How to Flush the Keyboard Typeahead Buffer

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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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) ;
}
Properties

Article ID: 43993 - Last Review: 1 Dec 2003 - Revision: 1

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