Question

I am using Delphi and trying to read from a bar code scanner over USB, so that it is just another Human Interface Device.

I get the digits correctly, but get each twice. I imagine that this is key down and key up.

I could; kludge it with a flag and ignore very second read, but would rather do it propery.

My code is adapted slightly from this link.

Can I specify that I only want key_up events when assigning the hook?

KBHook := SetWindowsHookEx(WH_KEYBOARD,
                           @KeyboardHookProc,
                           HInstance,
                           GetCurrentThreadId()) ;

or somehow check a flag within the hook function itself?

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Update: I tried to code for it, but looks like I got it wrong. Here’s what I tried at the start of my hook function

// http://msdn.microsoft.com/en-us/library/windows/desktop/ms644984%28v=vs.85%29.aspx
if Code < 0 then  
begin
   Result := CallNextHookEx(KBHook, Code, WordParam, LongParam);
   Exit;
end;

if (((LongParam and $80000000) <> $80000000)    (* key is not being released *)
and ((LongParam and $40000000) <>  $40000000))  (* key was not previously down *) then
begin
   Result := CallNextHookEx(KBHook, Code, WordParam, LongParam);
   Exit;
end;

---

[Further update] Five years after, and this still doesn’t help, but I find that my original follow-up question (q.v) does.

Answer 1

In your KeyboardHookProc, you need to check the high bit of the LongParam argument. If the high bit is zero, it is a key press. If the high bit is one, it is a key release.

For example:

KeyUp:boolean;

KeyUp := ((LongParam and (1 shl 31)) <> 0);