understanding a.out files (assembler's output) -- their structure

**jzacsh** · January 25th, 2010

Hey, I'm new to programming (taking my second class on C++) and I love it.

Where I stand in knowledge:
I've taken a course in assembly language (we talked about Microsoft Assembler). I'm reading www.linuxfromscratch.org -- this lead me to: http://www.tldp.org/HOWTO/Software-Building-HOWTO.html and some of the stuff I'm reading is new to me.

Can you recommend reading on...
So now that article's talk about ELF vs a.out file formats got me thinking about -- what exactly is in those files? Is it really all zeros and ones and just wouldn't look like anything to me? Can't be, because then where would strings I type in `cout' statements go??

I tried:

Code:

$ echo '#include<iostream>
using namespace std;

int main(){
cout << "Hello world" << endl;
} ' > ./sample.cpp
$ g++ ./sample.cpp
$ less ./a.out | strings | less
"./a.out" may be a binary file.  See it anyway? y

/lib/ld-linux.so.2
_UR[y=
CyIk
libstdc++.so.6
__gmon_start__
_Jv_RegisterClasses
_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
_ZSt4cout
_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
_ZNSt8ios_base4InitC1Ev
_ZNSt8ios_base4InitD1Ev
_ZNSolsEPFRSoS_E
__gxx_personality_v0
libm.so.6
libgcc_s.so.1
libc.so.6
_IO_stdin_used
__cxa_atexit
__libc_start_main
CXXABI_1.3
GLIBCXX_3.4
GLIBC_2.0
GLIBC_2.1.3
PTRhp
[^_]
Hello world         <-- aha! I knew it!
GCC: (Ubuntu 4.4.1-4ubuntu8) 4.4.1
GCC: (Ubuntu 4.4.1-4ubuntu9) 4.4.1
_IO_stdin_used
../sysdeps/i386/elf/start.S
/build/buildd/eglibc-2.10.1/csu
GNU AS 2.20
../sysdeps/i386/elf
start.S
3!4=%"
 YZ!"\[
init.c
/build/buildd/eglibc-2.10.1/csu
short unsigned int
GNU C 4.4.1
short int
_IO_stdin_used
long long unsigned int
unsigned char
init.c
long long int
.symtab
.strtab
.shstrtab
.interp
.note.ABI-tag
.note.gnu.build-id
.gnu.hash
.dynsym
.dynstr
.gnu.version
.gnu.version_r
.rel.dyn
.rel.plt
.init
.text
.fini
.rodata
.eh_frame_hdr
.eh_frame
.ctors
.dtors
.jcr
.dynamic
.got
.got.plt
.data
.bss
.comment
.debug_aranges
.debug_pubnames
.debug_info
.debug_abbrev
.debug_line
.debug_str
init.c
crtstuff.c
__CTOR_LIST__
__DTOR_LIST__
__JCR_LIST__
__do_global_dtors_aux
completed.6990
dtor_idx.6992
frame_dummy
__CTOR_END__
__FRAME_END__
__JCR_END__
__do_global_ctors_aux
sample.cpp
_ZStL8__ioinit
_Z41__static_initialization_and_destruction_0ii
_GLOBAL__I_main
_GLOBAL_OFFSET_TABLE_
__init_array_end
__init_array_start
_DYNAMIC
data_start
__cxa_atexit@@GLIBC_2.1.3
__libc_csu_fini
_start
__gmon_start__
_Jv_RegisterClasses
_fp_hw
_fini
_ZNSt8ios_base4InitC1Ev@@GLIBCXX_3.4
__libc_start_main@@GLIBC_2.0
_ZNSt8ios_base4InitD1Ev@@GLIBCXX_3.4
_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc@@GLIBCXX_3.4
_IO_stdin_used
__data_start
_ZSt4cout@@GLIBCXX_3.4
__dso_handle
__DTOR_END__
__libc_csu_init
__bss_start
_end
_ZNSolsEPFRSoS_E@@GLIBCXX_3.4
_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_@@GLIBCXX_3.4
_edata
__gxx_personality_v0@@CXXABI_1.3
__i686.get_pc_thunk.bx
main
_init

I think that ^ is *really*, really interesting! Can anyone with more knowledge suggest where I can read more about the structure of the assemblers' output files and the more advanced details about linking, etc.?

Thanks

**Simian Man** · January 25th, 2010

If you are interested in this kind of thing, the book to read is "Linkers and Loaders" by John R. Levine. It is pretty much *the* book in this area.

BTW that's an ELF file, not an a.out one. The a.out format is deprecated, though the name is kept.