Functions - Introduction

A function is a block of code that performs a specific task. In C, we can divide a large program into
the basic building blocks known as function.

A function can be called multiple times to provide reusability and modularity to the C program.

Reusability is the main achievement of C functions. By using functions, we can avoid rewriting same
logic/code again and again in a program.

There are two types of functions in C programming:

•     Library Functions: are the functions which are declared in the C header files such as scanf(), printf(), gets(), puts(), ceil(), floor() etc.

•     User-defined functions: are the functions which are created by the C programmer. It reduces the complexity of a big program and optimizes the code.

A function will have three aspects

1. Function Declaration / function prototype

2. Function Call

3. Function Definition.

1. Function Declaration / function prototype:

A function must be declared in a C program to tell the compiler about the function name, function
parameters, and return type.

A function prototype is simply the declaration of a function that specifies function's name, parameters
and return type. It doesn't contain function body.

Syntax of function prototype

returnType      functionName(type1 argument1, type2 argument2, ...);

Ex:

void     sum(int   x, int   y);

The function prototype is not needed if the user-defined function is defined before the main() function.

2. Function call: 

Function can be called from anywhere in the program. Control of the program is transferred to the
user-defined function by calling it.

Syntax of function call

functionName(argument1, argument2, ...);

Ex:

sum(a,b);

3. Function definition:

Function definition contains the block of code to perform a specific task. When a function is
called, the control of the program is transferred to the function definition. And, the compiler starts executing the codes inside the body of a function.

Syntax of function definition

returnType    functionName(type1 argument1, type2 argument2, ...)

{

    //body of the function

}

Ex:

void    sum(int x, int  y)

{

int  sum=x+y;

printf(“ sum is;%d”, sum);

}

Different aspects of function calling

There are four different aspects of function calls:

•     function without arguments and without return value

Ex: void sum( );

•     function without arguments and with return value

Ex: int sum( );

•     function with arguments and without return value

void   sum(int a,int b);

•     function with arguments and with return value

int   sum(int a,int  b);