Testing the keyboard buffer for MT/Not MT

Discussion in 'C Programming' started by NaN, May 2, 2008.

  1. NaN

    NaN Guest

    I've been trying to use _kbhit() but it didn't do what I thought it
    would from books, "Detects whether a keypress is available for reading."

    Herbert Schildt says,
    "If the user has pressed a key, this function returns
    true(non-0), but does not read the character.
    If no keystroke is pending, kbhit() returns false (0)."

    Here is the test code,

    #include<conio.h>
    void main(void) {
    int ig1=0, ig2=0, ik=-1;
    ig1 = _getch();
    ik = _kbhit();
    ig2 = _getch();
    _getch();
    printf("ig1=%d, ik=%d, ig2=%d", ig1,ik,ig2);
    return;
    }

    Whatever keys I press a runtime, ik returned as 0. Where else would a
    keystroke be "waiting" except in the buffer? If _kbhit() does not look
    at the buffer, what does it look at?
    NaN, May 2, 2008
    #1
    1. Advertising

  2. In article <fvg0rg$6tu$>, NaN <> wrote:
    >I've been trying to use _kbhit() but it didn't do what I thought it
    >would from books, "Detects whether a keypress is available for reading."


    _kbhit() is not part of standard C. Please check a newsgroup
    appropriate for your development environment if you wish to use it.

    There is no method in standard C that can be used to detect
    whether keypresses are available.
    --
    "Walter is a great man." -- Dennis Green
    Walter Roberson, May 2, 2008
    #2
    1. Advertising

  3. NaN

    user923005 Guest

    On May 2, 2:25 pm, NaN <> wrote:
    > I've been trying to use _kbhit() but it didn't do what I thought it
    > would from books, "Detects whether a keypress is available for reading."
    >
    > Herbert Schildt says,
    > "If the user has pressed a key, this function returns
    > true(non-0), but does not read the character.
    > If no keystroke is pending, kbhit() returns false (0)."
    >
    > Here is the test code,
    >
    > #include<conio.h>
    > void main(void) {
    >          int ig1=0, ig2=0, ik=-1;
    >          ig1 = _getch();
    >          ik = _kbhit();
    >          ig2 = _getch();
    >          _getch();
    >          printf("ig1=%d, ik=%d, ig2=%d", ig1,ik,ig2);
    >          return;
    >
    > }
    >
    > Whatever keys I press a runtime, ik returned as 0. Where else would a
    > keystroke be "waiting" except in the buffer?  If _kbhit() does not look
    > at the buffer, what does it look at?


    From the C-FAQ:

    19.1: How can I read a single character from the keyboard without
    waiting for the RETURN key? How can I stop characters from
    being echoed on the screen as they're typed?

    A: Alas, there is no standard or portable way to do these things in
    C. Concepts such as screens and keyboards are not even
    mentioned in the Standard, which deals only with simple I/O
    "streams" of characters.

    At some level, interactive keyboard input is usually collected
    and presented to the requesting program a line at a time. This
    gives the operating system a chance to support input line
    editing (backspace/delete/rubout, etc.) in a consistent way,
    without requiring that it be built into every program. Only
    when the user is satisfied and presses the RETURN key (or
    equivalent) is the line made available to the calling program.
    Even if the calling program appears to be reading input a
    character at a time (with getchar() or the like), the first call
    blocks until the user has typed an entire line, at which point
    potentially many characters become available and many character
    requests (e.g. getchar() calls) are satisfied in quick
    succession.

    When a program wants to read each character immediately as it
    arrives, its course of action will depend on where in the input
    stream the line collection is happening and how it can be
    disabled. Under some systems (e.g. MS-DOS, VMS in some modes),
    a program can use a different or modified set of OS-level input
    calls to bypass line-at-a-time input processing. Under other
    systems (e.g. Unix, VMS in other modes), the part of the
    operating system responsible for serial input (often called the
    "terminal driver") must be placed in a mode which turns off
    line-at-a-time processing, after which all calls to the usual
    input routines (e.g. read(), getchar(), etc.) will return
    characters immediately. Finally, a few systems (particularly
    older, batch-oriented mainframes) perform input processing in
    peripheral processors which cannot be told to do anything other
    than line-at-a-time input.

