Functions

This document covers Functions & Program Structure in C.

Functions are a fundamental building block in C programming that allow for modular, reusable, and maintainable code. Understanding function structure and program organization is key to writing efficient C programs.

In this section, we will cover:

• Function structure in modern C.
• Multi-function programs and how to split code into multiple functions.
• Function arguments, including pass-by-value and pass-by-reference.
• Using pointers with function arguments (briefly, as pointers will be covered in depth later).

Function Structure in Modern C

A function in C consists of:

• Return type: Specifies the type of value the function returns.
• Function name: Identifier used to call the function.
• Parameter list: Inputs passed to the function (optional).
• Function body: Contains the executable statements.
• Return statement: (Optional) Returns a value if the function is non-void.

Example of a Simple Function

#include <stdio.h>

// Function prototype (declaration)
int add(int a, int b);

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

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

info

• Function declaration (prototype) before main() allows the compiler to know about the function before it's defined.
• Function definition contains the actual logic.
• Function call in main() invokes the function.

Multi-Function Programs in Modern C

For larger programs, breaking code into multiple functions makes it more readable and maintainable.

#include <stdio.h>

void greet();
int square(int n);

int main() {
    greet();
    int num = 4;
    printf("Square of %d is %d\n", num, square(num));
    return 0;
}

void greet() {
    printf("Hello! Welcome to the program.\n");
}

int square(int n) {
    return n * n;
}

info

• greet() is a void function (does not return a value).
• square(int n) returns an integer value.
• The program remains structured and readable.

Function Arguments in C

Function arguments allow passing values to functions.

Pass-by-Value (Default Behavior)

The function receives a copy of the variable. Changes made inside the function do not affect the original variable. This is the most common way of passing arguments in C.

Example

#include <stdio.h>
void modify(int x);

int main() {
    int num = 10;
    modify(num);
    printf("Value after function call: %d\n", num); // Still 10
    return 0;
}

void modify(int x) {
    x = 20; // This change is local to the function
}

Advantages and Disadvantages of Pass by Value

Advantages:

• Prevents unintended modifications of the original variable.
• Ensures function encapsulation (modifications remain within the function).

Disadvantages:

• Copies the entire value, which can be inefficient for large data structures.

Pass-by-Reference Using Pointers

Instead of passing a copy, we pass a memory address. Changes inside the function affect the original variable. This method is used when modifying data directly is needed.

note

This section contain information about pointers. Pointers are covered in depth in the next chapter.

Example

#include <stdio.h>
void modify(int *x);

int main() {
    int num = 10;
    modify(&num);
    printf("Value after function call: %d\n", num); // Now 20
    return 0;
}

void modify(int *x) {
    *x = 20; // Modifies the actual variable
}

info

• int *x indicates a pointer parameter.

• modify(&num); passes the memory address of num.

• *x = 20; modifies the original num variable.

Advantages and Disadvantages of Pass by Reference

Advantages:

• Avoids unnecessary copying, making it more efficient for large data structures.
• Allows functions to modify the original variable directly.

Disadvantages:

• Requires explicit handling of pointers, increasing complexity.
• Modifications persist outside the function, which may lead to unintended side effects.

Return Values and Void Functions

Functions can return values or perform actions without returning anything.

Return a value

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

Void Functions (No Return Value)

void printMessage() {
    printf("This is a message.\n");
}

const in Function Parameters

Using const in function parameters prevents modification of input values.

void printMessage(const char *msg) {
    printf("%s\n", msg);
}

info

Prevents accidental modification of the input string. Helps in optimizing the code as the compiler knows the value won’t change.