Re: How to stop gcc padding structs???

[Grant Edwards <grante at visi dot com>]

On Mon, Jan 29, 2001 at 10:26:40AM +0000, Richard Earnshaw wrote:

> > I'm again fighting with gcc trying (and failing) to get it to
> > stop putting padding bytes into structs.  Has anybody figured
> > out how to prevent gcc from padding structs?
> > 
> > I ran into this problem before and gave up, finally having to
> > use hand-calculated constants instead of "sizeof (struct foo)"
> > in numerous places.  For example, it's impossible to define an
> > Ethernet header structure that ends up having a size of 14
> > bytes!
> 
> Relying on sizeof to do this is *very* non-portable.

It's a device driver for a specific board.  It doesn't really
need to be portable.

> Try:
> 
> typedef struct
> {
>   volatile unsigned char data;
>   volatile unsigned char _xxx;
> } __attribute((packed)) high;

Several people suggested that, but it didn't make any
difference:

======================================================================
typedef struct
{
  volatile unsigned char data __attribute__((packed));
  volatile unsigned char _pad __attribute__((packed));
}  __attribute__((packed)) high  __attribute__((packed)) ;

typedef struct
{
  high  a __attribute__((packed));
  high  b __attribute__((packed));
  high  c __attribute__((packed));
}  __attribute__((packed)) tDemo  __attribute__((packed)) ;

tDemo foo;

void *addr[] =
{
  &foo,
    &foo.a,
    &foo.b,
    &foo.c
};

int sizeofHigh = sizeof (high);
======================================================================

testit:     file format elf32-bigarm
Disassembly of section .data:

00008000 <addr>:
    8000:	00008018 	andeq	r8, r0, r8, lsl r0
    8004:	00008018 	andeq	r8, r0, r8, lsl r0
    8008:	0000801c 	andeq	r8, r0, r12, lsl r0
    800c:	00008020 	andeq	r8, r0, r0, lsr #32

00008010 <sizeofHigh>:
    8010:	00000004 	andeq	r0, r0, r4

-- 
Grant Edwards
grante@visi.com