C #ifndef

Syntax

#ifndef identifier
// code to be included if identifier is not defined
#endif

Example

#ifndef PI
#define PI 3.1415926535897
#endif

#include <stdio.h>

int main() {
   double radius = 2.5;
   printf("Area of the circle is: %f", radius * radius * PI);
   return 0;
}

Output

Area of the circle is: 19.634956

Explanation

#ifndef is a C preprocessor directive that checks whether a given identifier has been defined before inclusion of a certain block of code. The block of code between #ifndef and #endif is included only if the identifier has not been defined.

In the above example, the identifier PI is defined only if it was not defined before. This ensures that the value of PI is defined only once in the program, regardless of how many times the header file containing the definition is included.

Use

The #ifndef directive is commonly used to prevent multiple definitions of a variable, constant or function prototype. By surrounding the definition of the identifier with #ifndef and #endif, the code block is included only if the identifier is not already defined.

Important Points

• The #ifndef directive checks whether an identifier has been defined before its inclusion in a program.
• The code between #ifndef and #endif is included only if the identifier has not been defined.
• #ifndef is often used with #define to define constants that should only be defined once.
• #ifndef can also be used to include header files, with the header file contents between #ifndef and #endif guards to prevent multiple inclusion.

Summary

The C #ifndef directive is a valuable tool for preventing multiple definitions of a variable, constant or function prototype. It allows code blocks or header files to be included only once in a program, reducing the possibility of naming conflicts and memory leaks. It is important to use #ifndef directives with #define statements to define constants and to place header file contents between #ifndef and #endif guards. This ensures that header files are included only once, improving program efficiency and avoiding potential issues.