How to make stdin file descriptor ready when pressing a key?

Question

I have a custom _kbhit() function used for non-blocking character input. _kbhit() function uses select() to check if stdin is ready to read. But finally I have stdin ready if only I pressed RETURN. How to make it work when I press any key?

void set_conio_terminal_mode()
{
    struct termios new_termios;

    /* take two copies - one for now, one for later */
    tcgetattr(0, &new_termios);

    /* register cleanup handler, and set the new terminal mode */
    atexit(reset_terminal_mode);
    cfmakeraw(&new_termios);
    tcsetattr(0, TCSANOW, &new_termios);
}

int _kbhit()
{
    struct timeval tv = { 0L, 0L };
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(0, &fds);
    return select(1, &fds, NULL, NULL, &tv);
}

static char _getch(void) {
    int r;
    unsigned char c;
    if ((r = read(0, &c, sizeof(c))) < 0) {
        return r;
    } else {
        return c;
    }
}

int main( int argc, char *argv[] )
{
    set_conio_terminal_mode();
    while (1) {
        if (_kbhit()) {
            inputbuffer[inputlen] = _getch();
            if (inputbuffer[inputlen] >= 0x20 && inputbuffer[inputlen] < 0x7F) {
                putchar(inputbuffer[inputlen]);
                inputlen++;
            }
        }
    }
}

I expected that this code will output echo, but it outputs entire string just after I press RETURN.

Answer 1

I think the problem is that - despite your initialization of the terminal attributes using cfmakeraw() - stdout is still in line-buffered mode.

Try setting standard output to unbuffered mode.

setbuf(stdout, NULL);

which is equivalent to

setvbuf(stdout, NULL, _IONBF, 0);

This avoids the need to call fflush(stdout); after every putchar(inputbuffer[inputlen]);.

There may be a way of accomplishing this using tcsetattr() instead of setvbuf(), buf I am not familiar with it.