How Does a Compiler Work?

Table of Contents

Introduction

How does a compiler work? That is the question. A Compiler’s main task if to turn your source code into an executable file that computer understands. To achieve this, a compiler has to undergo several stages to build the final executable. The picture below describes difference compilation stages:

Preprocessor Stage

The first stage in the compilation process.
Expand preprocessor directives starting with a ‘#’ symbol and include commands like #include, #define, and #ifdef. See more details about preprocessor clauses here.
Performs text substitutions, file inclusions, and conditional compilation…etc
Removes comments, resolves macros, and expands header files included via #include.
The output of the preprocessor is an expanded source code file.

Compilation Stage

The compiler takes the preprocessed source code as input and translates it into assembly code or an intermediate representation.
It performs lexical analysis (tokenizing), syntax analysis (parsing), and semantic analysis (type checking).
The compiler generates machine-independent assembly code.
Some compilers may perform optimizations at this stage to improve the efficiency of the generated code.
The output of this stage is typically in the form of assembly code (which is still human readable).

Assembly Stage

Responsible for translating assembly code into machine code. (This is what a CPU understands).
It performs assembly-level optimizations, such as instruction scheduling for a particular CPU type.
The assembler generates an object file containing the machine code and information about program sections (data, text, etc.).
The object file is typically in a (.o) format on Unix-based operating system.

Linker Stage

The final stage of compilation, responsible for combining multiple object files and libraries into a single executable program.
It resolves references between different object files and ensures that functions and variables declared in one file are correctly linked to their definitions in other files.
The linker also resolves dependencies on external libraries and system functions.
It produces the final executable binary or shared library.
Additionally, the linker can perform optimizations and generate debug information for a debugger to trace. See more about using a debugger like GDB here.

Quick Summary

See below picture for a quick summary that answers question: “how does a compiler work”.

The Makefile Basics