The #ifndef Preprocessor Directive

The #ifndef Preprocessor Directive 

The syntax of the #ifndef directive is, as given below. 

#ifndef<identifier> 

{ 

statementl; 

statement2; 

} 

#else 

{ 

statement3; 

statement4; 

}

 #endif 

The #ifndef, works exactly opposite to that of # ifdef. The #ifndef preprocessor, tests whether the identifier has defined substitute text or not. If the identifier is defined then #else block, is compiled and executed and the compiler ignores #if block. If the identifier is not defined, then #if block, is compiled and executed.

The #error Preprocessor Directive 

The # error, is used to display user defined message during compilation of the program. 

The syntax is, as given below. 

# if !defined (identifier) 

# error<ERROR MESSAGE> 

#endif 

Write a program to display user-defined error message using #error directive. 

# include <stdio.h> 

# include <conio.h> 

void main ()

#if !defined (A) 

# error MACRO A IS NOT DEFINED 

#else 

printf ("Macro found"); 

#endif 

} 

Explanation: 

In the above program identifier 'A' is not defined. In the absence of the identifier, an error is generated and the # error directive displays the error message. The error message is user defined and displayed in the message box at the bottom of the editor.