C Preprocessor Directives (#include, #define, Macros)

Introduction

The C preprocessor runs before compilation. It handles #include, #define, macro expansion and conditional compilation. Every .c file you write already uses the preprocessor when you write #include <stdio.h>.

Common Directives

Directive	Purpose
#include	Insert header file contents
#define	Create macros and constants
#undef	Remove a macro
#ifdef / #ifndef / #endif	Conditional compilation
#if / #elif / #else	Numeric/string conditions

#include

#include <stdio.h>   /* system header — search standard paths */
#include "mylib.h"   /* local header — search current directory first */

#define Constants and Macros

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

int main(void) {
    printf("PI = %f\n", PI);
    printf("MAX(4, 9) = %d\n", MAX(4, 9));
    return 0;
}

Sample Output

PI = 3.141592
MAX(4, 9) = 9

Function-like Macros — Parentheses Matter

Always parenthesize macro parameters to avoid operator-precedence bugs:

#define SQUARE(x) ((x) * (x))   /* correct */
/* #define SQUARE(x) x * x      wrong for SQUARE(1 + 2) */

Conditional Compilation

#define DEBUG 1

#if DEBUG
    #define LOG(msg) printf("DEBUG: %s\n", msg)
#else
    #define LOG(msg)
#endif

Best Practices

• Prefer const variables over #define for typed constants when debugging matters.
• Keep macros short; use inline functions (C99+) for complex logic.
• Never put statements with side effects inside macro arguments unless you understand expansion rules.
• Use include guards in headers: #ifndef MY_HEADER_H / #define MY_HEADER_H / #endif.

Common Mistakes

• Forgetting parentheses in macro definitions.
• Using #define for long multi-line logic — hard to debug.
• Including the same header repeatedly without guards (duplicate symbol errors).

Related Pages

Continue learning with these related tutorials and programs:

• C Tutorials — Browse all C Tutorials.
• C Array — Concept hub — start with arrays before pointers.
• C Strings in C Programming — More in c concepts.
• C Functions — More in c concepts.