C/C++ Caesar Cipher on a .txt file?

Hi, How would I go about using the Caesar Cipher technique on a .txt file? E.g. shifting each letter by 3?

Here is my code?

Code:

#include <stdio.h>
#include <conio.h>

#define CNTRLZ 0x1A

char sourceFileName[] = "H:\\text.txt";
char commStreamName[] = "COM1";
FILE *commStream;
FILE *SouceFile;
char charToSend;
char ReadAccess[] = "r";
char WriteAccess[] = "w";
char errorMsg[] = "fail to open data file\n";
char fileopenflag[] = "r";
char EndOfFile[] = "EOF";

int main(void) {
FILE *fp;
FILE *dp;
int c;

if ((dp = fopen("COM1", "r")) == NULL) {
printf("fail to open COM port\n"); return 1;
}


if ((fp = fopen( sourceFileName, "w")) == NULL) {
printf("fail to open data file\n"); return 1;
}


while ((c = fgetc(fp )) != EOF) {
fputc( c, dp);
putch(c);

}

fputc('*', dp);
fclose(dp);
fclose(fp);
}

Any ideas where to start?

 

Did you even pay any attention to your other thread, with the same code?
http://www.go4expert.com/showthread.php?t=7849

 

You weren't no help as usual.