Constants

C has two different ways to define a constant, and they behave quite differently. const creates a real, typed variable the compiler prevents you from reassigning:

const int MAX_USERS = 100;

#define is a preprocessor macro — pure text substitution, happening before the compiler even sees types at all:

#define MAX_USERS 100

Prefer const for most cases — it's type-checked and visible to a debugger; reach for #define mainly for values needed across multiple files, or true compile-time constants.

Common mistakes

• Adding a semicolon to a #define: #define MAX_USERS 100; — the semicolon becomes part of the substitution, which can produce confusing errors wherever MAX_USERS is then used.