Handle values under Windows

[Paul Noble]

Does anyone know what range of values Windows chooses from when
creating a handle value ?

I ask, because while trying to discover why NT4 was randomly failing
to render images in my PBDLL6 program, I found that LoadBitmap() was
generating bitmap handles beyond the range of a LONG (2,835,677,702
was one such). This was easily cured by using a DWORD to reference the
loaded bitmap.

By comparison, Win98 SE has never yet (well, in 6 weeks' use) created
a bitmap handle that couldn't be referenced correctly by a LONG.

It seems however that dialog handles *can* always be represented by a
LONG, borne out for example by DDT's "DIALOG NEW ... TO hDlg&".

Either there's an underlying rule, or you have to check the range of
possible handle values for each type of object you reference. The Win32
SDK doesn't have the answer (AFAICS) - it just says HBITMAP in this case,
which leads to Windows header files, of which there are 315 in my VC++6
\Include folder... (the pointed-to WINUSER.H does not it seems contain
a definition of HBITMAP).

Anyway for the general case, there must be an easier way - any thoughts ?


Cheers,

Paul

[Chris Boss]

Paul;

I experienced this problem with Windows 2000.

Windows 95, 98 NT 3.5 and before use LONGS and the handle will
"always" be a positive number.

Windows 2000 (and likely later versions of NT) use DWORD.

What this means is that when you use a LONG to hold a handle value
, if the highest BIT is set your LONG will be a negative value.

As long as your code assumes that a handle can be negative, then
you should be OK. Where problems occur is when you test for a
valid handle with code like :

IF hWnd&>0 THEN 

END IF

' or

IF hFont&>0 THEN

END IF

Your code should assume the handle can be negative, so use :

IF hWnd&<>0 THEN 

END IF

' or

IF hFont&<>0 THEN

END IF

I had code that worked OK with WIN95, 98 and it failed on 2000
when creating Fonts. The reason was simply I was testing for
greater than zero, rather than not-equal to.



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[Paul Noble]

Thanks Chris, that's exactly how it came about in my application.

I guess that LONGs (rather than DWORDs) are used to represent handle values
in the general case so that comparisons with %INVALID_HANDLE_VALUE (-1) can
be performed (although as it happens this is not the case with LoadBitmap(),
which returns %NULL on failure).

However, I can't help feeling it would be simpler all round if handles were
always DWORDs, and %INVALID_HANDLE_VALUE was defined as its DWORD equivalent.
No big thing though.

Cheers,

Paul

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