RE: [lwip-users] Struct packing/alignment problems

[Mountifield, Tony]

Is it possible to override this behaviour, at the expense of less efficient 
code, by using the GCC command-line option -fpack-struct ?

I had to do a similar thing with the ST20 compiler I'm using: tell it not to 
pad or align structures to multiples of 4 bytes, but only to the minimum 
alignment requirements of its strictest member.

Cheers,
Tony

> -----Original Message-----
> From: Timmy Brolin [mailto:address@hidden
> Sent: 06 May 2004 01:09
> To: Mailing list for lwIP users
> Subject: Re: [lwip-users] Struct packing/alignment problems
> 
> 
> I just ran nested struct tests on gcc+arm. And unfourtunately, it did 
> not turn out very well.
> gcc+arm does indeed pad structs so that nested structs align 
> to a 4 byte 
> boundary. Also, it pads structs at the end to ensure the 
> struct size is 
> a factor of 4.
> This causes problems in both etharp and eth_hdr.
> The eth_addr struct is nested in the etharp struct and the eth_hdr 
> struct. Size of the eth_addr struct is obviously 6, so this 
> struct will 
> be padded to 8 by gcc.
> In etharp there is also the nested "sipaddr" struct. This 
> nested struct 
> is not on a 4 byte boundary, so there will be 2 bytes of 
> padding before it.
> 
> I see two solutions.
> 1: Remove the nested ip_addr2 and eth_addr structs, and use 
> the arrays 
> directly.
> 2: Put the PACK_STRUCT directives back in etharp .
> 
> I can see a potential problem with the second solution if 
> there is a ARM 
> compiler out there who does not support pack directives.
> I guess the decision is up to you Leon?
> 
> Stupid thing of me not to consider the nested structs in the 
> first place.
> Timmy
> 
> Leon Woestenberg wrote:
> 
> > Hello Jani,
> >
> > could you please test run the program below to see how the nested 
> > struct is aligned under arm-gcc?
> >
> >>>
> >> Actually, I think you are on to something here... Someone 
> said that 
> >> gcc for arm puts all structs on 4 byte boundarys in order 
> to use the 
> >> efficient load and store multiple instructions... I have not 
> >> considered this. It would surely break the etharp struct.
> >
> >
> > This piece of code should show the actual alignments under GCC when 
> > using nested structs.
> >
> > #include <stdio.h>
> > #include <stddef.h>
> >
> > #if 1
> > #define PACKED __attribute__ ((packed))
> > #else
> > #define PACKED
> > #endif
> >
> > struct A {
> >     u16_t x[2];
> > } PACKED;
> >
> > struct B {
> >     u16_t p;
> >     struct A q;
> >     u16_t r;
> > } PACKED;
> >
> > int main ()
> > {
> >         printf ("offsetof(p) = %d\n", offsetof(p));
> >         printf ("offsetof(q.x[0]) = %d\n", offsetof(q.x[0]));
> >         printf ("offsetof(q.x[1]) = %d\n", offsetof(q.x[1]));
> >         printf ("offsetof(r) = %d\n", offsetof(r));
> >         printf ("sizeof(struct B) = %d\n", sizeof(struct B));
> >         return 0;
> > }
> >
> > /*
> > Hopefully:
> >
> > offsetof(p) is 0
> > offsetof(q.x[0]) is 2
> > offsetof(q.x[1]) is 4
> > offsetof(r) is 6
> > sizeof(struct B) is 8
> > */
> >
> >
> >
> > Thanks, Leon.
> >
> >
> > _______________________________________________
> > lwip-users mailing list
> > address@hidden
> > http://mail.gnu.org/mailman/listinfo/lwip-users
> >
> >
> 
> 
> 
> _______________________________________________
> lwip-users mailing list
> address@hidden
> http://mail.gnu.org/mailman/listinfo/lwip-users
> 


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