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1. Writing Source Code

You begin by writing human-readable source code in a high-level language such as C:

int main() { return 0; }

This file (e.g., main.c) contains symbols, keywords, and logic understandable by humans, not machines.

2. Preprocessing

The preprocessor handles all lines starting with #, including:

  • Expanding header files (#include)

  • Replacing macros (#define)

  • Removing comments

  • Processing conditional compilation (#ifdef, #ifndef)

The output is still C code, but now fully expanded.

3. Compilation

The compiler translates the preprocessed C code into assembly language.
Important steps here:

  • Syntax analysis

  • Semantic analysis

  • Optimization

  • Generating assembly code (e.g., .s file)

This step maps your high-level constructs into low-level instructions.

4. Assembly

The assembler converts assembly code into machine code, producing an object file:

  • .o on Linux/macOS

  • .obj on Windows

This file contains binary instructions, but the program is still incomplete because external functions are unresolved.

5. Linking

The linker combines multiple object files and libraries into a complete executable:

  • Resolves external function references (e.g., printf)

  • Merges code and data sections

  • Performs relocations

  • Produces the final binary:

    • a.out or custom output on Linux

    • .exe on Windows

Static and dynamic linking may both occur.

6. Loading

When you run the executable, the operating system loader performs:

  • Creating a new process

  • Allocating virtual address space

  • Loading program segments into memory

  • Setting up stack, heap, and global data regions

  • Mapping dynamic libraries through the dynamic linker

  • Preparing CPU registers

  • Jumping to the entry point (_start)

At this point, the program is ready to execute.

7. Execution

Execution begins at _start, then proceeds through:

  1. Runtime environment initialization (usually from libc)

  2. Calling main()

  3. Running your program logic

  4. Returning the exit code

  5. Cleanup by the OS after the program terminates

The CPU executes the machine instructions step by step.

📌 Summary Diagram (Text Version)*

Source Code (.c)
       ↓ 
Preprocessing
       ↓ 
Compilation (→ Assembly)
       ↓
Assembly (→ Object File)
       ↓ 
Linking (→ Executable)
       ↓ 
Loading (OS loads the program)
       ↓ 
Execution (CPU runs instructions)