What are these extra parameters when calling SendInput via win32API - ruby

Question

I am interested in sending key input through Ruby using the win32API.
I have found the function that I would like to call: SendInput
MSDN describes the signature as follows:
UINT WINAPI SendInput(
_In_ UINT nInputs,
_In_ LPINPUT pInputs,
_In_ int cbSize
);
And INPUT looks like
typedef struct tagINPUT {
DWORD type;
union {
MOUSEINPUT mi;
KEYBDINPUT ki;
HARDWAREINPUT hi;
};
} INPUT, *PINPUT;
So there's one integer that I need showing the input type.
Since I'm interested in keyboard events, I looked at the KEYBDINPUT struct
typedef struct tagKEYBDINPUT {
WORD wVk;
WORD wScan;
DWORD dwFlags;
DWORD time;
ULONG_PTR dwExtraInfo;
} KEYBDINPUT, *PKEYBDINPUT;
And there's another 5 arguments: two 16-bit integers and three 32-bit integers.
Now, from what I have gathered, SendInput takes an integer, a pointer to an INPUT struct, and an integer representing the size of the INPUT struct. Creating the Win32API object looks like
SendInput = Win32API.new("User32", "SendInput", "LPL", "L")
INPUT_KEYBOARD = 1
Now I build my INPUT struct:
input = [INPUT_KEYBOARD, 0x5A, 0, 0, 0, 0].pack('ISSIII')
First argument is the input type, followed by the five arguments for the KEYBDINPUT struct: the virtual keycode for the key I want to press, and some other flags that I don't need for testing purposes.
So I run it:
SendInput.call(1, input, input.size)
Nothing happens.
When I check GetLastError, it returns an error code of 87, which means there were invalid arguments.
I then decided to search around and found that someone else is building their input struct like this:
input = [INPUT_KEYBOARD, 0x5A, 0, 0, 0, 0, 0, 0].pack('ISSIIIII')
I tried running it, and it executes successfully!
Difference? There's two extra arguments for the input.
Now I am confused: what are these two extra arguments for the input struct?
Is there something that I'm missing when I was reading the docs?
I'm thinking it might have something to do with the ULONG_POINTER type, but when I look up the data types, it's a 32-bit integer.
Reference: http://www.dsprobotics.com/support/viewtopic.php?p=7110&sid=cd256b848d00e64a2e74e093863f837f#p7110

Answer 1

The function call takes a union of those structs, I would wager it has something to do with that.
Looks like MOUSEINPUT is larger than KEYBOARDINPUT:
typedef struct tagMOUSEINPUT {
LONG dx;
LONG dy;
DWORD mouseData;
DWORD dwFlags;
DWORD time;
ULONG_PTR dwExtraInfo;
} MOUSEINPUT, *PMOUSEINPUT;
Dunno for sure though, that seems like a pretty terrifying way to bridge those calls!
edit: Yeah, looks like using that MOUSEINPUT size, you end up w/28 bytes of data sent to the function, which is what pack('ISSIIIII') would net you.