User-defined Functions in C

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A function in C is a block of organized, reusable code that is used to perform a single related action. In any C program, there are one or more functions − classified as library functions and user-defined functions.

There are two types of functions in C −

• Library functions
• User-defined functions

Any C compiler (e.g. GCC compiler, Clang, MSVC compiler, etc.) is distributed with a number precompiled header files (stdio.h, math.h, etc.), each consisting of one or more predefined library functions such as printf(), scanf(), pow(), sqrt(), etc. To be able to use the library function, the corresponding header file must be made available with the #include directive.

However, if you don’t find a suitable library function to serve your purpose, then you can define a customized function for the program. Normally, we find a C program with a main() function. Obviously, the main() function is a user-defined function, as it contains the instructions provided by the user. It can of course call the other library or user-defined functions.

What is User-Defined Function in C?

User-defined function is defined by the user to perform specific task to achieve the code reusability and modularity. To create and use the user-defined function, you do not need use any built-in library. These functions can be created either in the same program or in user-defined header file.

Creating a User-defined Function

For creating a user-defined function, first you need to understand the purpose of the function, that is, what do you want the function to do?

To create a user-defined function, you need to know about the following three parts of a function:

• Function declaration
• Function definition
• Function calling

Declaration of User-defined Function

In C language, it is necessary to provide the declaration of the prototype of any function. The prototype of a library function is present in the corresponding header file.

For a user-defined function, its prototype is present in the current program. The definition of a function and its prototype declaration should match.

Syntax

If you wish to define a function called add() that performs the addition of two integer arguments and returns the value as an integer, then the function declaration would be as follows −

int add(int, int);

Definition of User-defined Function

The definition of a function and its prototype declaration should match. The definition consists of a function header that matches the declaration and a function body.

Syntax

return_type function_name(arg1, arg2, ...){

   // Function body;

   return val;

}

Example

Using this template, you can write the user-defined function add() as follows −

int add(int a, int b){
   int c;
   c = a + b;
   return c;
}

Note that the order of definition of user-defined functions is not important in a C program. However, its prototype must be declared before calling the function.

In a program, the main() function is always the entry point, irrespective of whether it is the first function or not. We needn't provide the prototype declaration of the main() function.

Calling a User-defined Function

To call a function, you should use a statement that complies with the declaration of the function prototype. If the function is defined to receive a certain number of arguments, then the same number and type of arguments must be passed to call that function.

Example

The following statement calls the add() function that we defined above −

int result = add(10, 20);

Example of User-Defined Function

In this example, we are create two user-defined functions add() and sub() to find the addition and subtraction of the given numbers.

// C program to demonstrate an example of
// user-defined function
#include <stdio.h>

// Function declarations
int add(int, int);
int sub(int, int);

// Function definitions
int add(int a, int b) {
  return (a + b);
}

int sub(int a, int b) {
  return (a - b);
}

int main() {
  // Declaring two integer variables to
  // store the numbers
  // and resultant variables to store the result
  int num1 = 36, num2 = 24;
  int res_add, res_sub;

  // Calling the functions
  res_add = add(num1, num2);
  res_sub = sub(num1, num2);

  // Printing the results
  printf("Addition is : %d\n", res_add);
  printf("Subtraction is : %d\n", res_sub);

  return 0;
}

Addition is : 60
Subtraction is : 12

Formal and Actual Arguments in User-Defined Function

When a function is defined with arguments, the arguments in the parenthesis in front of the function name are called formal arguments. In the above example, the function is defined with "int a" and "int b" arguments; they are formal arguments.

When the function is called, the arguments passed to it are called the actual arguments. In the example below, the variables "x" and "y" are the actual arguments.

int x = 10, y = 20;
int result = add(x, y);

A user-defined function may be defined to have any type of variables as formal arguments. It includes primary types (int, float, char), array, pointer, or struct/union type variables.

A function should return a value to the calling environment. By default, the return type of a function is int type. However, it can return any data type − primary type, array, pointer, or a struct as well as a pointer. You can even define a function that returns a void type.

Example

If either the number or the type of actual and formal arguments or the return type as in the forward declaration of a function and its definition don’t match, then the compiler reports an error.

Look at the example below −

#include <stdio.h>

float divide (int, int);

int main(){

   int x = 15, y = 5;

   float z = divide (x, y);
   
   printf("%f", z);

   return 0;
}

int divide (int a, int b){

   int c = a/b;

   return c;
}

Output

In the code above, the declaration of the divide() function doesn’t match with its definition, hence the compiler shows the following error −

error: conflicting types for 'divide'

In C, any function can call any other function, any number of times. A function can call itself too. Such a self-calling function is called a recursive function.

The following program calls the main() function from inside main() itself −

#include <stdio.h>

int main(){

   printf("Hello");

   main();

   return 0;
}

When executed, the program goes into an infinite loop. In practice, recursion has to be used so that the program eventually terminates.