Respond to only the first WM_KEYDOWN notification?

Question

How can a Win32 application respond to only the first WM_KEYDOWN notification? The MSDN docs claim bit 30 "Specifies the previous key state. The value is 1 if the key is down before the message is sent, or it is zero if the key is up." but bit 30 is always 0 in my WndProc.

case WM_KEYDOWN:
    // ToDo - stop multiple notifications for repeating keys
    printf("WM_KEYDOWN %i %i", wParam, lParam & 30);
    return 0;

Is lParam & 30 the wrong way to ask for this? Am I doing something else wrong?

How did you end up implementing this? repeatCount=(lParam & 0xffff); if(repeatCount < 1) Action(); Doesn't seem to work for me. Nor does repeatCount < 2. I either get repeating keys or no key press at all. — Sam152, Oct 12 '09 at 04:16
I used: if ((lParam & (1 << 30)) == 0)... although if (lParam & 0x40000000) worked as well if I remember correctly. My choice was based on readability (1 << 30) makes more sense to me than 0x40000000 — Nick Van Brunt, Oct 13 '09 at 15:09

score 11 · Answer 1 · answered Sep 01 '09 at 14:33

11

To test bit 30 don't AND with 30, instead AND with 1 << 30.

const bool isBitSet = lParam & (1 << 30);

answered Sep 01 '09 at 14:33

sharptooth

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It probably does read better but it has to be computed every time (unless the compiler is optimizing this). 0x4000000 Will be faster. – kjfletch Sep 03 '09 at 13:19
2

VC++7 is optimizing it to a constant value. – sharptooth Sep 03 '09 at 13:40
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@kjfletch those kind of micro optimizations are certainly not needed anymore.. Any simple compiler could optimize that.. – Davy Landman Sep 11 '09 at 11:08

score 4 · Accepted Answer · answered Sep 01 '09 at 14:33

4

To get bit 30, you need this:

(lParam & 0x40000000)

An alternative would be to use bits 0-15 to get the repeat count:

int repeatCount = (lParam & 0xffff)

and only do anything if the repeat count is 0 (or possibly 1; I'm not sure whether the first message gets a repeat count of 0 or 1, and it's not clear from the documentation).

answered Sep 01 '09 at 14:33

RichieHindle

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This worked - thank you. My bit fiddling-fu is a little shaky. What exactly is 0x40000000? – Nick Van Brunt Sep 01 '09 at 14:36
2

That's a DWORD representing a bitmask consisting of bit 30 set and all other bits cleared. – sharptooth Sep 01 '09 at 14:37
You might want to read an introduction to hexadecimal, eg. http://www.learn-programming.za.net/articles_decbinhexoct.html It's not too hard, but not so trivial that I can explain it all here. – RichieHindle Sep 01 '09 at 14:40

Sanders · Answer 3 · 2014-11-10T15:52:16.787

Bitwise AND (lParam & 0x4000000) will work but you can easily make it more readable using the defines included with Windows.h (which you're already using for WM_KEYDOWN):

case WM_KEYDOWN:
    if((HIWORD(lParam) & KF_REPEAT) == 0) //process wParam
    return 0;

HIWORD takes the most significant 16 bits.

KF_REPEAT (= 0x4000) flags the location of the bit representing a repeat WM_KEYDOWN message.

The bitwise AND of these values equals 0 when the repeat flag is off (the initial keypress) and equals 1 every subsequent time (the autorepeat feature) until the key is released.

It's a small thing but it goes a long way to improving the readability of your code. 0x40000000 does not mean anything to the reader while much can be inferred from KF_REPEAT.

score 1 · Answer 4 · answered Sep 02 '09 at 18:35

The problem with doing lParam & 30 is that, '30' over here is considered to be in decimal, which when converted to binary would '11110'. Hence you are not testing bit 30 but just getting the result for lparam & 11110.

Hope this helps in clarifying the problem a bit.

Respond to only the first WM_KEYDOWN notification?

4 Answers4

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