OpenEuphoria

Re: Win32Bug

• Posted by Robert Craig <rds at EMAIL.MSN.COM> Nov 18, 1998
• 646 views

I've finally solved the Win32 bug.

It all comes down to one small mistake in Win32Lib.ew

-- Type PAINTSTRUCT
    psHdc               = allot( Long ),
    psErase             = allot( Long ),
    psPaintRect         = allot( SIZEOF_RECT ),  -- 16 bytes
    psRestore           = allot( Long ),
    psIncUpdate         = allot( Long ),
    psRgbReserved       = allot( Byte ),                    <----- !!!
    SIZEOF_PAINTSTRUCT  = allotted_size(),

SIZEOF_PAINTSTRUCT is set to 33 bytes. This
is wrong. According to Petzold Chapter 3, a PAINTSTRUCT
is defined as:
          typedef struct tagPAINTSTRUCT
              {
                  HDC hdc;
                  BOOL fErase;
                  RECT rcPaint;     // 16 bytes
                  BOOL fRestore;
                  BOOL fIncUpdate;
                  BYTE rgbReserved[32];
              } PAINTSTRUCT;

where I assume BOOL and HDC are 4 bytes.
The problem is that rgbReserved is 32 bytes, not 1.

I changed the last line to
    SIZEOF_PAINTSTRUCT  = allotted_size()  + 32,

and now everything works. (I guess +31 would work too.)

I ran Win32Lib using safe.e in place of machine.e and
nothing illegal was detected, so I assume that Windows
is storing stuff into the rgbReserved area, not David.

This bug actually happens in almost all Win32Lib programs,
it's just that the storage block that comes after the paint structure
is rarely critical. When I ran the Win32Lib demo programs
using my debugging version of exw (loaded with extra sanity checks),
heap corruption was detected in almost all cases
i.e. an allocated block of memory had been scribbled on.

The programs reported by Ad Rienks work now.
The 12-character thing was just a coincidence.
I guess it made the heap corruption evident.

I ran through the full set of demo programs in Win32Lib again,
and they all work flawlessly now, even with safe.e and
the debugging version of exw.

Regards,
     Rob Craig
     Rapid Deployment Software
     http://members.aol.com/FilesEu/