C structures and gas

C structures and gas

[Corey Holcomb-Hockin]

What is the best way to access C structures with gas assembly?  I 
haven't gotten any good ideas from looking at assembly generated from C 
code.  It looks like it keeps track of offsets and the size of 
structures on its own.  I would have a hard time keeping track of 
offsets and changing them if the structure changed.

Corey Holcomb-Hockin

Re: C structures and gas

[Scott Lanning]

On Mon, 11 Nov 2002, Corey Holcomb-Hockin wrote:
> What is the best way to access C structures with gas assembly?  I
> haven't gotten any good ideas from looking at assembly generated from C
> code.  It looks like it keeps track of offsets and the size of
> structures on its own.  I would have a hard time keeping track of
> offsets and changing them if the structure changed.

Yes. The way I did it, I use constants and "chain" them
one after another. The more constants the better (replace even
the numbers 0, 2, 4 below with constants). For example,

SIZEOF_SA = 16

SA = -SIZEOF_SA
SA_SIN_FAMILY = SA + 0
SA_SIN_PORT = SA_SIN_FAMILY + 2
# etc...

# always start the next variable declared on the stack based on
# the beginning of the previous one
SOCKET_ARGS = SA - 3*4
SOCKET_ARG0 = SOCKET_ARGS
SOCKET_ARG1 = SOCKET_ARG0 + 4
# etc..

# set STACK_TOP to the last variable
STACK_TOP = SOCKET_ARGS

# data here...

_start:
        // setup stack...
        pushl %ebp
        movl %esp, %ebp
        addl $STACK_TOP, %esp

# ...

Re: C structures and gas

[Randall Hyde]

Well, the best way to do it in Gas is to use HLA and have
HLA generate the Gas code for you :-)
Seriously, though, Gas doesn't have much in the way of
abstract data typing capabilities.  If you want to move
beyond the minimal features available in Gas or NASM,
you're a prime candidate to learn assembly under HLA
(the High Level Assembler).  Not only does HLA support
records (structures), unions, and classes, but it provides
some very flexible options for controlling the alignment of
objects within the structures so that your assembly structs
can be made compatible with just about any compiler
out there (C or otherwise).
You can check out HLA at http://webster.cs.ucr.edu
Randy Hyde
----- Original Message ----- 
From: "Corey Holcomb-Hockin" <crsntlvs@ipeg.com>
To: <linux-assembly@vger.kernel.org>
Sent: Monday, November 11, 2002 4:43 PM
Subject: C structures and gas


> What is the best way to access C structures with gas assembly?  I 
> haven't gotten any good ideas from looking at assembly generated from C 
> code.  It looks like it keeps track of offsets and the size of 
> structures on its own.  I would have a hard time keeping track of 
> offsets and changing them if the structure changed.
> 
> Corey Holcomb-Hockin
> 
> -
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Re: C structures and gas

[Christian Hildner]

Corey Holcomb-Hockin schrieb:

> What is the best way to access C structures with gas assembly?  I
> haven't gotten any good ideas from looking at assembly generated from C
> code.  It looks like it keeps track of offsets and the size of
> structures on its own.  I would have a hard time keeping track of
> offsets and changing them if the structure changed.
>
> Corey Holcomb-Hockin
>

Have a look how linux does this in the ia64 tree. See
/usr/src/linux/arch/ia64/tools/print_offset.c.
There are three steps:

1. compile print_offsets
2. run print_offsets (creates offsets.h)
3. include offsets.h into *.S

Christian