lkml.org 
[lkml]   [2009]   [Nov]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectEasier way to generate stuff like asm-offsets.
From
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 ...


\
 
 \ /
  Last update: 2009-11-21 01:59    [W:0.200 / U:0.532 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site