Date and Version

I started writing this paper on the 26th of August 2003. The kernel code quoted herein refers to version 2.4.22. The binutils code comes from binutils-1.4.

Notation

All kernel files that are referenced in this paper are specifed by a path name relative to the kernel's root directory. For example the setup.c file for the PowerPC architecture would be given as: arch/ppc/kernel/setup.c. A specific function (e.g. early_init) within a given file is expressed as arch/ppc/kernel/setup.c:early_init() regardless of what parameters it accepts (if any) and what it returns (if anything).

Architecture

As far as I know, this mechanism is not architecture-dependent. I actually found it while tracing through the boot process of the PowerPC architecture, but my code tests were performed on an x86-based machine. The code that makes this work is in the kernel's init/ directory, which (as is my understanding), contains initialization code which is used by all architectures. The ability to use this mechanism depends more on specific support given by the GNU tools and the ELF executable format rather than architecture-specific support.

Tools

Peering into the world of object files is made easier using the tools: * objdump * nm and * readelf

"objdump -t" output format

I often dump the symbols of a file using objdump -t. I can't seem to easily locate any documentation on the output format so I've included some quick notes here. A typical use of this tool would look something like the following:

[trevor]$ objdump -t add.o      

add.o:     file format elf32-i386

SYMBOL TABLE:
00000000 l    df *ABS*     00000000 add.c
00000000 l    d  .text     00000000 
00000000 l    d  .data     00000000 
00000000 l    d  .bss      00000000 
00000000 l    d  .comment  00000000 
00000000 g     F .text     0000000b add

Here is my understanding of the column descriptions. This information comes from browsing through bfd/syms.c:bfd_print_symbol_vandf() (where "vandf" stands for "value and flags").

attachment:object-t_output.png