What is File Inclusion in C pre-processor? Why we need file inclusion in our program? | C solutions

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What is File Inclusion in C pre processor? Why we need file inclusion in our program?
We have two cases:
1. If programmer is dealing with a very large program.
2. If similar functions and macro definitions are using frequently.
Syntax:
#include file_name
#include "file-name"

C pre processor- It is a program that processes our source program before it is passed to the compiler. (C Pre processor commands also known as directives)

There are four types of directives exists in C pre processor
1. MACO EXPANSION
2. FILE INCLUSION
3. CONDITIONAL COMPILATION
4. MISCELLANEOUS DIRECTIVES