Q43993: How to Flush the Keyboard Typeahead Buffer

Article: Q43993
Product(s): Microsoft Programming Utilities
Version(s): 1.0,1.5,5.1,6.0,6.0a,6.0ax; MS-DOS:7.0
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft C for OS/2, versions 5.1, 6.0, 6.0a 
- Microsoft C/C++ for MS-DOS, version 7.0 
- Microsoft Visual C++, versions 1.0, 1.5 
- Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax 
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SUMMARY
=======

To flush the BIOS keyboard typeahead buffer, the MS-DOS Interrupt 21 Function
0CH may be used. This function clears the keyboard typeahead buffer and then
invokes a reading function specified in the AL register. The AL register can be
0x01, 0x06, 0x07, 0x08, or 0x0A to specify a valid reading function. If you do
not intend to read after flushing the buffer, you may specify an invalid number
in AL.

Another method of flushing the BIOS buffer is to call the console I/O function
getch() until the function kbhit() becomes false. This method is demonstrated in
the program below and has the advantage of being usable under OS/2 as well as
MS-DOS.

MORE INFORMATION
================

The buffer implemented by the C run-time functions for the stream "stdin" is
different from the BIOS keyboard typeahead buffer. To clear the buffer for
stdin, use the function fflush(). However, this method will not flush the BIOS
buffer. To be totally flushed, you must both flush the BIOS buffer as described
above AND call fflush for stdin.

The following sample program is an example:

Sample Code
-----------

  /* Compile options needed: none
  */ 
  #include <stdio.h>
  #include <time.h>
  #include <conio.h>
  #include <dos.h>
  void main (void)
  {
  time_t start, work ;
  char str [50] ;
  puts ("type for getchar(). Go to stdin's buffer.") ;
  // user can type more than one character and an Enter.
  getchar () ;
  puts ("Type fast, 5 seconds. Go to BIOS buffer.") ;
  // user can type anything including multiple Enters.
  time (&start) ;
  work = start ;
  while ( (work - start) < 5 ) time (&work) ;
  bdos (0xC, 0, 0) ;       // clear BIOS keyboard buffer
  //  Alternative method:
  //  while (kbhit()) getch();
  fflush (stdin) ;         // clear stdin's buffer
  puts ("Should be waiting again.") ;
  gets (str) ;
  puts (str) ;
  }

Additional query words: kbinf 5.10 6.00 6.00a 6.00ax 7.00 1.00 1.50

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Keywords          : kb16bitonly 
Technology        : kbVCsearch kbAudDeveloper kbPTProdChange kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbCComp510DOS kbCComp600DOS kbCComp600aDOS kbCComp600axDOS kbCComp510OS2 kbCComp600OS2 kbCComp600aOS2 kbCVC700DOS
Version           : :1.0,1.5,5.1,6.0,6.0a,6.0ax; MS-DOS:7.0

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