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06 Intermediate Product — Relocatable Object File (.o)

1. Definition and Essence

The Relocatable Object File (suffix .o on Linux, .obj on Windows) is the output of the Assembly Stage.

  • Nature: It contains binary machine code that the CPU can understand.
  • Limitation: It is not executable. It is a "half-finished" module.

2. Why is it called "Relocatable"?

The term "Relocatable" refers to the fact that the addresses inside this file are temporary and floating.

  • Zero-Based Addressing: In an .o file, all code and data addresses start from 0x0000. This is a relative offset, not a valid physical or virtual memory address.
  • The Relocation Process: Since multiple .o files cannot all sit at address 0x0000 in the final program, the Linker will later "relocate" (move) this code to a specific, unique memory address.

3. Standard File Formats

Although binary, these files follow strict structural standards known as Object File Formats: * ELF (Executable and Linkable Format): The standard for Linux and Unix systems. * PE (Portable Executable): The standard for Windows (often derived from COFF (Common Object File Format),  the ancestor of modern object file formats, originally used in Unix System V.).

4. Internal Anatomy

An object file consists of several key sections. It is not just code; it contains metadata essential for linking. [[Memory Allocation 内存分配 | About memory allocation]]

Machine Instructions (.text Section)

  • Contains the actual compiled binary code (logic).
  • State: The instructions are valid, but the operands (addresses for jumps or variable access) are often placeholders.

What exactly is in .text?
It strictly contains Executable Instructions (Functions).

  • Does it contain Variables? No. Global/Static variables live in .data (if initialized) or .bss (if uninitialized). Local variables live on the Stack or Registers at runtime.

  • Does it contain Literals? Partially.

    • String Literals (e.g., "Hello") usually go to a separate Read-Only Data section (.rodata).

    • Immediate Values (small numbers like int a = 5;) are embedded directly into the instruction stream within .text.

Data (.data Section)

  • Contains initialized global and static variables (e.g., int global_var = 100;).

Symbol Table

This is the "inventory list" of the object file. It categorizes symbols into two types: 1. Exported (Defined) Symbols: Functions or variables defined in this file (e.g., main, my_func) that are available for other modules to use. 2. Imported (Undefined) Symbols: Functions or variables used in this file but defined elsewhere (e.g., printf, sin). The file is asking the linker: "Where are these?"

Relocation Table

This is the "instruction manual" for the linker. It lists exactly where in the .text section the code needs to be patched. * Content: "At offset 0x20 in the .text section, there is a call to printf. Please replace the 00 00 00 00 placeholder with the actual address of printf once you know it."

5. Visual Example: The Placeholder

Consider a C statement: call_func();

In the Relocatable Object File (.o)

  • Assembly: call 0x0
  • Machine Code: E8 00 00 00 00
    • E8: The Opcode for "CALL".
    • 00 00 00 00: A placeholder (32-bit offset) because the compiler does not know where call_func is located.
  • Relocation Entry: A note exists pointing to this 00 sequence, tagging it for the symbol call_func.

In the Final Executable

  • Action: The Linker calculates the distance between the call site and the actual function address.
  • Machine Code: E8 25 01 00 00
    • The placeholder has been replaced by the correct relative offset.