Messages in this thread | | | Date | Fri, 20 Nov 2009 16:56:14 -0800 | Subject | Easier way to generate stuff like asm-offsets. | From | Kaz Kylheku <> |
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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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