In C programming, function definition refers to writing the actual body of a function — the block of code that performs a specific task.
It tells the compiler what the function does when it is called.

A function definition includes:

Return Type
Function Name
Parameter List (optional)
Function Body enclosed in { }

1. Syntax of Function Definition

return_type function_name(parameter_list) {
    // statements
    // return value (if required)
}

2. Components Explained

a) Return Type

Specifies what type of value the function will return.

Common return types:

int
float
char
double
void (returns nothing)

b) Function Name

An identifier that follows C naming rules.

Examples:
add, display, calculateAverage

c) Parameter List

Represents input values passed to the function.

Examples:

int add(int x, int y)
void display(int n)
float average(float a, float b, float c)

If a function takes no parameters:

void greet()

d) Function Body

Contains the code statements that define the function’s behavior.

3. Example: Function Definition with Return Value

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

Here:

Return type → int
Function name → add
Parameters → a and b
Body → calculates and returns the sum

4. Example: Function Definition Without Return Value

void greet() {
    printf("Hello, welcome to C programming!");
}

The function returns nothing and only prints a message.

5. Example: Function Definition Without Parameters

int getNumber() {
    return 10;
}

No inputs are required — the function always returns 10.

6. Example: Function Definition With Multiple Parameters

float divide(float x, float y) {
    return x / y;
}

This function returns a floating-point result.

7. Full Program Demonstrating Function Definition

#include <stdio.h>

// Function definition
int square(int n) {
    return n * n;
}

int main() {
    int num = 5;
    int ans = square(num);   // Function call

    printf("Square of %d is %d", num, ans);
    return 0;
}

8. Notes About Defining Functions

A function can be defined above main() or below it.
If defined after main(), you must write a function declaration (prototype) before main().
Every C program must have one and only one main() function.
Function names must be unique within the same scope.

Citations

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

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

How to Define a Function in C? See Example