c - How can I flush unread data from a tty input queue on a UNIX system?

Question

My program has to read just ONE character from the standard input, and so I use read(0, buffer, 1). But if the user insert more than one single character, they remain in some buffer and when I call a read again they are still there.

So, how can I discard these characters? I want that when I call a read again, the buffer is filled with the new character, not with the old ones.

An example: I've a read(0, buffer, 1) and the user writes abcde. My buffer contains a (and it's right), but then I call read(0, buffer, 1) again and I want the next character written by the user from now, and not the b written before.

Answer 1

The POSIX answer is tcflush(): flush non-transmitted output data, non-read input data, or both. There is also tcdrain() which waits for output to be transmitted. They've been in POSIX since there was a POSIX standard (1988 for the trial-use version), though I don't recall ever using them directly.

Example program

Compile this code so the resulting program is called tcflush:

#include <stdio.h>
#include <unistd.h>
#include <termios.h>

int main(void)
{
    char buffer[20] = "";

    read(0, buffer, 1);
    printf("%c
", buffer[0]);
    tcflush(0, TCIFLUSH);
    read(0, buffer, 1);
    printf("%c
", buffer[0]);
    tcflush(0, TCIFLUSH);
    return 0;
}

Example dialog

$ ./tcflush
abc
a
def
d
$

Looks like what the doctor ordered. Without the second tcflush(), the shell complains that it can't find a command ef. You can place a tcflush() before the first read if you like. It wasn't necessary for my simple testing, but if I'd used sleep 10; ./tcflush and then typed ahead, it would make a difference.

Tested on RHEL 5 Linux on an x86/64 machine, and also on Mac OS X 10.7.4.