Hi Rohit,
"__attribute__((__aligned__(32)))" tells the compiler to align
whatever data type it is an attribute for, to a 32 byte boundary i.e the
data type will start from an address that is an exact multiple of 32.
In the first example
static struct s ss[32] __attribute__((__aligned__(32))) ;
says, align array 'ss' on a 32 byte boundary, so the compiler rightly
aligns it on a 32 byte boundary (80494e0 is an exact multiple of 32).
The compiler was never asked to align each element of the array, so the
next element rightly starts 8 byte after the start of the array.
In the 2nd example by putting the "__attribute__((__aligned__(32)))" at
the structure definition, you are asking the compiler to align any
instantiation of the structure to a 32 byte boundary, so all the array
elements are aligned on a 32 byte boundary.
try this example
struct s {
void (*action) (struct s *);
int *data;
}__attribute__((__aligned__(32)));
int main()
{
int k;
static struct s ss[32] __attribute__((__aligned__(64))) ;
printf("first %x second %x \n",&ss[0],&ss[1]);
}
Now the compiler will align array 'ss' on a 64 byte boundary, and then
each element on a 32 byte boundary.
Hope that helps.
tomar
On Wed, 27 Aug 2003, Rohit wrote:
> Hi,
> In softirq.c, statement
> static struct softirq_action softirq_vec[32]
> __cacheline_aligned;
>
> I am trying to figure out why this is cacheline_alligned.
> I wrote a small piece of code to see how this may work ....
>
> struct s {
> void (*action) (struct s *);
> int *data;
> };
>
> int main()
> {
> int k;
> static struct s ss[32] __attribute__((__aligned__(32))) ;
> printf("first %x second %x \n",&ss[0],&ss[1]);
> }
>
> on executing this I get
> >>>>first 80494e0 second 80494e8.
> Which means the two elements of ss[] are 8 bytes apart. (ie sizeof
> struct s)
>
> I achive the 32 byte alignment by modifing the code as below.
> struct s {
> void (*action) (struct s *);
> int *data;
> }__attribute__((__aligned__(32))) ;
>
> and declaring
> static struct s ss[32];
>
> on executing I get
> >>>>first 80494e0 second 8049500
> ie 2 elemnts are 32 byte apart.
>
> Please let me know what I am missing here.
>
> Regards,
> Rohit.
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