Conditional Compilation
Learn how to use preprocessor directives to compile code conditionally.
Conditional Compilation is a free C Academy lesson on CoddyKit — lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
1
Conditional Compilation
Conditional compilation allows code to be included or excluded based on specific conditions.
In this lesson, you will learn:
- How to use
#ifdefand#ifndeffor conditional compilation. - How to use
#if,#else, and#endifto control compilation flow. - How conditional compilation helps create platform-independent code.
2
Using #ifdef and #ifndef
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.
3
Example: Using #ifdef
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;
}4
Using #if, #else, and #endif
The #if directive allows conditional compilation based on expressions.
Example:
#if VERSION >= 2
printf("Version 2 or higher\n");
#else
printf("Old version\n");
#endif5
Example: Using #if and #else
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;
}6
Using #elif for Multiple Conditions
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");
#endif7
Example: Using #elif
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;
}8
9
Advantages of Conditional Compilation
Conditional compilation is useful for:
- Creating platform-independent code.
- Enabling debugging features without modifying the code.
- Compiling different versions of a program.
10
Summary
In this lesson, you learned:
- How to use
#ifdefand#ifndefto check macro definitions. - How to use
#if,#else, and#eliffor conditional compilation. - The advantages of conditional compilation in creating flexible programs.
This concludes the Preprocessor Directives and Macros section in C!
Frequently asked questions
Is the “Conditional Compilation” lesson free?
Yes — the full text of “Conditional Compilation” is free to read here on the web, and the C Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO.
What will I learn in “Conditional Compilation”?
Learn how to use preprocessor directives to compile code conditionally. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C Academy?
No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Conditional Compilation” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this C Academy lesson?
Yes. Every C Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Understanding the C Preprocessor
- Macros and Inline Functions
- Conditional Compilation