NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/os/linux/9725

Text File | 1992-09-02 | 2.9 KB | 82 lines

Path: sparky!uunet!mcsun!sun4nl!hgatenl!tedux!clifton!baron
From: [e-mail] (Wim `Blue Baron' van Dorst)
Newsgroups: comp.os.linux
Distribution: world
Subject: Re: Polling keyboard in linux. What works best?
References: <[e-mail]>
Message-ID: <[e-mail]>
Organization: Minix Hobbynet Host
Date: Wed,  2 Sep 1992 22:12:55 MDT
Lines: 70

> In article <[e-mail]>, [e-mail] (Karl Buck) writes:
> I want to check the keyboard for input (using gcc of course) without
> actually reading (ie: getc etc..). The unix FAQ suggests looking at
> the FIONREAD ioctl or using select() and the C FAQ states that this is
> a purely OS dependant question.
>
> What is best for Linux? Can this be non-blocking? A snippet of code
> for this would be great since I have not had to do this kind of thing
> before. Thanks.

[e-mail] (JAMES LEWIS NANCE) writes:
> I was given the following segement of code when I asked this question.
> [ stuff deleted -b-b- ]
> #include     <sgtty.h>
> #include    <fcntl.h>
> struct sgttyb normal_mode, cbreak_mode;
> [ stuff deleted -b-b- ]
>     ioctl(0, TIOCGETP, &normal_mode);
>     cbreak_mode = normal_mode;
>     cbreak_mode.sg_flags |= CBREAK;    /* make characters available as typed */
>     cbreak_mode.sg_flags &= ~ECHO;    /* turn off echo */

Sorry, but that won't work. I currently am working on a little game, and
of course you have to test for key-pressing. I had code like the above working
properly in Minix, but for Linux the implementation is purely Posix, which
the above is not.

I very easily changed my code to Termios (i.e., the Posix and thus Linux
way of doing such things) for the what I wanted, using indeed the non-blocked
read(). A very simple example is this:

#include <sys/types.h>
#include <termios.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>

main()
{
    int ch = 'a', counter;
    struct termios *tp;

    tcgetattr(0, tp);
    tp->c_lflag &= ~(ECHO | ICANON);
    tcsetattr(0, TCSANOW, tp);
    fcntl(0, F_SETFL, O_NONBLOCK);

    while (ch != 'q') {
        for (counter=0;counter<100000;counter++) /* */ ;
        if (read(0, &ch, 1) == -1)
            printf("Non-blocked ch=%c\n", (char) ch);
        else
            printf("Typed ch=%c\n", (char) ch);
    }

    tp->c_lflag |= (ECHO | ICANON);
    tcsetattr(0, TCSANOW, tp);
    fcntl(0, F_SETFL, O_ACCMODE);
}

Of course this code actually _reads_ the character: It is not a test
whether a character is available. Using code like this, however, such an
implementation could easily be made using a buffer for actually read
characters, and passing them on only when wanted.

Met vriendelijke groeten, Wim `Blue Baron' van Dorst
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