Functions In C | Questions, Types, Components, Examples

A function is a block of code that performs a specific task. In this lecture, we will learn identification, types, examples, and various questions about the function. After this lacture, you will be able to speak each and everything about a function in c programming. Let’s get started.

1. Identification of a Function

All elements of a function are listed below where some are compulsory and others are optional.

return_type function_name(parameter_list) {     // Body of function   // return Statement }
  • Return Type: Specify what type of value will be returned by the return statement; can be any valid type (i.e. int, float, char) or void.
  • Function Name (as variable name): Must be unique, descriptive, and follow naming conventions.
  • Arguments (0 to n): Must have types and names; must match in order and type during function call
  • Function body: block of code that performs a specific task. 
Note:

  • Compulsory Element: Return Type, Function Name, Parentheses (), and Function Body {} are the compulsory elements of a function
  • Optional Elements: A function can have zero or more parameters, and if the return type is void, the return statement is also optional.

2. Types of function 

There are two types of function in C programming:

  • Standard library functions
  • User-defined functions

1.  Standard library functions

The standard library functions are built-in functions in C programming. These functions are defined in header files. For example,

  • The printf() is a standard library function to send formatted output to the screen (display output on the screen). This function is defined in the h header file.
    Hence, to use the printf() function, we need to include the stdio.h header file using #include <stdio.h>.
  • The sqrt() function calculates the square root of a number. The function is defined in the h header file.  

2.  User-defined function

You can also create functions as per your need. Such functions created by the user are known as user-defined functions. In this article use all user define function as a example.

3. How a Function is called

When a function is called, control of the program is transferred to that user-defined function, and after the function completes its execution, the control is transferred back to the point where the function was called. This is the standard flow of control for function calls in C.

How a Function is called - Complete Procedure

4. Return Statement Explanation

The return statement terminates the execution of a function and returns a value to the calling function. The program control is transferred to the calling function after the return statement.

Function- Return Statement Explanation in C programming

Note: As soon as a return statement is executed in a function, the function exits, and no further code within the function is executed. So, a function can have multiple return statements, but only one return is executed during a single function call.

Important:  If a function in C is declared with any return type other than void, the function must contain a return statement that returns a value of the specified type. Failing to provide a return statement in such cases can lead to undefined behavior or a compilation warning/error, depending on the compiler. So,

  • If the return type is not void, you must use a return statement with a value matching the return type.
  • If the return type is void, you do not need a return statement (though you can use return; to exit the function early if necessary, but it doesn’t return a value).

5. Important FAQ Questions about a Function

Lets discuss some important frequently asked questions (FAQ) about function in C programming. 

Question 01: Can we add function inside function?

Clear cut answer is No. Nested function in C programming is not allowed.

Question 02: Can we call one function inside another function?

Yes, as many function which are created outside the function are called inside main function. But one function can call the other function, outside the main function as well.

Question 03: Do all functions in C have to be called inside the main() function?

No, it is not compulsory for all functions to be called inside the main() function. Functions can be called from other functions, but the program starts running from main(). However, at least one function must be called inside main() out of the many functions that may be calling one another.

#include <stdio.h>

// Function definitions
int calculate() {
int sum = add(5, 3); // calculate() calls add()
int product = multiply(4, 2); // calculate() calls multiply()

return sum + product; // Returns the sum of both
}

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

int multiply(int a, int b) {
return a * b;
}

int main() {
int result = calculate(); // main calls calculate()
printf(“The final result is: %d\n”, result);

return 0;
}

OUTPUT:  The Final Result is : 16

Explanation:

  • The main() function calls the calculate()
  • Inside calculate(), the add() and multiply() functions are called.
  • The add() and multiply() functions perform their calculations and return the results to calculate(), which then returns the final result to main().

Question 04: Where we can write a function, other than main()?

There are three main methods to place a function at right position in program.

  •  Method 01: Functions should defined at the top of main() function
  • Method 02: If you want to define a function after main(), you can do this by providing a function prototype (declaration) before main() function.
  • Method 03: Functions can be declared and define in header files (.h files)  and these header files are added file (.c) where main() function exist.

Following figure explain all these 3 cases of a function call

Where to use the function in C programming

6. Confusion in Some Terms of a Function

Function Signature, Function Declaration (Prototype) and Function definition are some important terms of a function. Everyone must have the clarity about these points before to proceed the function in any programming language.

i) Function Signature: The name and parameter types of the function (excluding the return type) is called function signature. In the function which hold function prototype int add(int a, int b); the function signature is

add(int, int)

This is especially useful in function overloading (like in C++).

ii) Function Declaration (Prototype): Specifies function details without the body; used for forward declarations.

 int add(int a, int b);

iii) Function declaration and definition:

A function definition contains both the function declaration (name, return type, parameters) and the actual code (body) that describes what the function does.

int add(int a, int b) {

return a + b;

 }

7. Benefits of using Function

Here are some key benefits of using functions in programming:

  • Code Reusability: Functions allow code to be reused multiple times without rewriting it.
  • Modularity: Breaks down large problems into smaller, manageable tasks.
  • Easier Debugging: Errors are easier to isolate and fix in smaller code blocks.
  • Improved Readability: Functions with descriptive names make code clearer and more understandable.
  • Reduced Code Redundancy: Prevents duplication by centralizing common tasks.
  • Facilitates Collaboration: Multiple developers can work on different functions independently.
  • Easier Maintenance: Updating a function in one place updates its behavior across all usage points.