Experiment 3

Q1)Write a program in C using system calls that lets users choose to copy either the first half or the second half of a file by entering 1 or 2
A1)#include <stdio.h>

#include <stdlib.h>

#include <fcntl.h>

#include <unistd.h>



#define BUF_SIZE 4096



int main(int argc, char *argv[]) {

    if (argc != 3) {

        fprintf(stderr, "Usage: %s <source_file> <destination_file>\n", argv[0]);

        exit(EXIT_FAILURE);

    }



    char *source_file = argv[1];

    char *dest_file = argv[2];



    int source_fd = open(source_file, O_RDONLY);

    if (source_fd == -1) {

        perror("Failed to open source file");

        exit(EXIT_FAILURE);

    }



    off_t file_size = lseek(source_fd, 0, SEEK_END);

    if (file_size == -1) {

        perror("Failed to get file size");

        close(source_fd);

        exit(EXIT_FAILURE);

    }



    off_t half_size = file_size / 2;

    off_t offset = 0;



    int choice;

    printf("Enter 1 to copy the first half or 2 to copy the second half: ");

    scanf("%d", &choice);



    if (choice == 1) {

        // Copy the first half

        offset = 0;

    } else if (choice == 2) {

        // Copy the second half

        offset = half_size;

    } else {

        fprintf(stderr, "Invalid choice\n");

        close(source_fd);

        exit(EXIT_FAILURE);

    }



    // Create destination file

    int dest_fd = open(dest_file, O_WRONLY | O_CREAT | O_TRUNC, 0666);

    if (dest_fd == -1) {

        perror("Failed to create destination file");

        close(source_fd);

        exit(EXIT_FAILURE);

    }



    // Set the offset in source file

    if (lseek(source_fd, offset, SEEK_SET) == -1) {

        perror("Failed to set offset in source file");

        close(source_fd);

        close(dest_fd);

        exit(EXIT_FAILURE);

    }



    // Copy data from source to destination

    char buffer[BUF_SIZE];

    ssize_t bytes_read, bytes_written;

    while ((bytes_read = read(source_fd, buffer, sizeof(buffer))) > 0) {

        bytes_written = write(dest_fd, buffer, bytes_read);

        if (bytes_written != bytes_read) {

            perror("Write error");

            close(source_fd);

            close(dest_fd);

            exit(EXIT_FAILURE);

        }

    }



    if (bytes_read == -1) {

        perror("Read error");

        close(source_fd);

        close(dest_fd);

        exit(EXIT_FAILURE);

    }



    printf("File copied successfully.\n");



    // Close file descriptors

    close(source_fd);

    close(dest_fd);



    return 0;

}

Q 2: Create a C program using system calls that keeps reading from the console until the user types '$'. Save the input data to a file called 'input.txt"
A2)

#include <stdio.h>

#include <stdlib.h>

#include <fcntl.h>

#include <unistd.h>



#define BUF_SIZE 4096



int main() {

    char buffer[BUF_SIZE];

    int input_fd;

    ssize_t bytes_read;



    // Open or create input.txt for writing

    input_fd = open("input.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666);

    if (input_fd == -1) {

        perror("Failed to open input.txt for writing");

        exit(EXIT_FAILURE);

    }



    printf("Type your input. Type '$' to end.\n");



    // Read from console until '$' is entered

    while ((bytes_read = read(STDIN_FILENO, buffer, sizeof(buffer))) > 0) {

        // Write to input.txt

        if (write(input_fd, buffer, bytes_read) != bytes_read) {

            perror("Write error");

            close(input_fd);

            exit(EXIT_FAILURE);

        }



        // Check if '$' is entered

        if (buffer[0] == '$') {

            break;

        }

    }



    if (bytes_read == -1) {

        perror("Read error");

        close(input_fd);

        exit(EXIT_FAILURE);

    }



    printf("Input saved to input.txt.\n");



    // Close input file

    close(input_fd);



    return 0;

}

Q3)Write a C program that encrypts a text file using a simple encryption technique and saves the encrypted content to a new file.

Requirements:

Input: Provide a text file named "input.txt" with plain text content

Encryption Technique: Shift each character in the file content by a fixed number
A3)
#include <stdio.h>



#define SHIFT_AMOUNT 3 // You can change this to any desired value



int main() {

    FILE *input_file, *output_file;

    char input_filename[] = "input.txt";

    char output_filename[] = "encrypted.txt";

    char ch;



    // Open input file for reading

    input_file = fopen(input_filename, "r");

    if (input_file == NULL) {

        perror("Error opening input file");

        return 1;

    }



    // Open output file for writing

    output_file = fopen(output_filename, "w");

    if (output_file == NULL) {

        perror("Error opening output file");

        fclose(input_file);

        return 1;

    }



    // Encrypt and write content to the output file

    while ((ch = fgetc(input_file)) != EOF) {

        // Check if character is a letter

        if ((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) {

            // Encrypt uppercase letters

            if (ch >= 'A' && ch <= 'Z') {

                ch = 'A' + (ch - 'A' + SHIFT_AMOUNT) % 26;

            }

            // Encrypt lowercase letters

            else if (ch >= 'a' && ch <= 'z') {

                ch = 'a' + (ch - 'a' + SHIFT_AMOUNT) % 26;

            }

        }

        // Write encrypted character to the output file

        fputc(ch, output_file);

    }



    // Close files

    fclose(input_file);

    fclose(output_file);



    printf("File encrypted successfully. Encrypted content saved to encrypted.txt.\n");



    return 0;

}