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.

[Richard Heathfield]

Bruce. said:

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++?

Not portably. Implementations may insert arbitrary padding after any
structure member. In a pathological implementation, you might have to
give var4 a negative number of elements (which is of course
impossible).

If you need exactly 1024 bytes, allocate an array of unsigned char, 1024
bytes wide, and dip into its bits as and when you need to.

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

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

On 2007-07-19 17:39, 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.

You could do something like this:

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

(Notice also the lack of typedef, it's not needed unless you require C
compatibility).

--
Erik Wikström

[Richard Heathfield]

Erik Wikström said:

On 2007-07-19 17:39, 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
automattical ly make the total structure 1024 bytes without having to
manually count the bytes of the other members myself.

You could do something like this:

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

And it might even work. But it might not. It depends on the
implementation.

(Notice also the lack of typedef, it's not needed unless you require C
compatibility).

It's not actually needed in C either, although admittedly its omission
does lead to wordier code.

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

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

On 2007-07-19 17:58, Erik Wikström wrote:

On 2007-07-19 17:39, 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.

You could do something like this:

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

As Mr. Heathfield pointed out, due to padding this might not work and
you could do as he said. However that's quite ugly, casting ranges of
memory to whatever you need. An alternative would be a union:
#include <iostream>

struct MyStruct {
int var1;
int var2;
int var3;
};

union MyUnion {
char buff[1024];
MyStruct theStruct;
};

int main()
{
MyUnion u;
u.theStruct.var 1 = 1;
std::cout << sizeof(MyUnion) ;
}

--
Erik Wikström

[LR]

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?

LR

[Bruce.]

"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
automattical ly 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.

Bruce.

[Bruce.]

"Richard Heathfield" <rj*@see.sig.in validwrote in message
news:Dq******** *************** *******@bt.com. ..

Bruce. said:

>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
automattical ly make the total structure 1024 bytes without having to
manually count the bytes of the other members myself.

Is that possible in c++?

Not portably. Implementations may insert arbitrary padding after any
structure member. In a pathological implementation, you might have to
give var4 a negative number of elements (which is of course
impossible).

In this case, portability is not a requirement and the compiler packing is
fixed a 1. I'm using MS Developer Studio C++ 2003. Does that change your
answer at all? Thanks.

Bruce.

[Richard Heathfield]

Bruce. said:

<snip>

>
In this case, portability is not a requirement

Then you're probably better off in a newsgroup devoted to your
implementation, where implementation-specific expertise can be brought
to bear on your problem.

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

[Bruce.]

"Richard Heathfield" <rj*@see.sig.in validwrote in message
news:rq******** *************@b t.com...

Then you're probably better off in a newsgroup devoted to your
implementation, where implementation-specific expertise can be brought
to bear on your problem.

If I change it to this:

#pragma pack( 1 )

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

Now do you know of a portable way to accomplish what I want?

Bruce.