How to do it…

1. Open the source file in read-only mode using the following code:

fp = fopen (argv [1],"r");

2. If the file does not exist or does not have enough permissions, an error message will be displayed and the program will terminate, as shown in the following code:

if (fp == NULL) {
    printf("%s file does not exist\n", argv[1]);
    exit(1);
 }

3. Open the destination file, the file where the encrypted text will be written, in write-only mode using the following code:

fq = fopen (argv[2], "w");

4. Read a line from the file and access each of its characters using the following code:

fgets(buffer, BUFFSIZE, fp);

5. Using the following code, subtract a value of 45 from the ASCII value of each of the characters in the line to encrypt that character:

for(i=0;i<n;i++)
    buffer[i]=buffer[i]-45;

6. Repeat step 5 until the line is over. Once all of the characters in the line are encrypted, write the encrypted line into the destination file using the following code:

fputs(buffer,fq);

7. Check whether the end of the file has been reached using the following code:

while (!feof(fp))

8. Close the two files using the following code:

fclose (fp);
fclose (fq);

The preceding steps are shown in the following diagram:

Figure 6.4

The encryptfile.c program to encrypt a file is as follows:

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

#define BUFFSIZE 255
void main (int argc, char* argv[])
{
    FILE *fp,*fq;
    int  i,n;
    char buffer[BUFFSIZE];

    /* Open the source file in read mode */
    fp = fopen (argv [1],"r");
    if (fp == NULL) {
        printf("%s file does not exist\n", argv[1]);
        exit(1);
    }
    /* Create the destination file.  */
    fq = fopen (argv[2], "w");
    if (fq == NULL) {
        perror ("An error occurred in creating the file\n");
        exit(1);
    }
    while (!feof(fp))
    {
        fgets(buffer, BUFFSIZE, fp);
        n=strlen(buffer);
        for(i=0;i<n;i++)
            buffer[i]=buffer[i]-45;
        fputs(buffer,fq);
    }
    fclose (fp);
    fclose (fq); 
}

Now, let's go behind the scenes.