Preprocessor Directives in C

The C Preprocessor processes source code before compilation. Directives start with # and are not C statements (no semicolon needed).

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1. #include

Includes the contents of another file.

#include <stdio.h>    /* system header (angle brackets) */
#include "myfile.h"   /* user-defined header (quotes) */

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2. #define — Macro Definition

Object-like Macros (Constants)

#define PI 3.14159
#define MAX_SIZE 100
#define TRUE 1
#define FALSE 0

float area = PI * r * r;

Function-like Macros

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

int s = SQUARE(5);       /* expands to ((5) * (5)) = 25 */
int m = MAX(3, 7);       /* expands to 7 */

Note: Always parenthesise macro arguments to avoid operator precedence bugs.

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3. #undef — Undefine a Macro

#define LIMIT 10
/* ... */
#undef LIMIT    /* LIMIT is no longer defined */

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4. Conditional Compilation

#ifdef / #ifndef / #endif

#define DEBUG

#ifdef DEBUG
    printf("Debug mode on\n");
#endif

#ifndef RELEASE
    printf("Not a release build\n");
#endif

#if / #elif / #else / #endif

#define VERSION 2

#if VERSION == 1
    printf("Version 1");
#elif VERSION == 2
    printf("Version 2");
#else
    printf("Unknown version");
#endif

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5. #pragma

Compiler-specific instructions.

#pragma once         /* include file only once (non-standard but widely supported) */
#pragma warning(disable: 4996)  /* suppress a specific warning */

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6. Predefined Macros

printf("File: %s, Line: %d\n", __FILE__, __LINE__);