[ILUG] gcc compile options

[David Neary]

RenWei wrote:
> 
> hi ,
>  i want to know how "align" works in gcc .
> i use this in my program, run in a X86 box, RedHat 6.2 full installed:
> 
> int a __attribute__ ((aligned(8)));
> 
> printf("size of a : %d \n", sizeof(a));
> 
> i want it's size be 8 bytes, by i get 4.
> anyone can tell me how to use align in detail?

The size of an int will always be 4 bytes. However, if you were to
define a struct with one integer element, I believe that the size of the
struct would be 8. align affects the amount of padding space set aside
in memory after the end of that variable, so if a is at address 0x0, the
next variable will be stored (at earliest) in 0x8. But the int is still
of size 4. If you want ints that are 8 bytes long, you should probably
use long.

As an aside, is an int always the same size as a memory address (ie. 4
bytes on 32 bit machines, 8 bytes on 64 bit), or is it always 4 bytes?

>     thanks
>              renwei.

Cheers,
Dave.