Re: Any pointer to Byte Alignment & Structure Padding?

["Amit Dang" <amit_dang@intersolutions.stpn.soft.net>]

Hi Vadiraj,
    Thanks for the explaination but when i try following structure
struct temp
 {
        char c;   /* 1 byte lenght */
        int i;      /* 4 byte length */
        char c1; /* 1 byte length */
        long long d /* 8 bytes  lenght */
 };
on a linux machine x86 32-bit with gcc 2.96. It gives its size = 20 bytes
not 24 bytes (as explained by you)

Regards,
Amit Dang

----- Original Message ----- 
From: "Vadiraj" <vadiraj.cs@gmail.com>
To: "Amit Dang" <amit_dang@intersolutions.stpn.soft.net>
Sent: Monday, August 01, 2005 5:44 PM
Subject: Re: Any pointer to Byte Alignment & Structure Padding?


> 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