Header and Source Files

As C programs grow, keeping everything in one .c file becomes hard to read and slow to compile.

C lets you split a project into multiple files: each handles one concern. This improves organization, lets you reuse code, and means changing one part doesn't force you to recompile everything.

A modular C project uses two file types.

Source files (.c) contain the actual implementation: function bodies and definitions.

Header files (.h) contain declarations: function prototypes, type definitions, and macros that other files need to know about.