Symbol Table understanding and reading

[linuxlittle]

Hi ,

I am ok in c programming but poor in internals of C.

the things like

internals of the compiler ( lexical analyzer , hashtables, parsers , symbol tables etc..)

please if any one has some idea or any links please share me .


especially i need the information of symbol table and contents of the symbol table how to view the symbol table for a function or a program .

i am working on linux so i am posting the query here .

i am very thank full to you .

Thanks,
Little

[GNU-Fan]

The nm program shows the symbols of a given object/executable file.

[linuxlittle]

i just tried to check the symbol table by using foll files .


I am using the following files

Head_Symb.h

#define LV 20
extern int Glb_Var;
extern int fun( char *p , char a[]);

Lib_Symb.c

#include "Head_Symb.h"
int fun( char *p , char a[])
{
        static int Stc_Var  ;
        int Lcl_Var2;

        printf("Ptr: = %s\n", p);
        printf("Arr: = %s\n", a);
        printf(" Glb_Var = %d \n", Glb_Var);
        return 0;

}

Main_Symb.c

#include "Head_Symb.h"
#include<stdio.h>
int Glb_Var = 10;
int main( void )
{
        int Local_Var = LV;
        char *Ptr = "pointer";
        char Arr[]= "array";
        fun ( Ptr , Arr);
return 0;
}

i am using following commands

gcc -c *.c
Lib_Symb.o and Main_Symb.o are generated

when i use the command
nm Lib_Symb.o
the following are listed :

00000000 T fun
                 U Glb_Var
                 U printf
00000000 b Stc_Var.0

and

when i use the command

nm Main_Symb.o

                 U fun
00000000 D Glb_Var
00000000 T main

gcc *c

and when i use the command
nm
or
nm a.out

i got following list

08049628 A __bss_start
080482e4 t call_gmon_start
08049628 b completed.1
08049528 d __CTOR_END__
08049524 d __CTOR_LIST__
08049618 D __data_start
08049618 W data_start
080484a4 t __do_global_ctors_aux
08048308 t __do_global_dtors_aux
0804961c D __dso_handle
08049530 d __DTOR_END__
0804952c d __DTOR_LIST__
08049538 D _DYNAMIC
08049628 A _edata
08049630 A _end
080484c8 T _fini
08049524 A __fini_array_end
08049524 A __fini_array_start
080484e4 R _fp_hw
0804833c t frame_dummy
08048520 r __FRAME_END__
08048368 T fun
08049624 D Glb_Var
08049604 D _GLOBAL_OFFSET_TABLE_
         w __gmon_start__
08048278 T _init
08049524 A __init_array_end
08049524 A __init_array_start
080484e8 R _IO_stdin_used
08049534 d __JCR_END__
08049534 d __JCR_LIST__
         w _Jv_RegisterClasses
08048460 T __libc_csu_fini
0804840c T __libc_csu_init
         U __libc_start_main@@GLIBC_2.0
080483b4 T main
08049620 d p.0
08049524 A __preinit_array_end
08049524 A __preinit_array_start
         U printf@@GLIBC_2.0
080482c0 T _start
0804962c b Stc_Var.0

the things i dont understand here is :

1) why the command nm Lib_Symb.o and Main_Symb.o are not having any address refereence, all address are zero there.

2) many new symbols __init_array_end , __init_array_start __preinit_array_end
__preinit_array_start

3) Does this have any relation with the assembly file

4) is there any relation between addresses
in the command nm a.out
because
there are different things that are specifeid at some address
ex : at
08049628 A __bss_start
08049628 b completed.1
08049628 A _edata

what all these specifies .
i understood little bit from the command nm.


but overall i dint understand completely what actually a symbol table of .0 files (Lib_Symb.o and Main_Symb.o ) consists of and .a.out consists of .

please just give a breif discussion or links

Thanks
Little

[GNU-Fan]

I am no expert either, so be skeptical in anything I say.

1) why the command nm Lib_Symb.o and Main_Symb.o are not having any address refereence, all address are zero there.

They do have. Pay attention to the characters. As the manual says, they refer to different sections. A section is kind of a reference frame. The address 0 is to be understood relative to that section. 0 just means "at the start". Introduce a second static variable in Lib_Symb.c and see what happens.

2) many new symbols __init_array_end , __init_array_start __preinit_array_end
__preinit_array_start

These are "operating system stuff". Your program isn't runnable on its own. It needs some init & cleanup work. For example, the command line parameters have to be put in the array argv so that your main function can read them. More details are to be found in the manual for the linker LD (from GNU binutils) and the C runtime library (the GNU libc).

3) Does this have any relation with the assembly file

Yes. Compile with the -S option to obtain the assembler code. You will find the different sections annotated there already.

4) is there any relation between addresses
in the command nm a.out
because
there are different things that are specifeid at some address
ex : at
08049628 A __bss_start
08049628 b completed.1
08049628 A _edata

what all these specifies .

It might mean that the bss section is of lenght 0. So _edata gets the same starting point.

but overall i dint understand completely what actually a symbol table of .0 files (Lib_Symb.o and Main_Symb.o ) consists of and .a.out consists of .

Prosaic, the object files annotate what they will need in order to work and how the variables should be named. The linker later does the job to provide them with these variables and tells where to find them in the final executable.