On Mon, 2007-02-12 at 14:34 +0100, Oleg Verych wrote:

quoted

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.