Header Files & Preprocessor Directives in C

1. What are Header Files?

Header files in C contain declarations of functions and macros. They allow you to reuse code and organize functions logically. They typically have a .h extension and are included using the #include directive.

2. Types of Header Files

• Standard Header Files – Provided by the C library.
  Example: #include <stdio.h>, #include <math.h>
• User-defined Header Files – Created by the programmer.
  Example: #include "myheader.h"

3. What are Preprocessor Directives?

Preprocessor directives are commands that begin with # and are processed before the compilation starts. They are used to include files, define macros, and conditionally compile code.

4. Common Preprocessor Directives

• #include – Includes a header file.
• #define – Defines a macro or constant.
• #undef – Undefines a macro.
• #ifdef / #ifndef – Conditional compilation if a macro is defined or not defined.
• #if / #else / #elif / #endif – Multi-branch conditional compilation.

5. Example Code

#include <stdio.h>
#define PI 3.14159

int main() {
    printf("Value of PI: %.2f\n", PI);
    return 0;
}

Explanation:
#include <stdio.h> tells the compiler to include standard input-output functions.
#define PI 3.14159 defines a macro constant named PI.

6. Why Use Header Files and Preprocessor Directives?

• They help organize large code into modular units.
• Prevents duplication of code.
• Makes it easier to maintain and understand the codebase.