#ifdef / #ifndef / #if / #endif

Conditional compilation controls whether specific parts of your source code are compiled based on whether a macro is defined and its value. It is commonly used for cross-platform support, toggling debug code, and preventing header files from being included more than once.

Syntax

// Compiles the block if the macro is defined.
#ifdef MACRO_NAME
    code
#endif

// Compiles the block if the macro is NOT defined.
#ifndef MACRO_NAME
    code
#endif

// Conditional branching combined with #else.
#ifdef MACRO_NAME
    code when the macro is defined
#else
    code when the macro is not defined
#endif

// Evaluates a condition based on the macro's value.
#if constant-expression
    code
#elif constant-expression
    code
#else
    code
#endif

// Header guard (idiom to prevent double inclusion).
#ifndef HEADER_NAME_H
#define HEADER_NAME_H
    // Header contents
#endif

Conditional Compilation Directives

Directive	Description
#ifdef	Compiles the block if the specified macro is defined.
#ifndef	Compiles the block if the specified macro is NOT defined. Used for header guards.
#if	Compiles the block if the constant expression is non-zero (true). Allows branching on macro values.
#elif	Adds another condition after #if / #ifdef / #ifndef.
#else	Defines the block to compile when the preceding condition is false.
#endif	Ends the conditional block.
defined(MACRO_NAME)	An operator used inside #if to check whether a macro is defined.

Sample Code

#include <stdio.h>

// Define a macro for debug mode (can also be passed at compile time with -DDEBUG).
#define DEBUG

// Compile-time branching based on the OS.
#if defined(_WIN32) || defined(_WIN64)
    #define PLATFORM "Windows"
#elif defined(__linux__)
    #define PLATFORM "Linux"
#elif defined(__APPLE__)
    #define PLATFORM "macOS"
#else
    #define PLATFORM "Unknown"
#endif

// Debug log macro (only active when DEBUG is defined).
#ifdef DEBUG
    #define LOG(msg) printf("[DEBUG] %s\n", msg)
#else
    #define LOG(msg) // Does nothing (removed in release builds).
#endif

int main(void) {
    printf("Platform: %s\n", PLATFORM);

    LOG("Program started."); // Printed because DEBUG is defined.

    int x = 42;

    #ifdef DEBUG
        // Check the variable value only in debug mode.
        printf("[DEBUG] x = %d\n", x);
    #endif

    // Branch on a constant value using #if.
    #define VERSION 2
    #if VERSION >= 2
        printf("Using features for version 2 or later.\n");
    #elif VERSION == 1
        printf("Using features for version 1.\n");
    #else
        printf("Unknown version.\n");
    #endif

    return 0;
}

Notes

Always add a header guard to every header file. If the same header is included multiple times, it will cause duplicate definition errors for types and functions. Use an all-uppercase, underscore-separated name based on the filename for the header guard macro (e.g., MYHEADER_H) to avoid collisions with other macros.

Some compilers support the non-standard #pragma once directive as a concise alternative to header guards, but since it is not part of the C standard, use the conventional #ifndef pattern when portability is a concern.

For details on #pragma, see #pragma / #error / #warning. For the basics of macro definitions, see #include / #define (constants).

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C Language Dictionary

#ifdef / #ifndef / #if / #endif

Syntax

Conditional Compilation Directives

Sample Code

Notes