Probable Bug in tcp.h

[Marc Lörner]

On Thursday 05 June 2008 17:56, Rui Paulo wrote:
> On Thu, Jun 05, 2008 at 05:12:47PM +0200, =?ISO-8859-1?Q?Marc_L=F6rner_ 
wrote:
> > Hello,
> > I probably found a bug in declaration of "struct tcphdr"!
> >
> > struct tcphdr {
> > 	u_short	th_sport;		/* source port */
> > 	u_short	th_dport;		/* destination port */
> > 	tcp_seq	th_seq;			/* sequence number */
> > 	tcp_seq	th_ack;			/* acknowledgement number */
> > #if BYTE_ORDER == LITTLE_ENDIAN
> > 	u_int	th_x2:4,		/* (unused) */						<---here
> > 		th_off:4;		/* data offset */						<---
> > #endif
> > #if BYTE_ORDER == BIG_ENDIAN
> > 	u_int	th_off:4,		/* data offset */
> > 		th_x2:4;		/* (unused) */
> > #endif
> > 	u_char	th_flags;
> >
> > First of all I have the problam of misalignment of th_off. Because in
> > this way always 4 bytes are read and the the bits of th_off are replaced.
> > Then the 4 bytes are written back.
> >
> > But should (th_x and th_off) not only be 1 byte in whole -> only read and
> > write 1 byte?
> >
> > I think if this was changed, my misalignment problems would go away!
>
> I'm not sure what you mean.
>
> Please supply more information, like:
> 1) Are you running on little endian? Or big endian?

I'm on itanium-architecture, therefore I can run big and little endian. But 
for now it is little endian.

> 2) th_x2 + th_off are 1 byte in size. What do you mean?

th_x2 and th_off are created as a bitfield. But C-Standard says that bitfields 
are accessed as integers => 4-bytes

On itanium integers are read with ld4-command but the address of th_x2/th_off 
may not be aligned to 4-bytes => we get an unaligned reference fault.

If we'd change to 1 byte-accesses => I won't get any misaligned faults 
anymore.


Hope this makes my dilemma a bit clearer,
Marc