View Full Version : [SOLVED] EOF (stdin) in Ubuntu 10.04 Lucid Lynx

July 30th, 2011, 11:36 PM
A friend learning C using KR asked me a question that pointed out a behavior that confuses me too. When going over the early book examples (e.g., ch. 1.5.1) in which you just mirror input and output such as (straight from book)....

#include <stdio.h>


int c;

while ( (c = getchar()) != EOF)

...you need to hit twice ^d to get out of the loop. The effect is as if one ^d only was hit. Any explanations?

July 30th, 2011, 11:44 PM
I only need to hit ^D once here... But the behavior of hitting ^D may differ in different terminal emulators or even different shells. Contrary to popular belief, ^D does not send EOF.

July 30th, 2011, 11:47 PM
Ah. So it could be just a terminal emulation oddity that one, for practical purposes, accepts and ignores? Or is there another way to communicate 'EOF' to a stdin stream?

July 30th, 2011, 11:51 PM
Does it get stuck if you use files? Something like this:

./yourprogram < afile > otherfile

July 30th, 2011, 11:53 PM
Basically, you do not "communicate" EOF to the process: EOF is by definition not a character, it is the name given to the value getchar will return when it can't read any character. So anything that causes the input to stop will make getchar return EOF.

July 31st, 2011, 12:01 AM
Good idea. Your piping suggestion copies the input file perfectly fine to the output file.

I played with it more on my Toshiba (grub Ubuntu/W7), and also under Lucid Lynx. For stdin, the behavior in slightly more involved (slightly) programs playing with stdin gets weird while more or less on target.

In sum, I take it as a terminal emulation quirk in this setup affecting only stdin, and am fine with it?

July 31st, 2011, 12:04 AM
@ Bachstelze (nice name, I'm German too while living away from home)

I don't understand how or if that explains an answer to my question though. If it does, kindly try again.

July 31st, 2011, 12:05 AM
In sum, I take it as a terminal emulation quirk in this setup affecting only stdin, and am fine with it?

Probably. When you use a file as input it's simple: the output ends when all the characters in the file have been read, so there is no ambigity as to when getchar will return EOF. When you're reading from a terminal, however, it basically depends on when exactly your terminal "cuts" the input stream. It rarely really matters, though, whether you have to press ^D once or twoce.

July 31st, 2011, 12:16 AM
Thanks everyone. Closing this now.

July 31st, 2011, 01:02 AM
Thanks everyone. Closing this now.

Sorry for continuing a closed thread, but I think the situation when you have to type ^D twice is when you did not just enter a newline.

Also, I think it's the terminal (pty) driver that has this behaviour, not the terminal emulator. It may be documented in some manual page in section 7.