OpenEuphoria

Re: Win32Lib bug

• Posted by Robert Craig <rds at EMAIL.MSN.COM> Nov 20, 1998
• 660 views

I've found the cause of the 12-character string bug in win32lib.ew.

Using an enhanced version of safe.e, it was easy to locate.

In getItem() we have:

    iLength = sendMessage( id, msg, item, 0 )

    -- extra line I added:
    iLength = iLength+1  -- temporary fix,  - David?

-- NEW! 0.14c
    if iLength = 0 then
        warnErr( "Get item text length is zero." )
        return ""
    end if

    -- allocate a buffer
    buffer = allocate( iLength )   -- not quite enough space!

  -- get the message, based on control type
  if      window_class[ id ] = LISTBOX then msg = LB_GETTEXT
  elsif   window_class[ id ] = COMBO   then msg = CB_GETLBTEXT
  end if

  -- move the text to a buffer
  ok = sendMessage( id, msg, item, buffer )

I found that the buffer allocated above was one character too small.
It did not allow for the 0 that C always adds to strings (but Euphoria
doesn't require). As a result, Windows would write the extra 0, sometimes
corrupting the next block of storage. If the string was an exact multiple
of 4 in length (such as 12) then Euphoria would allocate exactly the
number of bytes requested, but if it wasn't, Euphoria would round up
to the next multiple of 4 bytes, thereby protecting against the extra 0.

Maybe the fix should just be:
     buffer = allocate(iLength+1)
instead. I don't know. David should consider it.
There are other calls to SendMessage that should also be checked.

* * *

safe.e:

I've enhanced safe.e to make detection of these errors much easier.
safe.e now allocates extra space before and after each block that
you allocate and fills it with a known pattern. This makes it possible
to check for blocks of memory where data has been written just
before or just after the allocated block. Euphoria code can't
write out of bounds, because of safe.e checks in poke, mem_copy etc.,
but machine code, or Windows can do it.

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