Topic 11 of 15 · Advanced

Preprocessor & Macros

#define, macros, include guards and conditional compilation.

Concept

The preprocessor runs before compilation and does text substitution. #define creates constants and macros.

Function-like macros don't check types and can evaluate arguments twice — always wrap parameters and the whole body in parentheses.

Include guards (#ifndef/#define/#endif) or #pragma once stop a header being included twice. #ifdef enables debug-only code.

Code (C17)

#ifndef UTILS_H
#define UTILS_H

#define PI 3.14159
#define SQUARE(x) ((x) * (x))
#define MAX(a, b) ((a) > (b) ? (a) : (b))

#ifdef DEBUG
  #define LOG(msg) printf("[debug] %s\n", msg)
#else
  #define LOG(msg)
#endif

#endif
/* SQUARE(i++) still increments twice - prefer inline functions */

Remember

Parenthesise macro parameters.
Macros have no type safety.
Use include guards in every header.

Interview questions

Macro vs inline function?

Macros are text substitution without type checks and may evaluate args twice; inline functions are type-checked real functions the compiler may inline.

Output of #define SQ(x) x*x; SQ(2+3)?

2+3*2+3 = 11, not 25 — because the parameter wasn't parenthesised.

Practise C with the AI mentor

Lesson test

Take an 8-question test on “Preprocessor & Macros” — concepts, output prediction, bug spotting and interview questions.