Q41198: How ungetch() Works with getch() and getche()

Article: Q41198
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 16-MAY-1989

The Microsoft C run-time function ungetch() pushes back a character
to be read in by the next call to getch() or getche().

The character is pushed back to a special memory buffer defined in
getch(). The function getch() or getche() checks the special buffer
before it calls the DOS interrupt function to read input from the real
keyboard buffer. If the special buffer is not empty, it returns the
value in the buffer without calling the interrupt 21h function.

ungetch() does not put the character back to the keyboard buffer.

Pushing a character back to the keyboard buffer can be done on some
machines by calling BIOS keyboard service, interrupt 16H function 05H
with scan code in the register CH and the character's ASCII code in
the register CL.

Note: This BIOS function does not exist on many machines -- be sure to
check the machine you're running on before you try to use this
function.

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