objdump command

 

objdump (Object Dump)

Purpose

The objdump command in Linux displays detailed information about object files and executables.
It allows you to inspect:

• Machine code (assembly) generated by the compiler

• Headers and sections of binary files

• Symbol tables and debugging information

It’s extremely useful for learning:

• What happens after compilation

• How the assembler and linker organize executables

• How to read assembly code and understand binary structure

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Basic Syntax

objdump [options] filename

Example:

objdump -d a.out

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🧠 What It Works On

• .o files (object files produced by the compiler before linking)

• Executables (like a.out or your compiled programs)

• Static libraries (.a)

• Dynamic libraries (.so)

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1. Disassemble an Executable

objdump -d a.out

This shows the assembly code generated from your C source after compilation.

Example Output:

a.out:     file format elf64-x86-64

Disassembly of section .text:

0000000000001139 <main>:
    1139:   55                      push   %rbp
    113a:   48 89 e5                mov    %rsp,%rbp
    113d:   b8 00 00 00 00          mov    $0x0,%eax
    1142:   5d                      pop    %rbp
    1143:   c3                      ret

🧩 Explanation:

• The section .text contains the program’s machine instructions.

• Each line shows:

  • Address in memory

  • Hex code (machine code)

  • Assembly instruction

This helps students visualize how C code is translated into assembly.

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2. Display All Headers

objdump -x a.out

Shows all header information, including:

• ELF file type

• Entry point address

• Program headers

• Section headers

• Linked libraries

This output helps understand how executables are structured.

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3. Display Section Headers

objdump -h a.out

Example Output:

Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         0000013a  0000000000401139  0000000000401139  00001139  2**2
  1 .data         00000010  0000000000601040  0000000000601040  00001040  2**3
  2 .bss          00000008  0000000000601050  0000000000601050  00001050  2**3
  3 .comment      0000002a  0000000000000000  0000000000000000  00001058  2**0

🧩 Explanation:

• .text → executable code

• .data → initialized global/static data

• .bss → uninitialized data

• .comment → compiler information

This gives students insight into how memory is divided in a program’s binary.

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4. Display Symbol Table

objdump -t a.out

Lists functions and global variables known to the program.

Example Output:

SYMBOL TABLE:
0000000000401139 l    df *ABS*  0000000000000000 main.c
0000000000401139 g     F .text  0000000000000013 main
0000000000601040 g     O .data  0000000000000004 global_var

🧩 Explanation:

• F = function

• O = object (variable)

• .text, .data → section names where they are located

• Address shows where each symbol is stored in memory

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5. Show Only the Assembly of a Specific Function

objdump -d --disassemble=main a.out

This restricts disassembly output to the main function — useful for classroom demos.

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6. Show Source Code Along with Assembly (if compiled with -g)

objdump -S a.out

This displays C source code interleaved with assembly, which is excellent for teaching how high-level code maps to machine instructions.

Example:

int main() {
=>  1139:   55                      push   %rbp
    113a:   48 89 e5                mov    %rsp,%rbp
    printf("Hello");
=>  113d:   b8 00 00 00 00          mov    $0x0,%eax
}

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7. Show File Format

objdump -f a.out

Displays:

• File format (ELF, PE, etc.)

• Architecture (x86-64, ARM, etc.)

• Entry point address

Example Output:

a.out:     file format elf64-x86-64
architecture: i386:x86-64, flags 0x00000112:
EXEC_P, HAS_SYMS, D_PAGED
start address 0x0000000000401139

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Commonly Used Options Summary

Option	Description
-d	Disassemble executable code
-S	Show source code along with assembly
-t	Show symbol table
-h	Show section headers
-x	Show all headers
-f	Show file format info
--disassemble=function	Disassemble only a given function

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Simple Lab Exercise

Aim:

To study the use of objdump command for analyzing executable files.

Procedure:

1. Write and compile a simple program:

   #include <stdio.h>
   int main() {
       printf("Operating Systems Lab\n");
       return 0;
   }
   

2. Compile with debug symbols:

   gcc -g hello.c -o hello
   

3. Use these commands and observe the outputs:

   objdump -f hello
   objdump -h hello
   objdump -t hello
   objdump -S hello
   

4. Note the section names, symbol addresses, and assembly instructions.

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Learning Outcomes

Students will:

• Understand how a C program is translated into machine code.

• Learn how executables are organized internally (sections, headers, symbols).

• Connect the compilation and execution process with OS concepts like memory layout and process loading.