A function in C is a block of code designed to perform a specific task. Instead of writing the same code repeatedly, you can write a function once and call it whenever needed.

Functions make C programs modular, reusable, easy to maintain, and easier to debug.

1. Types of Functions in C

C supports two main types of functions:

a) Library Functions

These are predefined functions provided by C libraries.

Examples:

printf() – Output
scanf() – Input
sqrt() – Square root
strlen() – String length

Library functions require including header files like stdio.h, math.h, string.h, etc.

b) User-Defined Functions

These are custom functions created by the programmer.

Example:

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

2. Advantages of Using Functions

✔ Makes code shorter and cleaner
✔ Promotes reusability
✔ Improves readability
✔ Breaks complex problems into smaller parts
✔ Allows teamwork — different people can work on different functions

3. Function Structure in C

Every C function has three main parts:

a) Function Declaration (Prototype)

Tells the compiler about the function name, return type, and parameters.

int add(int, int);

b) Function Definition

Actual body of the function.

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

c) Function Call

Invokes the function.

int result = add(10, 20);

4. Syntax of a Function

return_type function_name(parameter_list) {
    // function body
}

Examples of return types:

void → returns nothing
int → returns an integer
float → returns a decimal value
char → returns a character

5. Parameter Types in Functions

a) Functions with No Parameters and No Return Value

void greet() {
    printf("Hello!");
}

Call:

greet();

b) Functions with Parameters but No Return Value

void display(int n) {
    printf("Number: %d", n);
}

c) Functions with Return Value but No Parameters

int getNumber() {
    return 100;
}

d) Functions with Parameters and Return Value

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

6. Example Program Demonstrating Functions

#include <stdio.h>

// Function declaration
int sum(int, int);

// Main function
int main() {
    int a = 5, b = 10;

    int result = sum(a, b);  // Function call

    printf("Sum = %d", result);

    return 0;
}

// Function definition
int sum(int x, int y) {
    return x + y;
}

7. Why Functions are Important in C

Reduce code duplication
Help organize large programs
Enable structured programming
Useful for modular development
Make debugging easier

8. Special Function: main()

The main() function is the entry point of every C program.

int main() {
    // code starts here
}

Without main(), a C program will not run.

Citations

🔗 View other articles about C Programming:
https://savanka.com/category/learn/c-programming/

🔗 External C Documentation:
https://www.w3schools.com/c/

What are Functions in C Programming? See Examples