C Preprocessor Overview

The C preprocessor is an integral part of the C programming language that performs macro substitutions and file inclusions before the source code is compiled. It is commonly used for performing simple code generation, conditional compilation, and feature testing.

Syntax

#define MACRO value

Example

#include <stdio.h>
#define PI 3.14159

int main() {
    double radius = 5.0;
    double area = PI * radius * radius;
    printf("The area of the circle is %f\n", area);
    return 0;
}

Output

The area of the circle is 78.539750

Explanation

In the example above, the #define statement defines a macro named PI with a value of 3.14159. The macro is then used in the main function to calculate the area of the circle.

Use

The C preprocessor is commonly used for the following tasks:

• Macro substitution: #define statements can be used to define macros that substitute code fragments.
• File inclusion: #include statements can be used to include other source code files.
• Conditional compilation: #ifdef, #ifndef, and #if statements can be used to selectively compile code based on predefined macros.
• Feature testing: Predefined macros such as __STDC__ and __GNUC__ can be used to test for specific compiler features.

Important Points

• Preprocessor directives begin with the # symbol.
• #define statements are used to define macros.
• Macros can be used to substitute code fragments or values.
• #include statements are used to include other source code files.
• Conditional compilation can be performed using #ifdef, #ifndef, and #if statements.
• Predefined macros can be used for feature testing.

Summary

The C preprocessor is a powerful tool that allows for simple code generation, conditional compilation, and feature testing. Understanding the basic syntax and capabilities of the preprocessor is essential for writing efficient and effective C programs.