Interference between the escape sequence to find cursor position and `stdin()` in raw mode

Question

I am writing a shell emulator in C, and put the terminal in raw mode. To get the cursor position and move using termcaps when necessary, I printed the escape sequence \033[6n and parsed its return.

The problem is that a high input rate (when I basically hit my keyboard), my input messes up with getting the return of the escape sequence, and ; characters appear on my screen.

This is how I parsed the return of the escape sequence :

static int fillup_cursor_position(int *x, int *y)
{
    int             result;

    result = 0;
    if ((result = read_cursor()) != 27)
        return 0;
    if ((result = read_cursor()) != '[')
        return 0;
    result = read_cursor();
    while (result >= '0' && result <= '9')
    {
        *y = 10 * *y + result - '0';
        result = read_cursor();
    }
    if (result != ';')
        return (0);
    result = read_cursor();
    while (result >= '0' && result <= '9')
    {
        *x = 10 * *x + result - '0';
        result = read_cursor();
    }
    if (result != 'R')
        return 0;
    return 1;
}

This is the function read_cursor I called several times :

static int      read_cursor(void)
{
    char buffer[4];
    int n;

    n = 0;
    while (1)
    {
        n = read(0, &buffer, 1);
        if (n > 0)
            return buffer[0];
        else
            return 0;
    }
}

And finally this is how I began by setting the terminal and raw-mode to have a non-blocking read() :

static int  set_non_canonical_input(void)
{
     struct termios termios_cpy;

    if (tcgetattr(0, &termios_cpy) != 0)
        return 0;
    cfsetispeed(&termios_cpy, B50);
    cfsetospeed(&termios_cpy, B50);
    termios_cpy.c_cc[VMIN] = 1;
    termios_cpy.c_cc[VTIME] = 0;
    termios_cpy.c_lflag &= (IGNBRK);
    termios_cpy.c_lflag &= (ICANON);
    if (tcsetattr(0, TCSANOW, &termios_cpy) != 0)
        return 0;
    return 1;
 }

I tried separating those two inputs by writing on the file stream, or by trying to lock the file access with fcntl(), or by modifying the baud rate, all to no avail.

I'm sure there is an easy way to do this and that I am confused somewhere, but I can't seem to find.

raw mode and `fread` absolutely do not mix **at all**. Use `read(2)` instead. — Antti Haapala -- Слава Україні, Jan 22 '18 at 06:34
Thank you. I had done so before and tried `fread()` as I thought that by writing on the file stream it would not appear on the file itself. I'll edit to reflect the version where I did not try interfering with the file stream. — Lazare Rossillon, Jan 22 '18 at 06:37
Also, please stop using the parentheses around the return value - `(0)` etc; the return value expression ends at the `;`, no exceptions; the parentheses just make it harder to read your code. — Antti Haapala -- Слава Україні, Jan 22 '18 at 06:42
Anyway; isn't it clear what you should do? Your logic is flawed, you expect the *first* character after the issuance of the command to be `27`. If it *isn't*, then you're going to need to read *more* characters, and retaining the others in the input buffer. — Antti Haapala -- Слава Україні, Jan 22 '18 at 06:46
I mean, it is unclear what it is that you're asking here? You're returning 0 too early. — Antti Haapala -- Слава Україні, Jan 22 '18 at 06:46
(for others wondering about the protocol, details are better laid out [in this question](https://stackoverflow.com/questions/44078725/how-to-get-the-cursor-position-in-a-c-program-using-termcap-without-writing-a-c)) — Antti Haapala -- Слава Україні, Jan 22 '18 at 06:47
Thank you for guidance, as you pointed out, I needed to continue reading instead of returning 0. — Lazare Rossillon, Jan 22 '18 at 12:09

Interference between the escape sequence to find cursor position and `stdin()` in raw mode

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