Defining and Calling Functions

Functions are the primary way C programs are structured. A function is a block of organized, reusable code that performs a single, related action. They are vital for breaking complex tasks into smaller, manageable parts, enhancing code modularity and readability.

1. Function Components

A function is defined by three main parts:

A. Function Prototype (Declaration)

A prototype is a declaration that tells the compiler the function’s name, the type of data it returns, and the types of parameters it accepts. Prototypes usually go at the beginning of the file, before the main() function, or in a separate header file (.h).

Syntax: returnType functionName(dataType parameter1, dataType parameter2);

// Prototype for a function that takes two ints and returns an int
int addNumbers(int a, int b);

B. Function Definition

The definition provides the actual body of the function—the code block that executes when the function is called.

Syntax:

returnType functionName(dataType parameter1, dataType parameter2) {
    // Body of the function
    return result; // Must match returnType
}

C. Function Call

The call (or invocation) is the statement that executes the function’s code.

Syntax: variable = functionName(argument1, argument2);

2. Example: A Simple Function

Here is a complete example demonstrating all three components:

#include <stdio.h>

// 1. Prototype (Declaration)
int multiply(int x, int y); 

int main() {
    int result;
    int num1 = 5;
    int num2 = 10;
    
    // 3. Function Call
    result = multiply(num1, num2); 
    
    printf("The result is: %d\n", result);
    return 0;
}

// 2. Function Definition
int multiply(int x, int y) {
    int product = x * y;
    return product; // Returns the integer product
}

3. Parameters and Arguments

• Parameters: The variables declared in the function prototype and definition (e.g., int x, int y in multiply). They are placeholders for the values the function expects.
• Arguments: The actual values passed to the function when it is called (e.g., num1, num2 in the call).

4. Functions Returning No Value (void)

If a function performs an action but does not need to return any data, its return type must be specified as void.

Example: void function

void print_message(void) {
    printf("This function performs a task but returns nothing.\n");
    // No return statement required
}

int main() {
    print_message(); // Call the function
    return 0;
}

Note: When a void function accepts no parameters, it is good practice to explicitly use (void) in the parameter list, though an empty list () is also common.