instructing gcc for alignment

[Megatron Eye <megatroneye at gmail dot com>]

Hi,

I seem not to be able to properly instruct gcc for producing struct
bytes aligned the way I expect.
I am using gcc 4.7.0, from the mingw32-64 distribution under Windows
7. My machine is a little-endian x86_64.

First let me state my expectations.

I have a memory region with these physical contents:

450002cf9fe5000040115a9fc0a8fe...

And I have a pointer to the struct defined below initialized to the
beginning of that region (the byte with contents "45" above).

typedef struct ip4 {
  unsigned int ihl :4;
  unsigned int version :4;
  uint8_t tos;
  uint16_t tot_len;
  uint16_t id;
  uint16_t frag_off; // flags=3 bits, offset=13 bits
  uint8_t ttl;
  uint8_t protocol;
  uint16_t check;
  uint32_t saddr;
  uint32_t daddr;
  /*The options start here. */
} ip4_t;

Now, gdb shows the following situation in runtime:

ip4:                   address=0x8da36ce
ip4->ihl:              address=0x8da36ce, value=0x5
ip4->version:          address=0x8da36ce, value=0x4
ip4->tos:              address=0x8da36d2, value=0x9f
ip4->tot_len:          address=0x8da36d4, value=0x0

But my expectations (basing on how the 32-bit-compiled version of this
code behaves) are:

ip4->tos:              address=0x8da36cf, value=0x0
ip4->tot_len:          address=0x8da36d0, value=0x02cf.

It seems gcc aligns the ihl and version bitfields to a 4-byte
boundary. That causes subsequent fields to be wrongly mapped. My goal
is to configure the padding/aligment so that the physical size of the
structure will be in agreement with the logical size.

I have tried the following options:

* -fpack-struct
* The aligned attribute
* The packed attribute
* The pragma pack (1)
* Combinations of the above

With no visible effects so far.

I will appreciate any hints on this issue.

Best regards,

D.