Difference between user defined function and library function in C/C++

Library function: These function are the built-in functions i.e., they are predefined in the library of the C. These are used to perform the most common operations like calculations, updation, etc. Some of the library functions are printf, scanf, sqrt, etc. To use this functions in the program the user have to use associate header file associated to the corresponding function in the program. 

For Example: If, the user have to use print the data or scan the data using input stream then we have to use functions printf() and scanf() in C program and cin and cout in C++ program. To use these functions the user have to include #include<stdio.h> preprocessor directive in C program and #include<iostream> preprocessor directive in C++ program. 

C

// C program to illustrate inbuilt function
#include <stdio.h>

// Driver Code
int main()
{

    // Print Statement
    printf("GeeksforGeeks!");

    return 0;
}

C++

// C++ program to illustrate inbuilt function
#include <iostream>
using namespace std;

// Driver Code
int main()
{

    // Print Statement
    cout << "GeeksforGeeks!";

    return 0;
}

Output:

GeeksforGeeks!

User-defined function: These functions are designed by the user when they are writing any program because for every task we do not have a library of functions where their definitions are predefined. To perform the according to the requirement of user the user have to develop some functions by itself, these functions are called user-defined functions. For such functions the user have to define the proper definition of the function. 

For Example: If we want to perform the addition of two numbers then below is the program to illustrate the addition of two numbers using user-defined functions: 

C

// C program to illustrate user-defined function
#include <stdio.h>

// Function Call to find the sum of a and b
void findSum(int a, int b)
{

    // Print the sum
    printf("Sum is: %d", a + b);
}

// Driver Code
int main()
{
    // Given two numbers
    int a = 3, b = 5;

    // Function Call
    findSum(a, b);
    return 0;
}

C++

// C++ program to illustrate inbuilt function
#include <iostream>
using namespace std;

// Function Call to find the sum of a and b
void findSum(int a, int b)
{

    // Print the sum
    cout << "Sum is: " << a + b;
}

// Driver Code
int main()
{
    // Given two numbers
    int a = 3, b = 5;

    // Function Call
    findSum(a, b);
    return 0;
}

Output:

Sum is: 8

Tabular Representation to illustrate the difference between library and user-define function:

User-defined Functions	Library Functions
These functions are not predefined in the Compiler.	These functions are predefined in the compiler of C language.
These function are created by user as per their own requirement.	These functions are not created by user as their own.
User-defined functions are not stored in library file.	Library Functions are stored in special library file.
There is no such kind of requirement to add the particular library.	In this if the user wants to use a particular library function then the user have to add the particular library of that function in header file of the program.
Execution of the program begins from the user-define function.	Execution of the program does not begin from the library function.
Example: sum(), fact(),…etc.	Example: printf(), scanf(), sqrt(),…etc.