Definition

A function in C is a block of code that performs a specific task. Instead of writing the same code many times, we can write it once in a function and use it whenever needed.

Example: A function add() can be used to add any two numbers in different parts of a program without rewriting the logic again.

Why are functions important?

Extra[You can skip this during writing on notebooks. It is just an extra knowledge.]
  • They bring structure and modularity to programs.
  • They allow collaborative development (different programmers can work on different functions).
  • They make debugging easier since issues can be isolated to specific functions.

Types of Functions

1. Library Functions

These are predefined functions provided by the C standard library. They are ready-to-use and save development time.

Examples: printf(), scanf(), sqrt(), strlen(), strcmp().

2. User-Defined Functions

These are functions created by programmers to perform specific tasks as required by the program.

Examples:

void greet();
void display();
int add(int num1, int num2);

Advantages of Functions

1. Reusability

Code written once can be reused multiple times, saving time and effort.

Example: Example: A function add() can be used to add any two numbers in different parts of the program.

2. Separation of Task

Each function performs a separate task, making programs modular and organized.

Example: Example: One function handles input, another handles calculation, and another displays output.

3. Readability

Programs become easier to read, understand, and maintain. Functions with meaningful names explain themselves.

Example: Example: A function named calculateSalary() makes the purpose clear.

4. Handling Complexity

Large problems are divided into smaller, manageable parts. Each function can be designed, tested, and debugged independently.

5. Collaboration

Extra[You can skip this during writing on notebooks. It is just an extra knowledge.]

In large projects, different team members can work on different functions simultaneously.

6. Reduced Redundancy

Extra[You can skip this during writing on notebooks. It is just an extra knowledge.]

Repeating code multiple times increases errors and maintenance issues. Functions reduce duplication.

Parts of a Function in C

1. Function Signature

Declares the return type, function name, and parameters.

Example: int add(int a, int b);

2. Parameters

Variables that receive values from the calling function.

Example: int add(int a, int b) → here a and b are parameters.

3. Return Value

The result produced by a function and sent back to the caller.

Example: Example: return sum;

4. Return Keyword

Used to send back a value or exit from a function.

Example: Example: return a + b;

5. Function Definition

The full implementation of the function.

Example:

int add(int x, int y) {
    return x + y;
}

6. Function Description

A short explanation of what the function does.

Example: "add() takes two numbers and returns their sum."

7. Function Body

The actual set of instructions inside { }.

void greet() {
    printf("Hello, Welcome to C Functions!\\n");
}

8. Function Header

The first line of the function which includes return type, function name, and parameters.

Example: int add(int a, int b)

Calling a Function

Calling a function means using the function in the program where it is needed. A function is called by its name. 

void greet() {
    printf("Hello, Welcome to C Functions!\n");
}

int main() {
    greet(); // function call
    greet(); // called again (reusability)
    return 0;
}

Arguments

Actual values passed to a function when it is called.

Example: add(5, 10); → here 5 and 10 are arguments.

Parts of Function Definition (Enlist)

  • Return Type
  • Function Name
  • Parameters (if any)
  • Function Body

Examples of Functions in C

Extra[You can skip this during writing on notebooks. It is just an extra knowledge.]

Example 1: Addition Function

#include <stdio.h>

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

int main() {
    int result = add(5, 7);
    printf("Sum = %d", result);
    return 0;
}

Example 2: Greeting Function

#include <stdio.h>

void greet() {
    printf("Hello, Welcome to Functions in C!\\n");
}

int main() {
    greet();
    return 0;
}

Interview Questions on Functions in C

  • What is the difference between call by value and call by reference?
  • Can a C function return multiple values?
  • What is recursion? Give an example of a recursive function.
  • Explain the difference between function declaration and function definition.
  • What are inline functions? Are they supported in C?
  • Why do we use header files in relation to functions?

Real-Life Applications of Functions

  • Banking Systems: A function to calculate interest, another to handle transactions.
  • Games: Functions to control movement, scoring, and rendering.
  • Healthcare: Functions to process patient data, calculate BMI, and generate reports.
  • Operating Systems: Functions manage memory allocation, file handling, and device drivers.

Best Practices for Writing Functions

  • Use meaningful names (e.g., calculateSalary() instead of cs()).
  • Keep functions short and focused on one task.
  • Document each function with comments.
  • Avoid global variables; prefer parameters and return values.
  • Test functions individually before integrating them.

MCQs – Functions in C

  1. What is a function in C?
    a) A storage unit
    b) A block of code that performs a specific task ✅
    c) A variable type
    d) None of the above
  2. Which of the following is a library function?
    a) add()
    b) greet()
    c) printf() ✅
    d) display()
  3. User-defined functions are:
    a) Predefined in C
    b) Created by programmers ✅
    c) Built-in to the compiler
    d) None of the above
  4. Which part of a function contains actual instructions?
    a) Function Header
    b) Function Body ✅
    c) Function Signature
    d) Return Type
  5. What is the keyword used to return a value from a function?
    a) send
    b) give
    c) return ✅
    d) exit
  6. Which of the following is true about recursion?
    a) A function calling itself ✅
    b) A function that never returns
    c) A loop inside a function
    d) None of the above
  7. What is the default return type of a function in C (if not specified)?
    a) void
    b) int ✅
    c) float
    d) char
  8. Why are header files important for functions?
    a) They provide function prototypes ✅
    b) They store variables
    c) They make functions run faster
    d) None of the above
  9. Which function is always present in every C program?
    a) printf()
    b) scanf()
    c) main() ✅
    d) exit()
  10. What is call by reference?
    a) Passing values
    b) Passing memory addresses ✅
    c) Copying arguments
    d) Ignoring arguments

Frequently Asked Questions (FAQs)

Functions in C are reusable blocks of code that perform specific tasks, making programs modular and efficient.

C has two main types of functions: Library Functions (predefined in the standard library) and User-Defined Functions (created by programmers).

Recursion is when a function calls itself directly or indirectly. It is useful for problems like factorial, Fibonacci, and tree traversal.

No, a function in C returns only one value directly. However, multiple values can be returned using pointers, arrays, or structures.

Header files contain function prototypes that allow the compiler to understand how to call a function before it is defined.