DOCUMENT:Q43993 06-MAY-2001 [utilities] TITLE :How to Flush the Keyboard Typeahead Buffer PRODUCT :Microsoft Programming Utilities PROD/VER::1.0,1.5,5.1,6.0,6.0a,6.0ax; MS-DOS:7.0 OPER/SYS: KEYWORDS:kb16bitonly ====================================================================== ------------------------------------------------------------------------------- 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 ------------------------------------------------------------------------------- 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 #include #include #include 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 ====================================================================== 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 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.