Re: structure size and structure element offset calculation issue

[Sivaprasad <sivaprasad dot pv at redpinesignals dot com>]

Jonathan Wakely,


I have a confusion about structure size calculation.

I have tried to compared  structure size result with
gcc version 4.7.2 20121109 (Red Hat 4.7.2-8)  generates code for x86 machine
and
cross compiler with gcc version 4.3.4  generate code for custom processor.

struct sample
{
  short int s;
}a;

sizeof(a) showing 2 bytes with x86 gcc and 4 bytes with cross compiler.

struct sample
{
  int i;
  short int s;
}a;

sizeof(a) showing 8 bytes with x86 gcc and 8 bytes with cross compiler.

When compiler will add hole at end of the structure ?
why cross compiler trying to make structure size multiple of 4 ?


Thanks,
Siva prasad



On 05/19/2014 07:14 PM, Jonathan Wakely wrote:
> On 19 May 2014 13:28, Sivaprasad wrote:
> > Hi Jonathan Wakely,
> >
> > Thanks for quick reply.
> >
> > Structure is included from same header file,So there is no difference in
> > structure.
>
> You can't conclude that. Defining a macro before including the header
> could alter the definition of the structure. A #pragma to turn on
> packed structs that comes before the header could alter the
> definition.
>
> You need to compare the preprocessed output for the two files.
>
>
>
> > How to get information related to alignments from preprocessor
> > output.Is there any additional options I need to give apart from "-E".
>
> You can't get that information from the preprocessor, you need to work
> it out by hand.