Extra argument being passed?

Jun 02, 2004

Malcolm Sharpe

Jun 02, 2004

Malcolm Sharpe

Jun 02, 2004

Jun 02, 2004

Jun 04, 2004

Jun 09, 2004

I am trying to port a small C++ program using DirectShow to D. Mostly, it has been working fine. However, one method call has problems with a null reference being passed, even though there is no argument. I'll snip out the relevent sections of code (minus some error-checking) to clarify: // dshow.d module dshow; ... interface IMediaEvent : IDispatch { extern (Windows): ... HRESULT WaitForCompletion(long msTimeout, long * pEvCode); ... } ... // test.d import dshow; ... IMediaEvent event = null; ... graph.QueryInterface(&IID_IMediaEvent, cast(void**)(&event)); ... long evCode; event.WaitForCompletion(INFINITE, &evCode); ... The program has an access violation on that last line (event.WaitForCompletion). It turns out it is a null reference problem. According to the Visual Studio debugger, this is the disassembly for that last line: 00402169 lea eax,[evCode] 0040216C push eax 0040216D push 0 0040216F push 0FFFFFFFFh 00402171 push dword ptr [event] 00402174 mov ecx,dword ptr [event] 00402177 mov edx,dword ptr [ecx] 00402179 call dword ptr [edx+24h] 0040217C add esp,20h 0040217F mov dword ptr [ebp-4],0 00402186 jmp +18Ch (40219Ch) The interesting line is 0040216D. It seems something is being pushed onto the stack in between INFINITE and &evCode. I'm still somewhat of a new programmer, so I don't know exactly what's going on here, but I can hazard a guess that the 0 is an argument that is being mistakenly passed to WaitForCompletiion. If I step to the offending line and then set the execution to the next line (thereby running every line except the offending line) the code works and evCode is assigned a value. Clearly something's up. Any ideas? I can post the full code of the program if I need to, but there are about 1000 lines between both files. - Malcolm Sharpe

See Comments Malcolm Sharpe wrote: > > 00402169 lea eax,[evCode] //Calculates relative address of evCode > 0040216C push eax //pushes that value onto the stack > 0040216D push 0 //pushes 0 on the stack > 0040216F push 0FFFFFFFFh //pushes infinite on the stack > 00402171 push dword ptr [event] //pushes event as dword onto stack > 00402174 mov ecx,dword ptr [event] //moves event into ecx > 00402177 mov edx,dword ptr [ecx] //moves ecx into edx (=event) > 00402179 call dword ptr [edx+24h] //call into vtable of event > 0040217C add esp,20h //Add 20 to esp ;-) Whatever is there > 0040217F mov dword ptr [ebp-4],0 //Move something > 00402186 jmp +18Ch (40219Ch) //Jump to 18C after 40218C > > > The interesting line is 0040216D. It seems something is being pushed onto the stack in between INFINITE and &evCode. I'm still somewhat of a new programmer, so I don't know exactly what's going on here, but I can hazard a guess that the 0 is an argument that is being mistakenly passed to WaitForCompletiion. If I step to the offending line and then set the execution to the next line (thereby running every line except the offending line) the code works and evCode is assigned a value. > > Clearly something's up. Any ideas? I can post the full code of the program if I need to, but there are about 1000 lines between both files. > > - Malcolm Sharpe I don't know what the 0 is for, too. Maybe my comments make it easier to read for the next one. Maybe this is a compiler bug?

