#ifdef and #ifndef in C: Conditional Compilation and Include Guards

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Lesson 142Mixed lesson

#ifdef and #ifndef in C: Conditional Compilation and Include Guards

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Lesson format12 sections1 video4 code blocks1 practice itemUpdated Aug 27, 2026

#ifdef checks whether a macro is defined. #ifndef checks whether it is not defined. The code between the directive and the matching #endif is included or excluded from compilation based on the result. This is how you write platform-specific code, enable debug features, and prevent header files from being included twice. The preprocessor runs before the compiler. Excluded code is not compiled, not syntax-checked, and does not exist in the binary.

#ifdef #ifndef Conditional Compilation Directives

#Platform-Specific Code#link

#include <stdio.h>

int main(void) {
#ifdef _WIN32
    printf("Running on Windows\n");
#elif defined(__linux__)
    printf("Running on Linux\n");
#elif defined(__APPLE__)
    printf("Running on macOS\n");
#else
    printf("Running on unknown platform\n");
#endif

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

    return 0;
}

Compile for Linux (normal)

root@vulnarex:~#gcc -Wall platform_detect.c -o platform_detect && ./platform_detect

Compile with DEBUG defined

root@vulnarex:~#gcc -Wall -DDEBUG platform_detect.c -o platform_debug && ./platform_debug

The -DDEBUG flag defines the DEBUG macro on the command line. The #ifdef DEBUG block is included. Without the flag, the block is excluded. The compiler never sees the printf inside the excluded block. It does not check its syntax. It does not allocate space for its string literal. The code does not exist.

#Include Guards#link

#ifndef CONFIG_H
#define CONFIG_H

#define MAX_CONNECTIONS 100
#define TIMEOUT_MS 5000

typedef struct {
    int port;
    int debug;
} Config;

#endif

The first time config.h is included, CONFIG_H is not defined. The #ifndef passes. The contents are processed. CONFIG_H is defined. The second time config.h is included (perhaps indirectly through another header), CONFIG_H is already defined. The #ifndef fails. Everything between it and #endif is skipped. The struct is not redefined. The constants are not redeclared. No duplicate symbol errors. This is the include guard pattern, and every header file you write must have one.

★ 30 XP

quiz BLOCK (★ 30 XP)

What is the purpose of an include guard in a header file?

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#ifdef and #ifndef are the gates of conditional compilation. They control what the compiler sees. Platform detection, feature flags, debug code, include guards: all built on these two directives. Next: #if, #elif, #else, which extend conditional compilation to arbitrary constant expressions.

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Platform-Specific Code Include Guards

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Last updated

August 27, 2026

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Laboratory sanity code

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#include <stdio.h>

int main(void) {

#ifdef _WIN32

printf("Running on Windows\n");

#elif defined(__linux__)

printf("Running on Linux\n");

#elif defined(__APPLE__)

printf("Running on macOS\n");

#else

printf("Running on unknown platform\n");

#endif

#ifdef DEBUG

printf("Debug mode enabled\n");

#endif

return 0;

}