On Mon, 2007-02-12 at 14:34 +0100, Oleg Verych wrote:
> > I'd like my own, private "asm-offsets.h". In this case, in
> > arch/i386/lguest/. I guess it's a matter of extracting the core of the
> > asm-offsets.h magic and generalizing it.
> >
> > Have a good break!
> > Rusty.
>
> If you will have time for newbie, to explain in a few words, what is it need
> for (whole idea, or key detail), and, maybe, why it is generated so ... interestingly:
>
> asm-offsets.c -> *.s -> *.h
> (but this looks like interconnecting C and assembler, obviously)
Hi Oleg,
Always happy to explain. There's often a need to access constants in
assembler, which can only be derived from C, such as the size of a
structure, or the offset of a certain member within a structure.
Hardcoding the numbers in assembler is fragile leading to breakage when
something changes.
So, asm-offsets.c is the solution: it uses asm() statements to emit
patterns in the assembler, with the compiler computing the actual
numbers, eg:
#define DEFINE(sym, val) \
asm volatile("\n->" #sym " %0 " #val : : "i" (val))
DEFINE(SIZEOF_FOOBAR, sizeof(foobar));
Becomes in asm-offsets.s:
->SIZEOF_FOOBAR $10 sizeof(foobar) #
This gets sed'd back into asm-offsets.h:
#define SIZEOF_FOOBAR 10 /* SIZEOF_FOOBAR # */
This can be included from .S files (which get passed through the
pre-processor).
Hope that helps!
Rusty.
