SIGSEGV in IDirectDrawImpl_EnumDisplayModes

Lionel Ulmer lionel.ulmer at
Fri Nov 1 08:54:14 CST 2002

> Okay, if it'll help you although my knowledge of x86 calling conventions and 
> post-286 instructions etc. (last x86 I wrote was in a BIOS five years ago) is 
> pretty hopeless :-)

Well, I am not very good at X86 ASM either, but well, the problem lies here :

>     2fe0:       8b 57 08                mov    0x8(%edi),%edx
             %edx is the address of the COM object (IDirectDraw)
>     2fe3:       8b 0a                   mov    (%edx),%ecx
             %ecx is the address of the VTable
>     2fe5:       6a 00                   push   $0x0
>     2fe7:       8d 47 10                lea    0x10(%edi),%eax
>     2fea:       50                      push   %eax
>     2feb:       53                      push   %ebx
>     2fec:       52                      push   %edx
             All arguments are pushed on the stack (with the last being the
	     pointer to the COM object itself).
>     2fed:       8b 41 20                mov    0x20(%ecx),%eax
             %eax is the address of the method
>     2ff0:       ff d0                   call   *%eax
             Which is called here.

The problem being that the method at the offset '0x20' in the VTable is NOT
CreateSurface (as you want to use in the code) but 'EnumDisplayModes' and
thus the crash.

I will now look at how the VTable is built for COM objects in the case of
C++ (you are building with g++, no ?) and try to understand why this
particular code is generated.


		 Lionel Ulmer -

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