[9fans] am I nuts? does 8c support packed structs?

[9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

consider this: 

struct a {
	ulong x;
	long y;
	short z;
	};

in GCC, you can add attribute packed and that struct will be 10 bytes. I'm 
getting some static from the Xen guys because IIRC 8c does not support 
packed structs -- that struct on 8c is 12 bytes. 

Am I nuts? Is there some 8c pragma that can force this to be packed?

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[Russ Cox]

#pragma hjdicks on

> struct a {
>         ulong x;
>         long y;
>         short z;
>         };

#pragma hjdicks off

Re: [9fans] am I nuts? does 8c support packed structs?

[C H Forsyth]

[-- Attachment #1: Type: text/plain, Size: 785 bytes --]

hj were only dicks on the mips it seems, but it's easy to add it to 8c.

diff /n/dump/2004/1022/sys/src/cmd/8c/swt.c /sys/src/cmd/8c/swt.c
589a590,591
> 		if(hjdickflg)
> 			w = hjdickflg;
597a600,601
> 		if(hjdickflg)
> 			w = hjdickflg;

t.c:

#include <u.h>
#pragma hjdicks dick
struct a {
	ulong x;
	long y;
	short z;
	};
#pragma hjdicks off
static int xxx = sizeof(struct a);
static struct a xx[2];
static int xxsize = sizeof(xx);

8c -S t.c

	DATA	xxx<>+0(SB)/4,$10
	DATA	xxsize<>+0(SB)/4,$20
	GLOBL	xxsize<>+0(SB),$4
	GLOBL	xx<>+0(SB),$20
	GLOBL	xxx<>+0(SB),$4

perhaps i should add "xen" as a synonym for "dick"
"on" will work too, if you'd like to be less rude.

also,
#pragma hjdicks xN
for some integer N sets the padding/rounding to N

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From: Russ Cox <russcox@gmail.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] am I nuts? does 8c support packed structs?
Date: Fri, 22 Oct 2004 11:30:21 -0400
Message-ID: <ee9e417a0410220830359e8a7a@mail.gmail.com>

#pragma hjdicks on

> struct a {
>         ulong x;
>         long y;
>         short z;
>         };

#pragma hjdicks off

Re: [9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

On Fri, 22 Oct 2004, C H Forsyth wrote:

> hj were only dicks on the mips it seems, but it's easy to add it to 8c.

but it's not there now, right? 

and I have to know, what's the whole hj thing? 

Is the name hjdicks a pejorative way of making a comment about packed 
structs :-)

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[Russ Cox]

I put padded structs into all the compilers.  The pragma is

#pragma pack on

(or pack yes) to enable it, and pack off
(or anything but on and yes) to disable it.
#pragma hjdicks will now be ignored like
any other unknown pragma.

The name hjdicks was a reflection of Ken's frustration 
at having to put the feature into the compiler
at all.  It's use (and the use of #pragma pack) is
greatly deprecated.

Russ

Re: [9fans] am I nuts? does 8c support packed structs?

[C H Forsyth]

>>but it's not there now, right? 

if you apply the diffs i gave in the email to the version of /sys/src/cmd/8c/swt.c on sources, against
which i had diffed, and recompile your copy, you will (should) then have it.

>>and I have to know, what's the whole hj thing? 

it's somewhere in the 9fans archive, but H&J (Harris & Jeffries i think) did protocol implementations
that relied on that effect extensively. oh here it is:

	...
	From: "Bruce Ellis" <brucee@chunder.com>
	Date: Wed, 7 May 2003 22:47:40 +1000

	pragmatism and a silly discussion over dinner.
	hj were dicks for assuming that they could
	lay out structs in an architecture independant way.
	we had to live with it.

Re: [9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

On Fri, 22 Oct 2004, C H Forsyth wrote:

> hj were only dicks on the mips it seems, but it's easy to add it to 8c.
> 

Here's a test of dicks on 8c today:


#include <u.h>
#include <libc.h>

#pragma hjdicks on
struct a {unsigned long x; char c; };
#pragma hjdicks off

struct a a;

main(){
	print("size %d\n", sizeof(a));
}


This yields 8, with dicks on or off.

In gcc, for this:

struct a {unsigned long x; char c; } __attribute__ ((packed));

struct a *a = 0;

main(){
        printf("size %d, addr %p\n", sizeof(*a), &a->c);
}

you get a size of 5. 


"How do you create TCP headers without packed structs?" is one question I
got. I guess Plan 9 is quite possibly the "here's how" except of course
everyone did this before packed structs (or even gcc, or even before C
compilers that aligned structs and struct members for that matter -- was
there a C compiler before gcc? one wonders. The mind boggles).

