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How does a Compiler Work? That is the Question.

c compiler


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:

On a Linux based environment, a common compiler is called gcc for C or g++ for C++ programs. On a Windows based environment, MinGW is a common compiler to use.

The Compiler Works in 4 Stages

Most executable programs are produced in these 4 stages of compilation.

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”.


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