Stephan Wienczny wrote: > See Comments > > Malcolm Sharpe wrote: > > > >> 00402169 lea eax,[evCode] //Calculates relative address of evCode >> 0040216C push eax //pushes that value onto the stack >> 0040216D push 0 //pushes 0 on the stack >> 0040216F push 0FFFFFFFFh //pushes infinite on the stack >> 00402171 push dword ptr [event] //pushes event as dword onto stack >> 00402174 mov ecx,dword ptr [event] //moves event into ecx >> 00402177 mov edx,dword ptr [ecx] //moves ecx into edx (=event) >> 00402179 call dword ptr [edx+24h] //call into vtable of event >> 0040217C add esp,20h //Add 20 to esp ;-) Whatever is there >> 0040217F mov dword ptr [ebp-4],0 //Move something >> 00402186 jmp +18Ch (40219Ch) //Jump to 18C after 40218C >> >> >> The interesting line is 0040216D. It seems something is being pushed onto the stack in between INFINITE and &evCode. I'm still somewhat of a new programmer, so I don't know exactly what's going on here, but I can hazard a guess that the 0 is an argument that is being mistakenly passed to WaitForCompletiion. If I step to the offending line and then set the execution to the next line (thereby running every line except the offending line) the code works and evCode is assigned a value. >> >> Clearly something's up. Any ideas? I can post the full code of the program if I need to, but there are about 1000 lines between both files. >> >> - Malcolm Sharpe > > > I don't know what the 0 is for, too. Maybe my comments make it easier to read for the next one. Maybe this is a compiler bug? That's about what I thought that code did. The jmp instruction is there because the method call is at the end of a try block. Maybe the add and mov are there as part of that as well. Here's some more information that might be helpful: if I replace long evCode; event.WaitForCompletion(INFINITE, &evCode); with OAEVENT hEvent; event.GetEventHandle(&hEvent); the event.GetEventHandle(&hEvent); statement produces the disassembly 0040213D lea edx,[hEvent] 00402140 push edx 00402141 push dword ptr [event] 00402144 mov ebx,dword ptr [event] 00402147 mov esi,dword ptr [ebx] 00402149 call dword ptr [esi+1Ch] 0040214C add esp,1Ch 0040214F mov dword ptr [ebp-4],0 00402156 jmp +15Ch (40216Ch) and there is no error. It is also clear that there is no extra argument being passed, unlike in the problem code.

The 'extra argument' you see is the hidden 'this' pointer. 'this' points to the instance of the interface object. But you've set 'event' to null. "Malcolm Sharpe" <masharpe2000@yahoo.ca> wrote in message news:c9j781$2r8t$1@digitaldaemon.com... > I am trying to port a small C++ program using DirectShow to D. Mostly, it has been working fine. However, one method call has problems with a null reference being passed, even though there is no argument. I'll snip out the relevent sections of code (minus some error-checking) to clarify: > > // dshow.d > > module dshow; > > ... > > interface IMediaEvent : IDispatch > { > extern (Windows): > > ... > > HRESULT WaitForCompletion(long msTimeout, long * pEvCode); > > ... > > } > > ... > > > // test.d > > import dshow; > > ... > > IMediaEvent event = null; > > ... > > graph.QueryInterface(&IID_IMediaEvent, cast(void**)(&event)); > > ... > > long evCode; > event.WaitForCompletion(INFINITE, &evCode); > > ... > > > The program has an access violation on that last line > (event.WaitForCompletion). It turns out it is a null reference problem. > According to the Visual Studio debugger, this is the disassembly for > that last line: > > > 00402169 lea eax,[evCode] > 0040216C push eax > 0040216D push 0 > 0040216F push 0FFFFFFFFh > 00402171 push dword ptr [event] > 00402174 mov ecx,dword ptr [event] > 00402177 mov edx,dword ptr [ecx] > 00402179 call dword ptr [edx+24h] > 0040217C add esp,20h > 0040217F mov dword ptr [ebp-4],0 > 00402186 jmp +18Ch (40219Ch) > > > The interesting line is 0040216D. It seems something is being pushed onto the stack in between INFINITE and &evCode. I'm still somewhat of a new programmer, so I don't know exactly what's going on here, but I can hazard a guess that the 0 is an argument that is being mistakenly passed to WaitForCompletiion. If I step to the offending line and then set the execution to the next line (thereby running every line except the offending line) the code works and evCode is assigned a value. > > Clearly something's up. Any ideas? I can post the full code of the program if I need to, but there are about 1000 lines between both files. > > - Malcolm Sharpe