Recording via System Calls C# Mono Linux

Trying to write an integer value to the Linux system address

/sys/class/backlight/backlight/brightness

I use Mono Posix. I'm doing something wrong, because the error Invalid argument [EINVAL] pops up. Perhaps I am not writing the value correctly.

Connecting dynamic libraries:

    [DllImport("libc.so.6", EntryPoint = "open")]
    public static extern int Open(string fileName, int mode);

    [DllImport("libc.so.6", EntryPoint = "fcntl", SetLastError = true)]
    private static extern int Fcntl(int fd, int request, int data);

private const string Path = "/sys/class/backlight/backlight/brightness";
    private int fd = -1;

Next, I open the "file":

private void Open()
    {
        fd = Syscall.open(Path, OpenFlags.O_RDWR);

        if (fd < 0)
        {
            CheckAndThrowUnixIOException();
        }
    }

And I try to write it down:

 public unsafe void WriteValue(int value)
    {
        Open();

        var ret = Fcntl(fd, 1, value); // что - то тут не так

        int count;
        var data = new char[1];
        data[0] = (char) value; // и тут тоже что-то не правильно? Нужно значение в IntPtr.

        fixed(char* p = data)
        {
            count = (int)Syscall.write(fd, p, (ulong) data.Length);
        }


        if (count < 0)
        {
            CheckAndThrowUnixIOException(); // Соответственно тут летит ошибка.
        }
    }

Sampling method exceptions:

 private void CheckAndThrowUnixIOException()
    {
        var error = Marshal.GetLastWin32Error();
        throw new UnixIOException(error);
    }

I can't figure out how to write the value to sys/class.... It is also not clear how one can read the address from the Path constant.