Functions in C Programming

Function in C: The block of code or some logic wrapped inside curly braces ({ }) that performs a specific operation. We already saw some C functions, which you may not have noticed them.

For instance, printf(), scanf(), strcpy() etc. are some of the built-in functions in C programming language. The following are the advantages of functions in C Programming

  1. It will help us to divide the large programs into small groups. So, we can understand and debug the program quicker and better.
  2. Multiple persons can work on the same program by assigning different functions to each of them.
  3. Functions in C prevent us from writing the same logic multiple times.
  4. It help us to call the same function over and over.

Syntax of a Functions in C Programming

The basic syntax of the Functions in C Programming is as shown below:

Return_Type Function_Name (Parameters)
  Local Variable Declaration;


  Executable Statement 1;

  • Return_Type: When the function called, it may or may not return value.
    1. If the function in C returns any value, then we have to replace the Return_Type with the appropriate data type. For example, int, float, char, etc., and also use the return keyword to return the value when we call it.
    2. If it does not return any value, then we have to replace the Return_Type with the void keyword. No return keyword is needed.
  • Function_Name: It can be any name you wish to give other than the system reserved keywords.
  • Parameters: Every method accepts 0 or more parameters; it entirely depends upon the user requirements. While declaring the parameters don’t forget the appropriate data type. For example (int a, int b)
  • Local Variable Declaration: Sometimes, we may need some temporary variable, which is required only for this particular method, then we can declare them inside it. It is not mandatory, purely depends upon user requirements. Remember, these variables are available to this particular method only; we can’t access them outside of it.
  • Logic: Any mathematical or any code or calculations you want to implement.
  • Executable Statement: Any printf statements to print some data from it.

Type of Functions in C Programming

There are two types of functions in C Programming language:

  • All the built-ins supported by the C Language called a Library functions. We don’t have to bother about the logic inside the Library functions in C because of those are stored in header files. In our previous articles, We used many library methods such as printf(), scanf(), sqrt(), strcpy() etc.
  • User Defined Functions: Instead of relying only on built-in, C language allows us to create our own called as user defined functions. For example, if we want to calculate the standard deviation or some mathematical calculations, then we can place them in separate methods with the proper name. Then we can call that method multiple times.

C User Defined Functions implementation

To implement the user defined function in C program, we have to follow a few rules such as:

C Function Declaration

It will inform the compiler about the return type, function name, and the number of arguments along with the data types.

Return_Type Function_Name(Parameters)

For example,

int Add(int, int)

The Function declaration is optional. As we all know, C programming starts executing from the main(). So, when the compiler reaches the method call, then it will travel upwards to check for the definition. It means,

  • If we placed the Definition before the main (), then there is no need to declare the function in C.
  • If we placed the Definition after the main() then, it is mandatory to declare the method because it will inform the compiler

Calling the Function in C Programming

Nothing but calling the original function with a valid number of arguments and valid data type. For example, Add (2, 3)

NOTE: User defined function name should exactly match with the calling name in C Programming.

C Function Definition

It is the place where we are going to put all the logics, calculations, etc. We can place the definition either Before the main () or after the main().

For example,

int Add (int a, int b)
  int Sum;
  Sum = a + b;
  return sum;

NOTE: Please don’t forget the return keyword if you defined the method other than the void return type.

Find Sum and Average of 3 Numbers using Functions in C Programming

In this program, the user asked to enter three numbers. Then by calling the function, we will calculate the Sum and Average of that three numbers.

#include <stdio.h>

// Declaration
void Average ( float, float, float );                               

int main( )               
  float a, b, c;
  int x = 4, y= 6, z =5;
  printf ("\nPlease Enter 3 Number to find Sum & Average \n");
  scanf ( "%f %f %f", &a, &b, &c ) ;
  Average (a, b, c);  
  Average (x, y, z);                     

void Average ( float x, float y, float z)   
  float Sum, Average;
  Sum = x + y + z;
  Average = Sum/3;
  printf ("\n Sum of %.2f, %.2f and %.2f = %.2f", x, y, z, Sum );
  printf ("\n Average of %.2f, %.2f and %.2f = %.2f \n", x, y, z, Average);

 Functions In C Programming

It is a C function declaration. If you forget this void Average ( float, float, float ) declaration, the compiler will throw an error.

The first printf statement will ask the user to enter 3 numbers. And the below statement will store the user input values in a, b, c variables.

In the next line, we called the Average(a, b, c). When the compiler reaches this point, it will traverse to the top to check for the Average() declaration. If the compiler fails to identify the method with the Average name, then it will throw an error.

In this case, while traversing upwards, it will stop at

void Average ( float, float, float );

The above declaration will take the compiler to the below method

void Average ( float x, float y, float z );

First, this method will check for the arguments, and it will only execute if

  • Number of arguments passing to the function is equal to the declared arguments
  • Data types of the arguments passing to this are equal to the declared arguments

Within the C function, we declared 2 local variables Sum and Average. In the next line, we calculated the sum and average of three numbers using Assignment Operators

Sum = x + y + z;

Sum = 10 + 20 + 30 = 60

Average = Sum / 3;

Average = 60 / 3 = 20;

The below printf statements are used to print the sum and average to the output

printf ("\n Sum of %.f, %.f and %.f = %.2f", x, y, z, Sum );

printf ("\n Average of %.2f, %.2f and %.2f = %.2f\n", x, y, z, Average);

In the next line, We called Average () one more time, this time, we passed local variables as method arguments. We called 2 times because it will help you to understand that we can call the method n number of times from other.