Why does C program print 0D instead of 0? (When EOF sent as Ctrl+D) [duplicate]

Question

OSX 10.6.8, GCC 4.2 86_64

#include <stdio.h>

/* count lines in input */
main()
{
    int c, nl;

    nl = 0;
    while ((c = getchar()) != EOF)
        if (c == '\n')
            ++nl;
    printf("%d\n", nl);
}

Run

./a.out

press ctrl+d to send EOF

0D

It should be just 0. Why does it append D? What does it mean?

Answer 1

I've seen this one - it confused me, too.

The terminal is echoing ^D and then the 0 is output from the program, overwriting the caret.

You can demonstrate this by changing the print format in your program to "\n%d\n".

---

When asked 'Why?', I went exploring. The answer is in the tty settings. For my terminal, the output from stty -a is:

speed 9600 baud; 65 rows; 120 columns;
lflags: icanon isig iexten echo echoe -echok echoke -echonl echoctl
    -echoprt -altwerase -noflsh -tostop -flusho pendin -nokerninfo
    -extproc
iflags: -istrip icrnl -inlcr -igncr ixon -ixoff ixany imaxbel iutf8
    -ignbrk brkint -inpck -ignpar -parmrk
oflags: opost onlcr -oxtabs -onocr -onlret
cflags: cread cs8 -parenb -parodd hupcl -clocal -cstopb -crtscts -dsrflow
    -dtrflow -mdmbuf
cchars: discard = ^O; dsusp = ^Y; eof = ^D; eol = <undef>;
    eol2 = <undef>; erase = ^?; intr = ^C; kill = ^X; lnext = ^V;
    min = 1; quit = ^\; reprint = ^R; start = ^Q; status = ^T;
    stop = ^S; susp = ^Z; time = 0; werase = ^W;

Notice the echoctl at the end of the second line - it is for 'echo control characters'.

$ stty -echoctl
$ cat > /dev/null
asdsadasd
$ stty echoctl
$ cat > /dev/null
asasada^D
$

You can't see it, but for each cat command, I typed a Control-D at the end of the line of asd characters, and a second one after hitting return. The prompt erased the second echoed ^D in the second example.

So, if you don't like the control characters being echoed, turn the echoing off:

stty -echoctl

The shell can also get in the way; I experimented with Control-R and my shell (bash) decided to go into

(reverse-i-search)`': aasadasdadadasdadadadadadsad

I'd typed the unoriginal sequence of 'asd' characters and then typed Control-R, and this is where I ended up in the shell. I interrupted; I'm not sure what a reverse-i-search is, but I suspect it is Emacs-ish; it was not what I expected.