    Therefore, when you need to do character-at-a-time input (or
    disable keyboard echo, which is an analogous problem), you will
    have to use a technique specific to the system you're using,
    assuming it provides one. Since comp.lang.c is oriented towards
    those topics that the C language has defined support for, you
    will usually get better answers to other questions by referring
    to a system-specific newsgroup such as comp.unix.questions or
    comp.os.msdos.programmer, and to the FAQ lists for these groups.
    Note that the answers may differ even across variants of
    otherwise similar systems (e.g. across different variants of
    Unix); bear in mind when answering system-specific questions
    that the answer that applies to your system may not apply to
    everyone else's.

    However, since these questions are frequently asked here, here
    are brief answers for some common situations.

    Some versions of curses have functions called cbreak(),
    noecho(), and getch() which do what you want. If you're
    specifically trying to read a short password without echo, you
    might try getpass(). Under Unix, you can use ioctl() to play
    with the terminal driver modes (CBREAK or RAW under "classic"
    versions; ICANON, c_cc[VMIN] and c_cc[VTIME] under System V or
    POSIX systems; ECHO under all versions), or in a pinch, system()
    and the stty command. (For more information, see <sgtty.h> and
    tty(4) under classic versions, <termio.h> and termio(4) under
    System V, or <termios.h> and termios(4) under POSIX.) Under
    MS-DOS, use getch() or getche(), or the corresponding BIOS
    interrupts. Under VMS, try the Screen Management (SMG$)
    routines, or curses, or issue low-level $QIO's with the
    IO$_READVBLK function code (and perhaps IO$M_NOECHO, and others)
    to ask for one character at a time. (It's also possible to set
    character-at-a-time or "pass through" modes in the VMS terminal
    driver.) Under other operating systems, you're on your own.

    (As an aside, note that simply using setbuf() or setvbuf() to
    set stdin to unbuffered will *not* generally serve to allow
    character-at-a-time input.)

    If you're trying to write a portable program, a good approach is
    to define your own suite of three functions to (1) set the
    terminal driver or input system into character-at-a-time mode
    (if necessary), (2) get characters, and (3) return the terminal
    driver to its initial state when the program is finished.
    (Ideally, such a set of functions might be part of the C
    Standard, some day.) The extended versions of this FAQ list
    (see question 20.40) contain examples of such functions for
    several popular systems.

    See also question 19.2.

    References: PCS Sec. 10 pp. 128-9, Sec. 10.1 pp. 130-1; POSIX
    Sec. 7.

    19.2: How can I find out if there are characters available for reading
    (and if so, how many)? Alternatively, how can I do a read that
    will not block if there are no characters available?

    A: These, too, are entirely operating-system-specific. Some
    versions of curses have a nodelay() function. Depending on your
    system, you may also be able to use "nonblocking I/O", or a
    system call named "select" or "poll", or the FIONREAD ioctl, or
    c_cc[VTIME], or kbhit(), or rdchk(), or the O_NDELAY option to
    open() or fcntl(). See also question 19.1.
    user923005, May 3, 2008
    #3
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Rob T

    Key ahead/keyboard buffer

    Rob T, Aug 1, 2005, in forum: ASP .Net
    Replies:
    1
    Views:
    614
    Bruce Barker
    Aug 1, 2005
  2. Pelle
    Replies:
    5
    Views:
    1,473
    Attila Feher
    Sep 19, 2003
  3. hokiegal99

    Keyboard Buffer

    hokiegal99, Sep 9, 2003, in forum: C Programming
    Replies:
    7
    Views:
    797
  4. herrcho
    Replies:
    23
    Views:
    1,306
  5. xingye
    Replies:
    9
    Views:
    274
    Michael Lu
    Apr 19, 2004
Loading...

Share This Page