printf printing extra "D" after getchar() call

Question

I'm trying to work through "The C Programming Language", and I'm running into some issues with printf and the EOF character. I'm working the the mac terminal and compiling with clang.

Running this code:

#include <stdio.h>

main()
{
    int val;
    while ((val = getchar()) != EOF)
        printf("%d\n", val);
    /*val = 5;*/
    /*printf("hi\n");*/
    /*printf("%d\n", val);*/
    printf("%d\n", val);
}

works like I would expect, blocking till I enter a character then printing: "*character code*\n10\n", except for ctrl-d, which prints "-1" then exits.

After uncommenting the "val = 5;" statement however, entering "ctrl-d" causes it to print: "5D".

I messed around with it and found that printing val a second time (the third commented statement) will result in only one "D": "5D\n5", and that printing a constant before the variables (the second commented statement) stops the "D" from appearing: "hi\n5\n5".

I absolutely do not want the D and if anyone could explain how to remove it, I would be very grateful.

Could you provide with one clear example of your code and expected behaviour? There are some inconsistencies between your text and your code which makes it hard to follow :) — Gerard van Helden, Jun 02 '15 at 06:23

score 2 · Accepted Answer · edited Jun 02 '15 at 06:47

2

So, what happens is the console input is printing what you type. Just like if you type the letter A, the letter A gets printed. The CTRL-D gets printed to the stdout as ^D.

If you print out 1 character, the ^ is overwritten. If you print out 2 characters, both the ^ and D are overwritten. So, -1 overwrites it, hi overwrites it, but 1 character will not.

edited Jun 02 '15 at 06:47

Jonathan Leffler

730,956
141
904
1,278

answered Jun 02 '15 at 06:42

Garr Godfrey

8,257
2
25
23

This is 'normal' on Mac; I'm not so sure about other systems, though I'm sure you can get it to happen on Linux too. The `stty` setting `echoctl` controls this, I believe (check with `stty -a`). – Jonathan Leffler Jun 02 '15 at 06:48
Actually, Linux (Ubuntu 14.04) does not echo `^D` when that is the EOF. It will echo `^A`, `^B`, etc for a number of other characters — but beware of those which have a special meaning. Mac does echo `^D` too. – Jonathan Leffler Jun 02 '15 at 07:05

printf printing extra "D" after getchar() call

1 Answers1