Re: How to stop gcc padding structs???

[Richard Earnshaw <rearnsha at arm 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
> 


Hmm, it looks to me like you are using a very old compiler (I fixed this 
donkeys ages ago -- probably before egcs tree forked).

Here's what I get with current CVS:

@ Generated by gcc 2.97 20010125 (experimental) for ARM/elf
        .file   "pack.c"
.gcc2_compiled.:
        .global addr
        .data
        .align  2
        .type   addr,object
        .size   addr,16
addr:
        .word   foo
        .word   foo
        .word   foo+2
        .word   foo+4
        .global sizeofHigh
        .align  2
        .type   sizeofHigh,object
        .size   sizeofHigh,4
sizeofHigh:
        .word   2
        .comm   foo, 8  @ 6

BTW splitting the struct declaration from the typedef should still work, 
even on the old compiler.  hence

struct foo {
  char a;
  char b;
} __attribute((packed));

typedef struct foo bar;

R.