How to stop gcc padding structs???

[Grant Edwards <grante at visi dot com>]

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!

In the following example, gcc insists that each of the "high"
structs occupies four bytes despite my putting a "packed"
attribute evryplace that doesn't generate a syntax warning.

$ arm-elf-gcc -v   
Reading specs from /usr/local/lib/gcc-lib/arm-elf/2.95.2/specs
gcc version 2.95.2 19991024 (release)


typedef struct
{
  volatile unsigned char data __attribute__((packed));
  volatile unsigned char _xxx __attribute__((packed));
} high __attribute((packed));

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

tDemo foo;

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

testit:     file format elf32-bigarm
Disassembly of section .data:
00008000 <addr>:
    8000:	00008010
    8004:	00008010
    8008:	00008014
    800c:	00008018

-- 
Grant Edwards
grante@visi.com