linux-kernel - Easier way to generate stuff like asm-offsets.

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Date:	Fri, 20 Nov 2009 16:56:14 -0800
From:	Kaz Kylheku <kkylheku@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Easier way to generate stuff like asm-offsets.

Hey all,

You can use declarations and the ``nm'' utility to discover sizes of
types, structure offsets and stuff like that.

There is no need to grep material from the assembly code itself.

I'm looking into this as a technique for to use in ./configure scripts
when a program is being cross-compiled.

Then I remembered that the Linux kernel does something to pull out
structure member offsets
for use in assembly code. How are they doing that, I wondered? I took
a look and saw that it
was done with a complicated hack.

Here is the simple trick:

  /* file test.c */

  #include <stddef.h>  /* for offsetof macro */

  struct foo { char padding[43]; int member; };

  char offsetof_foo_member[offsetof(struct foo, member)];

Now we compile test.c:

$ /path/to/toolchain/bin/arch-prefix-gcc test.c -c

Then we can use the ``nm'' tool (the toolchain should have binutils in it):

$ /path/to/toolchain/bin/arch-prefix-nm -t d -P foo.c   # decimal
output, posix format
offsetof_foo_member C 00000044 00000044

There we go, 44 bytes.  Simple.  This is almost an #include file;
we just have to do some simple filtering, such as:

$ /path/to/toolchain/bin/arch-prefix-nm -t d -P test.o | awk '{ print
"#define", $1, $4 }'
#define offsetof_foo_member 00000044

:)

Cheers ...
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