Structure member details from object symbol table - c

Question

Trying to use objdump command in linux to display symbol table information in executable.
i have tried a simple program below.
#include<stdio.h>
int global = 0;
typedef struct global_struct{
int a;
int c;
}global_struct;
global_struct gs;
int main()
{
printf("%d\n",global);
printf("%d\n",gs.a);
return 0;
}
compiled with -g option in gcc compiler
The output of objdump looks like
00000000004005b0 l F .text 0000000000000000 __do_global_ctors_aux
0000000000000000 l df *ABS* 0000000000000000 symboltable.c
0000000000600870 l O .got.plt 0000000000000000 _GLOBAL_OFFSET_TABLE_
00000000006006ac l .ctors 0000000000000000 __init_array_end
00000000006006ac l .ctors 0000000000000000 __init_array_start
00000000006006d8 l O .dynamic 0000000000000000 _DYNAMIC
0000000000600898 w .data 0000000000000000 data_start
00000000006008b4 g O .bss 0000000000000008 gs
0000000000000000 F *UND* 0000000000000000 printf##GLIBC_2.2.5
0000000000400510 g F .text 0000000000000002 __libc_csu_fini
00000000004003e0 g F .text 0000000000000000 _start
0000000000000000 w *UND* 0000000000000000 __gmon_start__
0000000000000000 w *UND* 0000000000000000 _Jv_RegisterClasses
00000000004005e8 g F .fini 0000000000000000 _fini
0000000000000000 F *UND* 0000000000000000 __libc_start_main##GLIBC_2.2
00000000006008b0 g O .bss 0000000000000004 global
00000000004005f8 g O .rodata 0000000000000004 _IO_stdin_used
0000000000600898 g .data 0000000000000000 __data_start
0000000000400600 g O .rodata 0000000000000000 .hidden __dso_handle
00000000006006c8 g O .dtors 0000000000000000 .hidden __DTOR_END__
0000000000400520 g F .text 0000000000000089 __libc_csu_init
000000000060089c g *ABS* 0000000000000000 __bss_start
00000000006008c0 g *ABS* 0000000000000000 _end
My requirement is gs being C structure, I want to know the data members of gs{a,b}. How can i know structure member details from object files. Thanks for your support

Answer 1

Structure member details are not detailed in the object file. The object file will only have enough memory allocated to hold the struct and an offset telling the linker where to find it.
The compiler knows at compile time at which offsets from the base struct pointer to find the members and hard codes those into the program text on each use.

Answer 2

If you passed -g to gcc when building the program, it should have DWARF debug information compiled in:
$ objdump -t prog | grep debug
0000000000000000 l d .debug_aranges 0000000000000000 .debug_aranges
0000000000000000 l d .debug_info 0000000000000000 .debug_info
0000000000000000 l d .debug_abbrev 0000000000000000 .debug_abbrev
0000000000000000 l d .debug_line 0000000000000000 .debug_line
0000000000000000 l d .debug_str 0000000000000000 .debug_str
Then you can read it with objdump:
$ objdump --dwarf=info prog
[...]
<1><65>: Abbrev Number: 5 (DW_TAG_base_type)
<66> DW_AT_byte_size : 4
<67> DW_AT_encoding : 5 (signed)
<68> DW_AT_name : int
[...]
<1><2f8>: Abbrev Number: 8 (DW_TAG_structure_type)
<2f9> DW_AT_name : (indirect string, offset: 0x22f): global_struct
<2fd> DW_AT_byte_size : 8
<2fe> DW_AT_decl_file : 1
<2ff> DW_AT_decl_line : 3
<300> DW_AT_decl_column : 16
<301> DW_AT_sibling : <0x31c>
<2><305>: Abbrev Number: 17 (DW_TAG_member)
<306> DW_AT_name : a
<308> DW_AT_decl_file : 1
<309> DW_AT_decl_line : 4
<30a> DW_AT_decl_column : 9
<30b> DW_AT_type : <0x65>
<30f> DW_AT_data_member_location: 0
<2><310>: Abbrev Number: 17 (DW_TAG_member)
<311> DW_AT_name : c
<313> DW_AT_decl_file : 1
<314> DW_AT_decl_line : 5
<315> DW_AT_decl_column : 9
<316> DW_AT_type : <0x65>
<31a> DW_AT_data_member_location: 4
<2><31b>: Abbrev Number: 0
<1><31c>: Abbrev Number: 2 (DW_TAG_typedef)
<31d> DW_AT_name : (indirect string, offset: 0x22f): global_struct
<321> DW_AT_decl_file : 1
<322> DW_AT_decl_line : 6
<323> DW_AT_decl_column : 2
<324> DW_AT_type : <0x2f8>
[...]
Here we can see that global_struct's DIE (Debug Information Entry) has two leaves (two DW_TAG_members, look at <2> prefixes which I believe it level in the tree). The members are called a and c. Both of the members reference type 0x65, which is defined above as a signed int.
A good place to learn more about DWARF is this official tutorial: http://www.dwarfstd.org/doc/Debugging%20using%20DWARF-2012.pdf