On 8/2/05, Amit Dang <[EMAIL PROTECTED]> wrote:
> Do you mean that max byte alignment on a 32-bit machine is 4 bytes? If we
> move to 64-bit machine will the size of this structure be 24 bytes?
32-bit machines' maximum alignment is 4 bytes and 64-bit machines
align up to 8 bytes. This does not mean that all integral types are
padded to 8-byte boundaries. Yes, the size of the structure will
change to 24 bytes, because the most efficient offset for the long
member is at byte 16 and not byte 12. It is very likely that a
padding of 4 bytes will be inserted after the 3rd structure member.
As a result the size of the structure will increase to 24 bytes.
On IA64 long and long long, both signed and unsigned, are 8-byte
aligned, while int is 4-byte, short 2-byte and char 1-byte aligned.
Nevertheless, padding is needed for short and char to align to 4-byte
boundaries. Therefore the size of the structure would not change,
since the member of type long is 4-byte aligned already, but
unfortunately its layout in memory is very inefficient. The
increasing size of the structure in this particular case is not a
result of the alignment itself but of the compiler's efforts to
optimize memory layout.
Normally a structure is also padded at the end in order to make the
size a multiple of the alignment if needed. Rules for alignment are
compiler and platform specific.
Regards
\Steve
> ----- Original Message -----
> From: "wwp" <[EMAIL PROTECTED]>
> To: <[email protected]>
> Sent: Monday, August 01, 2005 7:41 PM
> Subject: Re: Any pointer to Byte Alignment & Structure Padding?
>
>
> > Hello Amit,
> >
> >
> > On Mon, 1 Aug 2005 17:57:48 +0530 "Amit Dang"
> > <[EMAIL PROTECTED]> wrote:
> >
> > > 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)
> >
> > See below (this applies to both members sorting examples from Vadiraj):
> >
> > struct padded offset
> > member size size range
> > -------------------------------
> > c 1 4 0-3
> > i 4 4 4-7
> > c1 1 4 8-11
> > d 8 8 12-19
> >
> > So, 20 bytes. Isn't it right?
> >
> >
> > Regards,
> >
> > > ----- Original Message -----
> > > From: "Vadiraj" <[EMAIL PROTECTED]>
> > > To: "Amit Dang" <[EMAIL PROTECTED]>
> > > Sent: Monday, August 01, 2005 5:44 PM
> > > Subject: Re: Any pointer to Byte Alignment & Structure Padding?
> > >
> > >
> > > > On 8/1/05, Amit Dang <[EMAIL PROTECTED]> 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
> > >
> > > -
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> > >
> >
> >
> > --
> > wwp
> >
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