From:	"John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Simple Getch problem
Date:	Sat, 23 Nov 1996 23:33:13 -0800
Organization:	Three pounds of chaos and a pinch of salt
Lines:	63
Message-ID:	<3297FA39.15DD@cs.com>
References:	<5783b1$gbk AT yama DOT mcc DOT ac DOT uk>
Reply-To:	fighteer AT cs DOT com
NNTP-Posting-Host:	ppp102.cs.com
Mime-Version:	1.0
To:	mfepsdp AT afs DOT mcc DOT ac DOT uk
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Mr David Potts wrote:
> 
> I installed djgpp v2.01. I then found the following odd
> behavior. I compiled the following program:
  [snip]
> But when I ran it, it waited for a key press *then* printed
> the text rather than the other way round. What is going on?

DJGPP uses buffered output for stdin/stdout, which means that unless you
explicitly flush the output buffer, it doesn't print anything on the
screen until you a) print a newline, b) call any stdin-based input
function.  This is where your program fails, because you use getch()
instead of getc() for your input.  getch() is part of the <conio.h>
package, which uses BIOS calls to interface directly with the text
screen and keyboard; thus they completely bypass the <stdio.h>
functions.  It is in general a bad idea to mix functions from <stdio.h>
and <conio.h> unless you are very careful to flush all the stdin/stdout
buffers first.

To fix your code, do one of these things:

/* use the newline to flush stdout */
    printf( "Before getch\n" );
    getch( );

/* explicitly flush stdout */
    printf( "Before getch" );
    fflush( stdout );
    getch( );

/* use all stdio functions */
    printf( "Before getc" );
    getc( );

/* use all conio functions */
    cprintf( "Before getch" );
    getch( );

/* (dangerous) instead of stdout, use stderr which is not buffered */
    fprintf( stderr, "Before getch" );
    getch( );

Here's a brief list of the differences between stdio and conio
functions:

- stdio uses DOS i/o calls; conio writes to text memory and reads from
BIOS.
- stdio is line buffered by default; conio is not and cannot be
buffered.
- stdio can be redirected with DOS redirection calls; conio CANNOT be
redirected.  (*)
- stdio uses a DOS file buffer; conio uses the BIOS keyboard buffer.

(*) Actually, you can, but only by writing interrupt handlers to hook
the BIOS keyboard interrupt.  I know of no way at all to "redirect"
output short of constantly scanning text video memory.

-- 
---------------------------------------------------------------------
| John M. Aldrich, aka Fighteer I |        fighteer AT cs DOT com          |
| Proud owner of what might one   |   http://www.cs.com/fighteer    |
| day be a spectacular MUD...     | Plan: To make Bill Gates suffer |
---------------------------------------------------------------------

- Raw text -