Explain different categories of Pre-Processor directives used in C.

Preprocessor Directives in C

Preprocessor directives are commands in C that are processed before the actual compilation starts.

They begin with a # (hash symbol) and are used to instruct the compiler to perform certain tasks like including files, defining constants, and controlling compilation.

These directives do not end with a semicolon.

Categories of Preprocessor Directives

Preprocessor directives are mainly divided into three categories:

• Macro Substitution
• File Inclusion
• Conditional Compilation

1. Macro Substitution (#define)

This is used to define constants or macros.

Instead of writing values again and again, we use macros.

#include <stdio.h>
#define PI 3.14

int main() {
    float r = 5;
    float area = PI  r  r;
    printf("Area = %f", area);
    return 0;
}

Explanation: PI is replaced by 3.14 before compilation.

2. File Inclusion (#include)

This is used to include header files in a program.

Header files contain predefined functions.

#include <stdio.h>

int main() {
    printf("Hello World");
    return 0;
}

Types:

• #include <file.h> → system file
• #include "file.h" → user-defined file

3. Conditional Compilation

This is used to compile specific parts of code based on conditions.

#include <stdio.h>

#define NUM 10

int main() {

#if NUM > 5
    printf("Number is greater than 5");
#else
    printf("Number is small");
#endif

    return 0;
}

Explanation: Only the true condition block is compiled.

Other Important Directives

Important Points for Exams

• Preprocessor runs before compilation
• No semicolon is used
• Macros improve readability
• Conditional compilation helps debugging

Final Tip

This is a theory question that appears frequently in exams. Focus on categories and examples to score full marks.