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File Handling in C

File handling allows us to store data permanently in files instead of just keeping it in memory. In C, we use the FILE pointer and functions like fopen(), fprintf(), fscanf(), fclose(), etc.

File Handling modes in C

Mode Description
"r" Read-only mode. The file must exist.
"w" Write mode. Creates a new file or overwrites an existing file.
"a" Append mode. Creates a new file if it doesn’t exist, otherwise appends to the existing file.
"r+" Read and write mode. The file must exist.
"w+" Read and write mode. Creates a new file or overwrites an existing file.
"a+" Read and append mode. Creates a new file if it doesn’t exist.
"rb" Read mode for binary files. The file must exist.
"wb" Write mode for binary files. Creates a new file or overwrites an existing file.
"ab" Append mode for binary files. Creates a new file if it doesn’t exist, otherwise appends to the existing file.
"rb+" Read and write mode for binary files. The file must exist.
"wb+" Read and write mode for binary files. Creates a new file or overwrites an existing file.
"ab+" Read and append mode for binary files. Creates a new file if it doesn’t exist.
Writing in File
#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "w");  // Open file in write mode and also creates if not created
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;  // Exit if file opening fails
    }

    char name[50];
    int age;

    printf("Enter name: ");
    scanf("%s", name);
    printf("Enter age: ");
    scanf("%d", &age);

    fprintf(file, "Name: %s\nAge: %d\n", name, age);  // Writing to file
    fclose(file);  // Close the file

    printf("Data written successfully!\n");
    return 0;
}
Reading Data from a File
#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r");  // Open file in read mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char name[50];
    int age;

    fscanf(file, "Name: %s\nAge: %d\n", name, &age);  // Read data from file
    fclose(file);

    printf("Data from file:\n");
    printf("Name: %s\nAge: %d\n", name, age);
    
    return 0;
}
Append (add) in to File
#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "a"); // Open file in append mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    fprintf(file, "This is a new line added to the file.\n"); // Append data
    fclose(file);

    printf("Data appended successfully!\n");
    return 0;
}
Deleting File
#include <stdio.h>

int main() {
    if (remove("data.txt") == 0) {
        printf("File deleted successfully!\n");
    } else {
        printf("Error deleting file!\n");
    }
    return 0;
}
File Handling + Structure Example(Employee Management)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Define structure for Employee
struct Employee {
    int id;
    char name[50];
    float salary;
};

// Function to add an employee
void addEmployee() {
    FILE *fp = fopen("employees.dat", "ab"); // Open file in append mode
    struct Employee emp;
    
    printf("Enter Employee ID: ");
    scanf("%d", &emp.id);
    getchar(); // To consume newline

    printf("Enter Employee Name: ");
    fgets(emp.name, sizeof(emp.name), stdin);
    emp.name[strcspn(emp.name, "\n")] = 0; // Remove newline character

    printf("Enter Salary: ");
    scanf("%f", &emp.salary);

    fwrite(&emp, sizeof(struct Employee), 1, fp); // Write to file
    fclose(fp);

    printf("Employee added successfully!\n");
}

// Function to view all employees
void viewEmployees() {
    FILE *fp = fopen("employees.dat", "rb");
    struct Employee emp;

    if (fp == NULL) {
        printf("No records found!\n");
        return;
    }

    printf("\n----- Employee Records -----\n");
    while (fread(&emp, sizeof(struct Employee), 1, fp)) {
        printf("ID: %d | Name: %s | Salary: %.2f\n", emp.id, emp.name, emp.salary);
    }
    fclose(fp);
}

// Function to delete an employee
void deleteEmployee() {
    FILE *fp = fopen("employees.dat", "rb");
    FILE *temp = fopen("temp.dat", "wb");
    struct Employee emp;
    int id, found = 0;

    if (fp == NULL) {
        printf("No records found!\n");
        return;
    }

    printf("Enter Employee ID to delete: ");
    scanf("%d", &id);

    while (fread(&emp, sizeof(struct Employee), 1, fp)) {
        if (emp.id != id) {
            fwrite(&emp, sizeof(struct Employee), 1, temp);
        } else {
            found = 1;
        }
    }
    fclose(fp);
    fclose(temp);

    remove("employees.dat");
    rename("temp.dat", "employees.dat");

    if (found)
        printf("Employee deleted successfully!\n");
    else
        printf("Employee ID not found!\n");
}

// Main menu
int main() {
    int choice;
    
    do {
        printf("\nEmployee Management System\n");
        printf("1. Add Employee\n");
        printf("2. View Employees\n");
        printf("3. Delete Employee\n");
        printf("4. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
        getchar(); // To consume newline

        switch (choice) {
            case 1:
                addEmployee();
                break;
            case 2:
                viewEmployees();
                break;
            case 3:
                deleteEmployee();
                break;
            case 4:
                printf("Exiting program...\n");
                break;
            default:
                printf("Invalid choice! Please try again.\n");
        }
    } while (choice != 4);

    return 0;
}