Last character is not being overwritten while copying one file into another file containing pre existing text

Question

I am trying to write a simple C program that encrypts a text file using the offset cipher.

Here is what I want the program to do:

• Take a text file, read it character by character and after incrementing each character by some integer value (which increments the ASCII value of the character, resulting in some completely different characters to be obtained) overwrite the original text file such that it gets encrypted.

Here is my code: I am using an additional file named text.txt to store modified data and copy it back to the original file (named file.txt)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void encrypt(char *filename) {
    FILE *fp, *ft;
    int key = 1;
    char ch;
    fp = fopen(filename, "r+");
    ft = fopen("temp.txt", "w+");
    if (fp == NULL) {
        printf("Unable to open file");
        exit(1);
    }
    if (ft == NULL) {
        printf("Unable to generate encryption key");
        exit(2);
    }
    //modifying ASCII values and storing it in temp.txt
    while (1) {
        ch = fgetc(fp);
        if (ch == EOF)
            break;
        ch = ch + key; //offset
        fputc(ch, ft);
    }
    rewind(fp);
    rewind(ft);
    //copying temp to original file
    while (1) {
        ch = fgetc(ft);
        if (ch == EOF)
            break;
        fputc(ch, fp);
    }
    printf("The file has been encrypted successfully");
    fclose(fp);
    fclose(ft);
}

void decrypt(char *filename) {
    FILE *fp, *ft;
    char ch;
    fp = fopen(filename, "r+");
    ft = fopen("temp.txt", "w+");
    if (fp == NULL) {
        printf("Unable to open file");
        exit(1);
    }
    if (ft == NULL) {
        printf("Unable to access encryption key");
        exit(2);
    }
    //decrementing the chars by the same ASCII value to get back original text into temp.txt
    while (1) {
        ch = fgetc(fp);
        if (ch == EOF)
            break;
        ch = ch - 1;
        fputc(ch, ft);
    }
    rewind(fp);
    rewind(ft);
    //copying decrypted data to original file
    while (1) {
        ch = fgetc(ft);
        if (ch == EOF)
            break;
        fputc(ch, fp);
    }
    printf("File Decrypted Successfully");
}

int main(void) {
    int choice;
    char name[100], ch;
    FILE *fs, *ft;
    printf("To encrypt/decrypt a file: ");
    while (1) {
        printf("\nMENU");
        printf("\n1.Encrypt\n2.Decrypt\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
        fflush(stdin);
        switch (choice) {
          case 1:
            printf("Enter the name of the file to be encrypted: ");
            fgets(name, 99, stdin);
            strtok(name,"\n");
            encrypt(name);
            return 0;
            break;
          case 2:
            printf("Enter the name of the file to be decrypted: ");
            fgets(name, 99, stdin);
            strtok(name,"\n");
            decrypt(name);
            return 0;
            break;
        }
    }
}

But, the problem is that the encypt function works as expected while modifying the original file and storing it in temp.txt, but when it is copying the temp.txt file back into the original file, the last character is not being overwritten.

Below are some links to the output screenshots

What is wrong with my code? does it have to do something with the EOF of temp.txt or am I missing something obvious?

Answer 1

I tried out your code and ran into an initial problem.

First, let me preface my problems and solution with the fact that I tested out your code on a Linux virtual machine.

The use of "fget" with the "stdin" handle did not work when I ran your code. It would skip right by that prompt without allowing me to enter a file name, say that no file name was found and end. So, to compensate for that, I switched out the fget input statements with "scanf" statements, as you were using that function earlier in your code.

        switch (choice)
        {
        case 1:
            printf("Enter the name of the file to be encrypted: ");
            scanf("%s", name);
            //strtok(name,"\n");
            encrypt(name);
            return 0;
            break;
        case 2:
            printf("Enter the name of the file to be decrypted: ");
            scanf("%s", name);
            //strtok(name,"\n");
            decrypt(name);
            return 0;
            break;
        }

When I made that change and entered in the name of my test file ("Notepad.txt"), the file encryption data in the temporary file matched what was stored in the input file that was updated.

I don't know if you are testing this out on Windows system or a Linux system, but all I can say it worked for me. Others with more experience might chime in as to the difference in file opening, file puts, and so forth between the operating systems, but if you are curious and have access to a Linux machine (or virtual machine) you might try it out over there. And to eliminate the fget statements as a "suspect", you might try out the code switch I did when prompting for a file.

At the very least, food for thought.