Mastering C Programming: A Complete Tutorial for Beginners and Beyond

📘 Chapter 6: File Handling and Advanced Topics in C

🧠 Introduction

C isn’t just limited to screen-based input/output operations. Real-world applications often need to read from and write to files—think of saving logs, storing user data, or exporting reports. C handles files using file pointers and standard I/O functions defined in the <stdio.h> header.

In this chapter, we’ll cover:

• File handling basics
• Reading and writing files
• Error handling with files
• Structures and unions
• Enums and typedef
• Preprocessors and macros

📂 Part 1: File Handling in C

🔹 What is File Handling?

File handling is a way to store data permanently on storage devices such as hard drives or SSDs. It allows a program to create, read, write, append, and modify data in files.

✅ File Pointer

All file operations in C are done via a special pointer:

FILE *fp;

Defined in <stdio.h>, this FILE structure tracks information like the current position, the file descriptor, and buffering.

📋 File Modes

🔹 Opening a File

FILE *fp = fopen("data.txt", "r");

🔹 Closing a File

fclose(fp);

🔹 Writing to a File

FILE *fp = fopen("data.txt", "w");

fprintf(fp, "Hello World!\n");

fclose(fp);

🔹 Reading from a File

FILE *fp = fopen("data.txt", "r");

char line[100];

fgets(line, sizeof(line), fp);

printf("%s", line);

fclose(fp);

✅ Example: Writing and Reading Numbers

#include <stdio.h>

int main() {

    FILE *fp = fopen("numbers.txt", "w");

    for (int i = 1; i <= 5; i++) {

        fprintf(fp, "%d\n", i);

    }

    fclose(fp);

    fp = fopen("numbers.txt", "r");

    int num;

    while (fscanf(fp, "%d", &num) != EOF) {

        printf("%d ", num);

    }

    fclose(fp);

    return 0;

}

🔒 Error Handling

Always check if the file pointer is NULL:

if (fp == NULL) {

    printf("File not found or cannot be opened.\n");

    return 1;

}

🧱 Part 2: Structures and Unions

🔹 Structures in C

Structures allow grouping different types of variables under one name.

struct Person {

    char name[50];

    int age;

};

✅ Usage:

struct Person p1 = {"Alice", 30};

printf("%s is %d years old", p1.name, p1.age);

📋 Table: Structure Example

🔹 Array of Structures

struct Person people[2] = {

    {"Alice", 30},

    {"Bob", 25}

};

🔹 Nested Structures

struct Date {

    int day, month, year;

};

struct Employee {

    char name[50];

    struct Date dob;

};

🔹 Pointers to Structures

struct Person *ptr = &p1;

printf("%s", ptr->name);

🔹 Unions in C

A union uses the same memory for all its members. Only one member is active at a time.

union Data {

    int i;

    float f;

    char str[20];

};

union Data d;

d.i = 10;

📋 Table: Structure vs Union

🧮 Part 3: Enums and Typedef

🔹 Enums (Enumerations)

Enums assign names to a set of integer constants.

enum Color { RED, GREEN, BLUE };

• RED = 0, GREEN = 1, BLUE = 2 by default
• Can be customized:

enum Color { RED = 5, GREEN = 10, BLUE = 20 };

✅ Example:

enum Day { MON, TUE, WED };

enum Day today = TUE;

printf("%d", today);  // Output: 1

🔹 Typedef

Used to create new names for existing data types.

typedef unsigned int uint;

✅ Use Case:

uint age = 25;

⚙️ Part 4: Preprocessor Directives and Macros

🔹 Preprocessors

These are instructions executed before compilation.

✅ Examples:

🔹 Macros

Macros are constants or functions replaced at compile time.

#define PI 3.14159

#define AREA(r) (PI * r * r)

✅ Example:

#include <stdio.h>

#define SQUARE(x) ((x)*(x))

int main() {

    printf("%d", SQUARE(5));  // Output: 25

    return 0;

}

🧪 Sample Program: Store Struct Data in File

#include <stdio.h>

#include <stdlib.h>

struct Student {

    char name[50];

    int age;

};

int main() {

    struct Student s1 = {"John", 20};

    FILE *fp = fopen("student.dat", "wb");

    fwrite(&s1, sizeof(s1), 1, fp);

    fclose(fp);

    struct Student s2;

    fp = fopen("student.dat", "rb");

    fread(&s2, sizeof(s2), 1, fp);

    fclose(fp);

    printf("Name: %s, Age: %d", s2.name, s2.age);

    return 0;

}

🧠 Real-World Applications

📋 Chapter Summary Table