>I don't think you can use offsetof on an incomplete type.

As poor Bruce found out.

If you can't find a sane way to do it, think outside the box.

struct dummy{
int var1, var2, var3;
char data[1];
};

struct mystruct{
int var1, var2, var3;
char data[ 1024 - offsetof( dummy, data ) ];
};

// cout<<"sizeof dummy="<<sizeof (dummy)<<std::e ndl;
// cout<<"sizeof mystruct="<<siz eof(mystruct)<< std::endl;
// sizeof dummy=16
// sizeof mystruct=1024
We'd have to assume (or prey) that the compiler would construct the two
structs the same (up to 'data').

No, it wasn't my bath-water I drank!
<G>
--
Bob R
POVrookie