User defined function

C allows users to define their functions as their requirements, these functions which are defined by user are known as user defined functions.

For example:

#include <stdio.h>
using namespace std;
void add(); // declaration of function(prototype)
int main()
	add(); //function call
void add() // body of the function 
		float a,b,c;
	printf("\tenter two numbers to add\n\t");
	printf("\t addtion is:%f",c); 

Standard library functions:

The standard library functions are built in functions in C programming language which are already included in the header file that you are using, you don’t have to define them separately.

For example, printf() which is defined in the header file stdio.h and some other built in functions of stdio.h are scanf()