Functions

Introduction

In C, a function is a self-contained block of code designed to perform a specific task. Functions increase code modularity, promote reusability, and allow division of work in larger programs. They enable you to break a large program into smaller, manageable chunks.

Function Structure

A function in C typically involves the following components:

1. Function Declaration (Prototype)

2. Function Definition

3. Function Call

1. Function Declaration (Prototype)

A prototype tells the compiler about the function’s return type, name, and parameter list. This is useful when the function is defined after the main() function.

Syntax:

return_type function_name(type1, type2, ...);

Example:

int add(int, int);  // Prototype

2. Function Definition

It provides the actual body of the function — what the function does.

Syntax:

return_type function_name(parameter_list) {
    // body of the function
}

Example:

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

3. Function Call

To execute the function, we use a function call, usually from the main() function.

Example:

int main() {
    int result = add(5, 10);
    printf("%d", result);
    return 0;
}

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Return Type

• A function can return only one value.
• The return type of a function should be the same as the data type of the value to be returned.
• If no value is to be returned, the return type should be void.

Example:

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

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Parameters and Arguments

• Parameters are placeholders defined in the function definition.

• Arguments are actual values passed when calling the function.

Example:

void display(int x, float y) { // Parameters -> x, y
    printf("%d %.2f", x, y);
}
 
int main() {
    display(3, 1.5); // Arguments 3, 1.5
}

Parameter Passing Techniques

a) Call by Value

Default in C. A copy of the argument is passed; original value remains unchanged.

void modify(int x) {
    x = 10;
}

b) Call by Reference (via pointers)

We pass the address of variables using pointers. The ==actual value can be changed==.

void modify(int *x) {
    *x = 10;
}

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Types of Functions

a) User-defined Functions

Functions created by the programmer to perform specific tasks.

b) Library Functions

Predefined in C libraries.

Examples:

• printf(), scanf() — in <stdio.h>

• sqrt(), pow() — in <math.h>

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Recursion in Functions

Recursion is when a function calls itself to solve a problem.

Example:

int factorial(int n) {
    if (n == 0) return 1;
    return n * factorial(n - 1);
}

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Memory Management

Each function call uses a stack frame on the function call stack. This contains:

• Local variables

• Return address

• Parameters

When the function finishes, its frame is popped off the stack, and memory is released. This is particularly useful when using recursion or back-tracing.

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Function Pointers

Used to store addresses of functions and call them dynamically. They essentially point to the starting address of a function in memory. Later, these pointers can be used as regular functions and can be called using the () parenthesis notation.

Example:

void hello() {
    printf("Hello");
}
 
int main() {
    void (*fptr)() = hello;
    fptr();  // Calls hello()
}