Library functions are those functions which are defined by C library, example printf(), scanf(), strcat() etc. You just need to include appropriate header files to use these functions. These are already declared and defined in C libraries.
 

User-defined functions are those functions which are defined by the user at the time of writing program. Functions are made for code reusability and for saving time and space.
 

#include<stdio.h>

#include<conio.h>

void main()

{

    int a,b;

    clrscr();

    printf("Enter value of a:");

    scanf("%d",&a);

    printf("Enter value of b:");

    scanf("%d",&b);

    

    if(a>=b)

    {

        printf("

The value %d is greater than %d",a,b);

    }

    else if(b>=a)

    {

        printf("

The value %d is greater than %d",b,a);

    }

    else

    {

        printf("

The value %d is equal to %d",a,b);

    }

    getch();

}