=== Date and Version ===
The original version of this paper was started on the 26th of August 2003; the kernel code quoted refered to version 2.4.22 and the binutils utility came from binutils-1.4. This document was rechecked on the 11th of October, 2006. At that time small modifications were made to the text and all output and program examples were re-written. The kernel being used was 2.6.18, binutils 2.15.94.0.2.2, gcc 4.0.0.

=== 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

The flags which are described above are part of a larger set of symbols and attributes which are defined in `bfd/bfd.h`. The entire set of flags (or attributes) and their meanings are given below. '''NOTE:  `objdump -t` doesn't try to display the values of all the possible flags, just the ones mentioned above.'''

||'''Definition'''|| ||'''Symbol'''||'''Description'''||
||0x00000|| ||BSF_NO_FLAGS||placeholder for no defined flags||
||0x00001|| ||BSF_LOCAL||The symbol has local scope (i.e. a static in C). VALUE(1) is this symbol's offset into the data section.||
||0x00002|| ||BSF_GLOBAL||The symbol has global scope (i.e. initialized data in C). VALUE(2) is this symbol's offset into the data section.||
||BSF_GLOBAL|| ||BSF_EXPORT||This symbol has global scope and is exported. Same as BSF_GLOBAL.||
||0x00008|| ||BSF_DEBUGGING||The symbol is a debugging record. The VALUEs are arbitrary, unless BSF_DEBUGGING_RELOC is set.||
||0x00010||ELF||BSF_FUNCTION||Function entry point.||
||0x00020|| ||BSF_KEEP||used by the linker||
||0x00040|| ||BSF_KEEP_G||used by the linker||
||0x00080|| ||BSF_WEAK||Weak global symbol. This symbol is overridable (without warning) by a regular global symbol of the same name.||
||0x00100||ELF||BSF_SECTION_SYM||This symbol points to a section.||
||0x00200|| ||BSF_OLD_COMMON||This symbol used to be *COM*, but is now allocated.||
||0x00400||COFF||BSF_NOT_AT_END||This symbol appears where it is declared and not at the end of a section.||
||0x00800|| ||BSF_CONSTRUCTOR||This symbol indicates the start of the constructor section.||
||0x01000|| ||BSF_WARNING||The presence of this symbol acts to indicate that there is a warning on the next symbol.||
||0x02000|| ||BSF_INDIRECT||This symbol is an indirect pointer to the symbol with the same name as the next symbol.||
||0x04000||ELF||BSF_FILE||This symbol contains a filename.||
||0x08000||ELF||BSF_DYNAMIC||This symbol is associated with dynamic linking.||
||0x10000||ELF||BSF_OBJECT||This symbol denotes a data object.||
||0x20000|| ||BSF_DEBUGGING_RELOC||This is a debugging symbol. VALUE(1) is the offset into the data section. BSF_DEBUGGING should be set too.||
||0x40000||ELF||BSF_THREAD_LOCAL||This symbol is used for thread local storage.||