Unless there's another 8c pragma I have not heard of, I'll stuck to 
marshalling data by hand.

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

On Fri, 22 Oct 2004, Russ Cox wrote:

> #pragma pack on

no change in that program I sent. Still prints 8 for the size. 
Interesting, maybe the feature rotted away due to it being unloved :-)

No big deal, I'm going to ignore it. I come down on the side of not using 
it anyway, and not worrying about what gcc and linux guys thing. 

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[Russ Cox]

> was there a C compiler before gcc?

is there a C compiler now besides gcc?

> Unless there's another 8c pragma I have not heard of, I'll stuck to
> marshalling data by hand.

you should do this anyway.

Re: [9fans] am I nuts? does 8c support packed structs?

[jmk]

On Fri Oct 22 13:21:11 EDT 2004, rminnich@lanl.gov wrote:
> ...
> "How do you create TCP headers without packed structs?" is one question I
> got. I guess Plan 9 is quite possibly the "here's how" except of course
> everyone did this before packed structs (or even gcc, or even before C
> compilers that aligned structs and struct members for that matter -- was
> there a C compiler before gcc? one wonders. The mind boggles).
> 
> Unless there's another 8c pragma I have not heard of, I'll stuck to 
> marshalling data by hand.
> 
> ron

Next you'll tell me they use bitfields in packed structs.

Re: [9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

On Fri, 22 Oct 2004, Russ Cox wrote:

> you should do this anyway.


agreed. Now if I could convince the xen guys, but that's another story.

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

On Fri, 22 Oct 2004 jmk@plan9.bell-labs.com wrote:

> Next you'll tell me they use bitfields in packed structs.


hey man, I'll keep you posted.

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[Brantley Coile]

At a prior job I was in charge of the embedded kernel for a piece of
telco gear.  Another group was writing the application and doing so on
Sun Motorola boxes.  I warned them that my kernel would be aligning
structures to 32 bits, not the 16 bits that Sun OS was forced to use
because of all the SunOS structures that assumed they overlayed
protocol headers.  My warning had the effect of causing the
application developers to write a bunch of code to marshal things into
and out of their inter-machine messages.  But somehow they still
managed to screw it up and had to spend two weeks fixing their code
that some how still depended on 16=bit structure alignment.

I assume Sun fixed their code when the Sparc arrived.

  Brantley

Re: [9fans] am I nuts? does 8c support packed structs?

[Charles Forsyth]

>>This yields 8, with dicks on or off.

i get 5 with the changes i posted (which should be the same
as russ's updates, except for the change of name).

Re: [9fans] am I nuts? does 8c support packed structs?

[C H Forsyth]

> Unless there's another 8c pragma I have not heard of, I'll stuck to 
> marshalling data by hand.

perhaps the pickling code might help, though if it's only a little
i shouldn't bother.

Re: [9fans] am I nuts? does 8c support packed structs?

[boyd, rounin]

> "How do you create TCP headers without packed structs?" is one question I
> got.

iirc tcp is pretty much 16 bit aligned.  failing that you can do 32 bit tricks.

Re: [9fans] am I nuts? does 8c support packed structs?

[Ronald G. Minnich]

On Fri, 22 Oct 2004, boyd, rounin wrote:

> iirc tcp is pretty much 16 bit aligned.  failing that you can do 32 bit tricks.

yeah but the point the person was trying to make, which surprised me, was
that in this New Era it is seen to be impossible to create correct tcp
headers without __packed__.

ron

Re: [9fans] am I nuts? does 8c support packed structs?

[boyd, rounin]

> yeah but the point the person was trying to make, which surprised me, was
> that in this New Era it is seen to be impossible to create correct tcp
> headers without __packed__.

who cares?  we have 64 bit processors now :(

idiots.

Re: [9fans] am I nuts? does 8c support packed structs?

[Charles Forsyth]

>hj weren't the most egregious dicks, but yeah it was a hack
>(that saved months of work).

it's actually a reasonable example of the convenience of having fairly
complete control of the tools!

Re: [9fans] am I nuts? does 8c support packed structs?

[Bruce Ellis]

hj weren't the most egregious dicks, but yeah it was a hack
(that saved months of work).  just don't tell jon stewart
about them!  i was particularly impressed by some protocol
code which not only used packed structs with glee but managed
to byte swap headers twice, as it had been endian ported
twice by different monkeys.  babel-fish this!

brucee