How to read a text file 3 characters at a time? - c

Question

I'm learning to program in C, and right now my homework is like this. I am supposed to be able to take a text file with words/letters and my program should print the ascii codes+1 of the letters it gets. It's "encoding" it. So for example the letter "A" would be printed as 066. So my problem is the program needs to also be able to decode those ascii codes back to letters, but I don't know how to get my program to read 3 numbers at once. I used
c = fgetc(pF);
while (c != EOF) {
fprintf(pF2,"%03i",c+1);
c = fgetc(pF);
to read the file one character at a time. Should I use something similar to get the 3 numbers I need, or would it be something completely different?
#include "stdio.h"
#include "stdlib.h"
int encode(){
char * fName = "testR.txt";
char * fName2="testW.txt";
FILE * pF;
FILE * pF2;
char c, cArray[500];
int i=0;
pF = fopen(fName, "r");
if ( pF == NULL ) {
printf("Error: The specified file could not be opened.\n");
return -1;
}
pF2 = fopen(fName2, "w");
c = fgetc(pF);
while (c != EOF) {
fprintf(pF2,"%03i",c+1);
c = fgetc(pF);
}
fclose(pF);
fclose(pF2);
return;
}
char decode(){
char * fName = "testW.txt";
char * fName2="testW2.txt";
char * buf;
FILE * pF;
FILE * pF2;
char cArray[500];
char buffer[4];
buffer[3] = '\0';
pF = fopen(fName, "r");
if ( pF == NULL ) {
printf("Error: The specified file could not be opened.\n");
return -1;
}
pF2 = fopen(fName2, "w");
while (fread(buffer, 3, 1, stdin)){
putchar((atoi(buffer)-1) & 0xFF);
}
fclose(pF);
fclose(pF2);
}
int main (int argc, char *argv[]){
char c;
if ( argc != 2 ) {
printf("Incorrect number of arguments.\n");
exit(-1);
}
if (strcmp(argv[1], "-e") == 0){
encode();
}
if (strcmp(argv[1], "-d") == 0){
decode();
}
return 0;
}
I am aware that my code is a mess and there are probably things that shouldn't be there from all the changes I've been trying to make =[

Answer 1

Encoding:
int c;
while ((c = getchar()) != EOF)
printf("%.3d", (c+1)&0xFF);
Decoding:
char buffer[4];
buffer[3] = '\0';
while (fread(buffer, 3, 1, stdin))
putchar((atoi(buffer)-1) & 0xFF);
Technically, if the output does not end with a newline when encoding, the output is not a text file. There's no error checking on the input. The fread() will return 1 when it reads a triplet of characters; otherwise, it will return 0.
Working Programs
e3.c — encryption
#include <stdio.h>
int main(void)
{
int c;
while ((c = getchar()) != EOF)
printf("%.3d", (c+1)&0xFF);
return 0;
}
d3.c — decryption
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char buffer[4];
buffer[3] = '\0';
while (fread(buffer, 3, 1, stdin))
putchar((atoi(buffer)-1) & 0xFF);
return 0;
}
Sample output
$ e3 < e3.c
036106111100109118101102033061116117101106112047105063011011106111117033110098106111041119112106101042011124011033033033033106111117033100060011011033033033033120105106109102033041041100033062033104102117100105098115041042042033034062033070080071042011033033033033033033033033113115106111117103041035038047052101035045033041100044050042039049121071071042060011033033033033115102117118115111033049060011126011011$
$
The dollar at the end of the line is the prompt.
$ e3 <e3.c | d3
#include <stdio.h>
int main(void)
{
int c;
while ((c = getchar()) != EOF)
printf("%.3d", (c+1)&0xFF);
return 0;
}
$ e3 <e3.c | d3 | diff e3.c -
$
Basic round-tripping the encoding and decoding shows that the code works.

Answer 2

while (fread(buf, 1, 3, pF) == 3)
{
...
}