What is conditional compilation in C language?

In C programming language, several directives control the selective compilation of different parts of the program code. Conditional Compilation is a set of preprocessing directives that allows us to exclude and include parts of a program based on specified conditions. Each directive in the compilation is processed across different platforms, including debug and release versions. They are as follows ?

• #if
• #else
• #elif
• #endif

The "#if" Directory

The #if is a directive preprocessor that evaluates the expression or condition. If the condition is true then this directory executes the code otherwise #elseif or #endif code is executed.

The general form of #if is as follows ?

#if constant_expression
 statement sequence
#endif

The #else statement defines a block of code that runs when the condition in an if statement is false. This works similarly to the other keyword in C.

#elif stands for "else if" and creates an if-else-if compilation chain. The #elif statement is used within an #if statement. This includes a section of text that is determined if its condition is True. Among other users, #if provides an alternative "commenting out" code method.

For example, this code snippet is a preprocessor directive that comments out the printf statement, this disables it during the compilation.

#if 0
 printf("#d", total);
#endif

Here, the compiler will ignore printf("#d", total);

Example

This C program checks if NUMBER is equal to 0 using the #if directive. If true, it prints "Value of Number is:0" and then waits for user input getchar() before generating.

#include <stdio.h>
#define NUMBER 0

void main() {
#if (NUMBER == 0)
    printf("Value of Number is: %d", NUMBER);
#endif       
    getchar();
}

Output

The result is obtained as follows ?

Value of Number is: 0

The "#else" Directory

The #else preprocessor determines the expression on condition if condition of #if is false. This can be used with #elif, #ifndef directive.

Syntax

Following is the syntax of #else directive ?

#else  
//else code  

Example

In the following example, we print "Value of Number is non-zero" because NUMBER is set to 1. This C program checks if NUMBER is equal to 0. If true, it prints the value of NUMBER. Otherwise, it prints "Value of Number is non-zero" and waits for the user.

#include <stdio.h>
#define NUMBER 1

void main() {
#if NUMBER == 0
    printf("Value of Number is: %d", NUMBER);
#else
    printf("Value of Number is non-zero");
#endif
    getchar();
}

Output

We will get the result as follows ?

Value of Number is non-zero

The "elif" Directory

The #elif directive is a unique action when it is combined with the other directives. This condition specifies after #if is false, the processor will specify the condition after #elif. The code will be blocked after #elif is compiled if the corresponding test is true.

Syntax

Following is the syntax of the #elif directive ?

#elif condition_expression

Example

In this example, we print "Suresh is good boy" because YEARS_OLD is 15, which is less than or equal to 20. This C program checks if YEARS_OLD is 15 and prints "Suresh is a good boy".

#include <stdio.h>  
#define YEARS_OLD 15  
int main()  
{  
   #if YEARS_OLD <= 20  
printf("Suresh is a good boy.
");  
   #elif YEARS_OLD > 20  
printf("Suresh is %d years old.
", YEARS_OLD);  
   #endif  
   return 0;  
}  

Output

The result is produced as follows ?

Suresh is a good boy.

The "#endif" Directory

In the following #endif directive specifies the end of the #if, #ifdef, or #ifndef. When the #endif directive is encountered, it completes the preprocessing of the determined opening directive.

Syntax

Following is the syntax of the #endif ?

#endif

Example

In this example we print "TutorialsPoint is great Learning Platform" because WINDOWS is defined as 1, this enables the conditional code.

#include <stdio.h>

#define WINDOWS 1

int main()
{
   printf("TutorialsPoint is a great");

   #if WINDOWS
   printf("Learning ");
   #endif

   printf("Platform.
");

   return 0;
}

Output

We will produce the following result ?

TutorialsPoint is a greatLearning Platform.

Basic Usage

Here is a C program demonstrating the use of #ifdef, #ifndef, #else, and #endif. This uses conditional compilation to check if a is defined. If a is specified, it prints "Hello I am here." Otherwise, it prints "Not defined".

#include <stdio.h>
#define a 10

void main() {
#ifdef a
    printf("Hello I am here..");
#endif
#ifndef a
    printf("Not defined ");
#else
    printf("R u There ");
#endif
}

Output

When the above program is executed, it produces the following result ?

Hello I am here..
R u There