A function is a group of statements that together perform a task. Every C program has at least one function, which is main(), and all the most trivial programs can define additional functions.

You can separate your code into functions. Dividing your code into functions is up to you, but logically the division is such that each function performs a specific task.

A function basically declares that the compiler is about a function’s name, return type, and parameters. A function definition provides the actual body of the function.

The C standard library provides numerous built-in functions that your program can call. For example, strcat() to concatenate two strings, memcpy() to copy one memory location to another location, and many more functions.

Method or subroutine or procedure are also called as a function.

Defining a Function

The general form of a function definition in C programming language follows,

return_type function_name( parameter list ) {
   body of the function

Functions in C programming consists of a function header and a function body. To describe the meaning of these, we can say that

  • Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
  • Function Name − This is the actual name of the function. The function name and the parameter list together constitute the function signature.
  • Parameters − A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
  • Function Body − The function body contains a collection of statements that define what the function does.

For Instance

In the below source code, the function called maximum(). This function takes two parameters num1 and num2 and returns the maximum value between the two −

/* function returning the maximum between two numbers */
int maximum(int num1, int num2) {

   /* local variable declaration */
   int result;
   if (num1 > num2)
      result = num1;
      result = num2;
   return result; 

Declaring a Function

A function declaration tells the compiler about a function name and how to call the function. In other words, the compiler only needs to know the function’s name and then the actual body of the function can be defined.

A function declaration has the following parts.

return_type function_name( parameter list );

For the above defined function max(), the function declaration is as follows −

int max(int num1, int num2);

Parameter names are not important in function declaration only their type is required, so the following is also a valid declaration −

int max(int, int);

Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.

Calling a Function

When you create your function in C, you give a definition of what the function has to do. To use a function, you will have to call that function to perform the defined task.

When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program.

To call a function, you simply need to pass the required parameters along with the function name, and if the function returns a value, then you can store the returned value. For example −

#include <stdio.h>
/* function declaration */
int max(int num1, int num2);
int main () {

   /* local variable definition */
   int a = 100;
   int b = 200;
   int ret;
   /* calling a function to get max value */
   ret = max(a, b);
   printf( "Max value is : %d\n", ret );
   return 0;
/* function returning the max between two numbers */
int max(int num1, int num2) {

   /* local variable declaration */
   int result;
   if (num1 > num2)
      result = num1;
      result = num2;
   return result; 

We have kept max() along with main() and compiled the source code. While running the final executable, it would produce the following result −

Max value is : 200

Example 2: Swapping numbers using Function Call by Value

#include <stdio.h>

void swapnum( int var1, int var2 )
   int tempnum ;
   /*Copying var1 value into temporary variable */
   tempnum = var1 ;

   /* Copying var2 value into var1*/
   var1 = var2 ;

   /*Copying temporary variable value into var2 */
   var2 = tempnum ;

int main( )
    int num1 = 35, num2 = 45 ;
    printf("Before swapping: %d, %d", num1, num2);

    /*calling swap function*/
    swapnum(num1, num2);
    printf("\nAfter swapping: %d, %d", num1, num2);


Before swapping: 35, 45
After swapping: 35, 45

Why variables remain unchanged even after the swap?
Understanding the point of the functions starts with their names. The function is called by value for num1 & num2. So actually var1 and var2 gets swapped (not num1 & num2). As in call by value actual parameters are just copied into the formal parameters.

Types of function

There are two kinds of function. They are:

  1. Library functions
  2. User-defined functions

1. Library functions

Library functions are the built-in function that is already defined in the C library. The prototype of these functions is written in header files. So we need to include respective header files before using a library function. For example, the prototype of math functions like pow(), sqrt(), etc is present in math.h, the prototype of exit(), malloc(), calloc() etc is in stdlib.h and so on.

2. User-defined functions

Those functions that are defined by the user to use them when required are called a user-defined function. The function definition is written by the user. main() is an example of a user-defined function.

Function Arguments

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function.

Formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

End Notes

Functions sometimes are the main components of codes. Therefore, developers use their function in common languages because of the advantages. Functions avoid repetition of codes, increases program readability, divides a complex problem into simpler ones, reduces chances of error, and most importantly modifies a program becomes easier by using a function.

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