C Programming Conditional Compilation Ifdef Ifndef Else Endif Complete Guide

Here are some step-by-step examples that cover beginner-level scenarios:

Example 1: Basic Use of #ifdef

This example demonstrates how to conditionally compile code based on whether a macro is defined.

#include <stdio.h>

#define DEBUG

int main() {
    int value = 42;

#ifdef DEBUG
    // This block will be compiled only if DEBUG is defined
    printf("Debug mode is enabled.\n");
    printf("The value is: %d\n", value);
#endif

    printf("Program execution continues here regardless of debug mode.\n");

    return 0;
}

Steps:

1. Include the standard input-output library with #include <stdio.h>.
2. Define the macro DEBUG using #define DEBUG. You can comment out or define this line to toggle debug mode.
3. Use #ifdef DEBUG to check if DEBUG is defined.
4. Inside the #ifdef and #endif, you can write code that will be compiled only if DEBUG is defined.
5. Add some general-purpose code outside the #ifdef block that will execute regardless of the debug mode.

Output when DEBUG is defined:

Debug mode is enabled.
The value is: 42
Program execution continues here regardless of debug mode.

Output when DEBUG is not defined:

Program execution continues here regardless of debug mode.

Example 2: Using #ifndef

This example shows how to compile code only if a macro is not defined.

#include <stdio.h>

//#define RELEASE

int main() {
    int version = 1;

#ifndef RELEASE
    // This block will be compiled only if RELEASE is NOT defined
    printf("Running in development mode.\n");
    printf("Version: %d\n", version);
#else
    // This block will be compiled if RELEASE is defined
    printf("Running in release mode.\n");
    printf("Version: %d\n", version);
#endif

    printf("Common functionality here...\n");

    return 0;
}

Steps:

1. Similar to Example 1, start by including the stdio.h library.
2. Optionally define the RELEASE macro using #define RELEASE.
3. Use #ifndef RELEASE to check if RELEASE is not defined.
4. Write the development-mode-specific code inside the #ifndef block.
5. Use #else to provide an alternative block for when RELEASE is defined.
6. Close the conditional section with #endif.

Output when RELEASE is defined:

Running in release mode.
Version: 1
Common functionality here...

Output when RELEASE is not defined:

Running in development mode.
Version: 1
Common functionality here...

Example 3: Conditional Compilation Based on Multiple Conditions

This example demonstrates how to compile different sections of code based on multiple conditions.

#include <stdio.h>

#define PLATFORM_LINUX
// #define PLATFORM_WINDOWS

int main() {
    int version = 2;

#ifdef PLATFORM_LINUX
    // This block will be compiled if PLATFORM_LINUX is defined
    printf("Platform: Linux\n");
    printf("Version: %d\n", version);
#elif defined PLATFORM_WINDOWS
    // This block will be compiled if PLATFORM_WINDOWS is defined
    printf("Platform: Windows\n");
    printf("Version: %d\n", version);
#else
    // This block will be compiled if neither PLATFORM_LINUX nor PLATFORM_WINDOWS is defined
    printf("Unknown Platform\n");
    printf("Version: %d\n", version);
#endif

    printf("Shared code here...\n");

    return 0;
}

Steps:

1. Include the stdio.h library.
2. Define one of the platform macros, such as PLATFORM_LINUX or PLATFORM_WINDOWS.
3. Use #ifdef PLATFORM_LINUX to check if PLATFORM_LINUX is defined.
4. Inside this block, write platform-specific code for Linux.
5. Use #elif defined PLATFORM_WINDOWS to check another condition if the first one fails.
6. Inside the #elif block, write platform-specific code for Windows.
7. Use #else to handle cases where none of the above conditions are met.
8. Finally, close the conditional section with #endif.

Output when PLATFORM_LINUX is defined:

Platform: Linux
Version: 2
Shared code here...

Output when PLATFORM_WINDOWS is defined:

Platform: Windows
Version: 2
Shared code here...

Output when neither is defined:

Unknown Platform
Version: 2
Shared code here...

Example 4: Toggling Functionality

You can use these directives to enable or disable specific functionalities, such as logging.

#include <stdio.h>

#define LOGGING_ENABLED

void log(const char *message) {
#ifdef LOGGING_ENABLED
    // The log function will do logging if LOGGING_ENABLED is defined
    printf("Log: %s\n", message);
#else
    // If LOGGING_ENABLED is not defined, the log function does nothing
    (void)message;  // Prevent unused parameter warnings
#endif
}

int main() {
    int count = 10;

    log("Initializing...");
    // Main program logic here
    printf("Count initialized to: %d\n", count);
    return 0;
}

Steps:

1. Include the stdio.h library.
2. Define the LOGGING_ENABLED macro to control whether logging is enabled or not.
3. Create a log function that prints messages when LOGGING_ENABLED is defined.
4. Inside the log function, use #ifdef LOGGING_ENABLED to compile the logging code.
5. Use (void)message; in the #else block to prevent any warnings related to unused function parameters.
6. Use the log function in your main program to leave traces if logging is enabled.

Output when LOGGING_ENABLED is defined:

Log: Initializing...
Count initialized to: 10

Output when LOGGING_ENABLED is not defined:

Count initialized to: 10

Example 5: Nested Conditional Compilation

Conditional directives can also be nested within each other.

#include <stdio.h>

#define FEATURE_A
#define FEATURE_B
// #define FEATURE_C

int main() {
    printf("Feature overview:\n");

#ifdef FEATURE_A
    printf("Feature A is enabled.\n");

#ifdef FEATURE_B
    printf("FEATURE_B is also enabled within Feature A.\n");

#ifdef FEATURE_C
    printf("FEATURE_C is enabled within Feature A and Feature B.\n");
#else
    printf("FEATURE_C is disabled within Feature A and Feature B.\n");
#endif

#else
    printf("Feature B is disabled within Feature A.\n");
#endif

#else
    printf("FEATURE_A and its subfeatures are all disabled.\n");
#endif

    return 0;
}

Steps:

1. Include the stdio.h library.
2. Define the desired features (FEATURE_A, FEATURE_B, FEATURE_C).
3. Use #ifdef FEATURE_A to check if FEATURE_A is defined and start a new block.
4. Inside FEATURE_A, use another #ifdef FEATURE_B to check for FEATURE_B.
5. You can continue nesting #ifdef blocks for more complex dependencies.
6. Properly use #else and #endif to manage code flow and terminate conditional blocks.

Output when FEATURE_A and FEATURE_B are defined but FEATURE_C is not:

Feature overview:
Feature A is enabled.
FEATURE_B is also enabled within Feature A.
FEATURE_C is disabled within Feature A and Feature B.

Output when all features are defined:

Feature overview:
Feature A is enabled.
FEATURE_B is also enabled within Feature A.
FEATURE_C is enabled within Feature A and Feature B.

Output when only FEATURE_A is defined but the others are not:

Feature overview:
Feature A is enabled.
Feature B is disabled within Feature A.

Output when all features are disabled: