Conditional Compilation

Conditional Compilation

Conditional compilation allows code to be included or excluded based on specific conditions.

In this lesson, you will learn:

• How to use #ifdef and #ifndef for conditional compilation.
• How to use #if, #else, and #endif to control compilation flow.
• How conditional compilation helps create platform-independent code.

The #ifdef directive includes code only if a macro is defined.

Example:

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

The #ifndef directive works oppositely, including code only if a macro is NOT defined.

This program demonstrates how to enable debugging messages only when DEBUG is defined.

#include <stdio.h>

#define DEBUG

int main() {
    #ifdef DEBUG
        printf("Debugging mode enabled\n");
    #endif
    printf("Program running...\n");
    return 0;
}

The #if directive allows conditional compilation based on expressions.

Example:

#if VERSION >= 2 printf("Version 2 or higher\n"); #else printf("Old version\n"); #endif

This program demonstrates version-based compilation using #if and #else.

#include <stdio.h>

#define VERSION 2

int main() {
    #if VERSION >= 2
        printf("Version 2 or higher\n");
    #else
        printf("Old version\n");
    #endif
    return 0;
}

The #elif directive checks multiple conditions.

Example:

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

This program demonstrates how to handle multiple compilation conditions using #elif.

#include <stdio.h>

#define VERSION 2

int main() {
    #if VERSION == 1
        printf("Version 1\n");
    #elif VERSION == 2
        printf("Version 2\n");
    #else
        printf("Unknown version\n");
    #endif
    return 0;
}

Conditional compilation is useful for:

• Creating platform-independent code.
• Enabling debugging features without modifying the code.
• Compiling different versions of a program.

In this lesson, you learned:

• How to use #ifdef and #ifndef to check macro definitions.
• How to use #if, #else, and #elif for conditional compilation.
• The advantages of conditional compilation in creating flexible programs.

This concludes the Preprocessor Directives and Macros section in C!