Any pointer to Byte Alignment & Structure Padding?

[Vadiraj <vadiraj.cs@gmail.com>]

On 8/1/05, Amit Dang <amit_dang@intersolutions.stpn.soft.net> wrote:
> Hi,
>    Can any body provide some light on Byte Alignment & Structure Padding
> for gcc linux x86 32-bit?

 The system expects the address of a variable to be multiple of
its size. Meaning for 32 bit x86 int being 4 bytes. The address
location of a int variable is expected to be at multiple of 4.
ex 0 4 8 12 16. if  its double then its expected it to be multiple of 8.
0 8 16 ...

 In case of structure allignment... this is achieved by padding.
if this is the structure
struct temp
{
      char c;   /* 1 byte lenght */
      int i;      /* 4 byte length */
      char c1; /* 1 byte length */
      long long d /* 8 bytes  lenght */
};

 c starts at offset x( x is assured 4 byte alligned by gcc), i should
start at x+4 as it has to be multiple of 4 3 bytes of padding will be
done by gcc.
c1 starts at x+9, no padding is required char is 1 byte.
d starts at x+16,7 bytes of padding to get multiple of 8.

It would differ if you re arrange the struct like this.
struct temp
{
      char c;   /* 1 byte lenght */
      int i;      /* 4 byte length */
      long long d /* 8 bytes  lenght */
      char c1;
};

for same base offset...i will be from x+4 d would start from x+8,
there would be no padding for d  and c1 at x+16.

I hope it helps.
--
cheers,
Vadi