How is this program meant to work?

Question

I've been reading The C Programming Language by Kernighan and Ritchie and very early own I came across a program that didn't work, even though I copied it directly from the book. Here is a screen cap of the description - https://i.stack.imgur.com/OjbtJ.png

It gets stuck in an infinite loop because anything I enter is obviously a keyboard entry, and it's checking in against EOF which is clearly not a keyboard entry.

#include <stdio.h> 
/* copy input to output; 1st version  */ 
main() 
{ 
    int c; 
    c = getchar(); 
    while (c != EOF) { 
        putchar(c); 
        c = getchar(); 
    } 
}

Surely an authority book on C like this can't have an error, am I missing something?

Tyler Brock · Accepted Answer · 2012-09-02T21:09:21.857

4

You would run it and use ctrl + d to signal the EOF (end of file) when reading from the command line.

If you were reading a stream of bytes from a file then it would have that in there automatically. However, since you are reading from the command line, one needs to signal the end of file manually by using the key sequence above.

How it actually works

EOF usually represents -1 in (glibc at least) which is why you can't just say while(c) { do work; } because any non-zero value is true -- hence EOF is true, just like any other positive number character returned by the call to getchar(). This is why you have to directly check to see if c matches EOF (-1) with the code c != EOF which appears in the stream when you send the EOF signal with ctrl + d.

edited Sep 02 '12 at 21:09

answered Sep 02 '12 at 20:41

Tyler Brock

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On Windows, it'd be Ctrl+Z, iirc. – Daniel Fischer Sep 02 '12 at 20:42
If I also remember correctly, and if it has not changed, in Windows and in DOS, ctrl-Z is placed as an actual character in the stream. In Unix, ctrl-D is not actually sent as a character, and it does not actually mean end-of-file, it means flush. That's what I wrote in http://stackoverflow.com/questions/1516122/how-to-capture-controld-signal anyway… – Pascal Cuoq Sep 02 '12 at 20:49
1

Thanks, ` ctrl + d ` worked a trick! ` ctrl + z ` didn't work – Geesh_SO Sep 02 '12 at 20:58
@Geesh_SO Yes, Ctrl-D for normal OSs, Ctrl-Z for Windows. Though on a normal OS, Ctrl-Z should suspend your programme to be continued per `fg`. – Daniel Fischer Sep 02 '12 at 21:37

score 2 · Answer 2 · answered Sep 02 '12 at 20:40

2

EOF is End-Of-File. Try Ctrl-D.

answered Sep 02 '12 at 20:40

corgichu

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Ctrl-D on most proper operating systems. – Paul R Sep 02 '12 at 20:41
1

Ctrl+C does something different than Ctrl+D. – Makoto Sep 02 '12 at 20:42
haha yeah. I just remembered and edited it. What's considered a "proper" OS? – corgichu Sep 02 '12 at 20:42
1

"Proper" = pretty much any OS other than Windows. ;) – Paul R Sep 02 '12 at 21:17

How is this program meant to work?

2 Answers2