RE: "sizeof" get unwanted result

[<john dot gill at quadrics dot com>]

Alignment is the key here as previously mentioned.
Using the packed attribute that gcc offers will force
the compiler to use the minimum space like so.

typedef struct a
{
    long data1 __attribute__ ((packed));
    unsigned short data2 __attribute__ ((packed));
    long data3 __attribute__ ((packed));

} A ;

This may be what you
want, but it does have side effects. Take your struct and 
look at the address data3 will sit at. It will always be
on a 2 byte boundary (assuming ia32 arch). This may cause
performance issues when accessing the misaligned 
member - on other architechures misaligned accesses 
are not supported by the cpu and you'll receive a 
bus error for your trouble. 

You can rearrange the members of the struct so each member
is on the correct alignment like this:

typedef struct b
{
    long data1 __attribute__ ((packed));
    long data3 __attribute__ ((packed));
    unsigned short data2 __attribute__ ((packed));
} B;

But again, this comes with caveats - if you create
an array of these structs then the alignment of subsequent
structs will be changed.



-----Original Message-----
From: Kaul, Martin [mailto:martin.kaul@leuze.de]
Sent: 19 April 2005 07:42
To: gcc-help@gcc.gnu.org
Subject: AW: "sizeof" get unwanted result




> -----Ursprüngliche Nachricht-----
> Von: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org]Im
> Auftrag von Xiaoshan Sun
> Gesendet: Dienstag, 19. April 2005 04:48
> An: gcc-help@gcc.gnu.org
> Betreff: "sizeof" get unwanted result
> 
> 
> hi,
>    i use "sizeof" to get the size of a struct ,defined as:
>    typedef struct _A {
>      long data1;
>      unsigned short data2;
>      long  data3;
>    }A;
>  i guess the sizeof(A) should be 10 .however when i compile 
> and print the
> value of "sizeof(A)" ,the reult is 12!!
>  so where's the extra 2 bytes? how can i get the gcc to calculate the
> desired size ??
> an aligned issue??
On a 32bit machine the compiler align each element on a 32bit even address. So between data2 and data3 there be 2 Byte filldata. On a 64bit machine the compiler align addresses to 64bit so he fill with 6 Bytes. 

To avoid use the packed attribute as proposed by Kevin.

regards
  Martin