a.out (Assembler Output Format)

a.out is one of the earliest binary file formats used on Unix systems for executables, object code, and core dumps. While largely obsolete today, it provides a historical foundation for understanding binary file formats. Here's a breakdown of what the a.out header contains and how you can find where the code begins:

Structure of the a.out Header

The a.out header typically contains the following fields:

Magic Number:
- Identifies the file as an a.out executable.
- Common values:
  - 0x0107: Normal executable.
  - 0x0108: Read-only text segment.
  - 0x010B: Demand-paged executable.
Header Fields:
- a_text (4 bytes):
  - Size of the text (code) segment.
- a_data (4 bytes):
  - Size of the initialized data segment.
- a_bss (4 bytes):
  - Size of the uninitialized data segment (BSS).
- a_syms (4 bytes):
  - Size of the symbol table.
- a_entry (4 bytes):
  - Entry point address where execution begins.
- a_trsize (4 bytes):
  - Size of the text relocation table.
- a_drsize (4 bytes):
  - Size of the data relocation table.

Sections in an a.out File

An a.out file is divided into several segments:

Text Segment:
- Contains the executable code.
Data Segment:
- Contains initialized variables and constants.
BSS Segment:
- Contains uninitialized variables.
Symbol Table:
- Stores symbols for debugging and linking.
Relocation Tables:
- Used to adjust addresses in the text and data segments during linking or loading.

Finding Where the Code Begins

To determine where the code starts in an a.out file:

Check the Entry Point (a_entry):
- The a_entry field in the header specifies the address where execution begins.
Locate the Text Segment:
- The text segment starts immediately after the header.
- Offset of the text segment in the file can be calculated as:
  text_offset = sizeof(a.out header)
Disassemble the Code:
- Use tools like objdump or gdb to disassemble the binary and verify the code.

Commands to Analyze a.out Files

objdump -f <file>: Displays the file header, including entry point and segment sizes.
objdump -d <file>: Disassembles the text (code) segment.
nm <file>: Lists symbols in the file.
gdb <file>: Debugs the a.out binary, allowing inspection of the header and code.

Example

# Display header information
objdump -f example

# Disassemble to find the entry point
objdump -d example

# List symbols
nm example

These tools can help you analyze the a.out structure and understand how the binary is organized and executed.

Last modified: 19 January 2025