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Conditional compilation

  • C course
  • Beginner
  • Lesson 108/116
  • 5 min

Learn conditional compilation with #ifdef, #ifndef and #endif.

Conditional compilation: #ifdef, #ifndef, #endif

#ifdef compiles code if a macro is defined; #ifndef compiles if it is not. #endif closes the block.

#define DEBUG

#ifdef DEBUG
    printf("debug");
#endif

Here if DEBUG is defined, the printf is compiled; otherwise it is skipped. This is useful to remove debug code from production builds. With #ifndef you can run custom code when a macro is missing. This allows producing different builds from a single source.

Conditional definition with #ifndef

#ifndef compiles its content if a macro is not defined. This is useful to give a constant a default value.

#ifndef VERSION
    #define VERSION 1
#endif

Here if VERSION was not previously defined, it becomes 1. The compiler can give a different value from the command line with -DVERSION=2. This is also used as include guard in header files; we will see it later. It is an important tool for writing flexible and portable code.

Practice Conditional print with the DEBUG macro

Define a macro with #define DEBUG. In main use #ifdef DEBUG to print printf("debug\n"); close the block with #endif.

Starter code

main.c
#include <stdio.h>

// Define the DEBUG macro
int main(void) {
    // If DEBUG is defined print debug with printf; close the block
    return 0;
}

Sample solution

main.c
#define DEBUG
#ifdef DEBUG
printf("debug\n");
#endif

Expected output

debug

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Fill in the blank: #ifdef block

Question 
#ifdef DEBUG
    printf("debug");
____
Answer 
#ifdef DEBUG
    printf("debug");
#endif

Explanation: #endif closes the #ifdef block.

2 Match directive to meaning

Term Meaning
#ifdef X If X is defined
#ifndef X If X is not defined
#endif Ends the block
#define X Defines X

3 What does #endif do?

  • Starts an #ifdef block
  • Terminates the program
  • Declares a variable
  • Ends an #ifdef block Correct answer

Explanation: #endif closes an #ifdef or #ifndef block.

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together