为什么单个数字会附加“D”（在 C 输出中）？

Question

Why single digit numbers are appended with "D" (in C output)?

The following code

#include <stdio.h>

int main(void)
{
    int c = 0;

    printf("%d\n", c); 

    return 0;
}

once compiled & ran, outputs 0 , as I would expect it to.
Though, this code

#include <stdio.h>

int main(void)
{
    int c = 0;

    while (getchar() != EOF) {
        ++c;
    }

    printf("%d\n", c); 

    return 0;
}

once compiled & ran, after triggering EOF right away -- outputs 0D for some reason, though value of c (as far as I can see) should be absolutely the same as in the first case.
Same happens for all the single digit numbers (ie 1D , 2D , 3D ... 9D ), starting with 10 the appending D is not seen anymore.

I'd like to know:

1. Why D is appended to the output in the second case, but not in the first (even though c should hold the same value)?

2. Is it possible to avoid this D appending (and how, if it is)?

Answer 1

Your code simply cannot output a "D" for whatever reason. Not unless something really iffy happens, like a bug in the compiler or glibc. Or maybe a bitflip in memory.

The "D" is very likely due to the terminal you're using, but your code simply CANNOT output it.

Well, there is a very small chance. If you read more than INT_MAX characters, then the signed integer c will overflow, thus invoking undefined behavior. It's not likely that this would output a "D", but it's possible.

Answer 2

When posting the question, I thought this behaviour has something to do with cases like this , but, as it turned out, it has nothing to do with the code and is solely terminal behaviour .
Suggestion by Some programmer dude in a comment, is exactly right: terminal simply outputs ^D and output of the program then overwrites ^ , but not D , thus for single-digit numbers output ends up being appended with D .
Two (or more) digit numbers simply overwrite the whole ^D .

I was able to verify this by changing EOF triggering sequence to Ctrl + L (by stty eof ^L ), then the output becomes 1L , 2L , 3L ... 9L .
As Jonathan Leffler said in comment above disabling echoing of control characters (by stty -echoctl ) would solve the issue.