Why do I need to press CTRL+D twice to break out of `while ((c=getchar())!=EOF)` in Ubuntu 14.10?

问题

I am new to C Programming and Ubuntu. I was reading the "The C Programming Language" by D.M Ritchie where I found the following code:

#include <stdio.h>

int main()
{
    int c;
    int nc=0;
    while((c = getchar()) != EOF)
    {
        nc++;
    }

    printf("%d Characters \n",nc);

    return 0;
}

But while running the program I enter "Hello" ,then CTRL+D twice to get the actual number of characters which is 5.

But when I enter "Hello" then CTRL+D once, nothing happens, the terminal still waits for input.

Why?

回答1:

Quoting @Veritas's comment,

On linux Ctrl-D only works when the buffer is already empty otherwise it just flushes it. Therefore unless he has pressed enter without any characters after that, he will have to press Ctrl-D twice.

This explains the issue. You have to press it twice because you , after typing Hello, did not press the Enter to flush the input into the stdin. So the first time you press CTRL+D, it flushes the data into the stdin. The second time you press it, EOF is sent.

来源：https://stackoverflow.com/questions/30137434/why-do-i-need-to-press-ctrld-twice-to-break-out-of-while-c-getchar-eof