Unit II — Linkers & Loaders

After a program is compiled, it goes through two more stages before it can run: linking and loading. These are critical steps in converting source code into a running process.

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The Software Translation Pipeline

Source Code (.c)
      ↓  [Compiler / Assembler]
Object Code (.obj / .o)
      ↓  [Linker]
Executable File (.exe / a.out)
      ↓  [Loader]
Running Process (in RAM)

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Linker

A linker (also called link editor) combines one or more object files produced by the compiler/assembler and resolves references between them into a single executable file.

What Does a Linker Do?

1. Symbol Resolution — Matches function calls in one module with their definitions in another (e.g., printf defined in C standard library).
2. Relocation — Assigns actual memory addresses to symbols.
3. Library Linking — Incorporates library code into the executable.

Types of Linking

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Loader

A loader is part of the operating system that takes an executable file from disk and loads it into main memory (RAM) for execution.

Functions of a Loader

1. Allocation — Allocates sufficient memory for the program.
2. Loading — Copies the program's machine code and data from disk to RAM.
3. Relocation — Adjusts addresses in the program to reflect its actual position in memory.
4. Linking — (In dynamic loading) Loads required shared libraries.
5. Initialization — Sets up the program counter (PC) to the program's entry point and transfers control to the OS.

Types of Loaders

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Linker vs Loader

Key Takeaway: The linker merges compiled object files into a runnable executable, resolving all cross-module references. The loader brings that executable into RAM and hands control to the CPU. Together they complete the journey from source code to a running program.