Callback parameters gets shifted

Question

I have a video c++ callback function where the parameters get suddently shifted after a few hours. In debug, it will assert on this:

void CCameraInstance::VideoCallback(void *pContext, unsigned char *pBuffer, long nWidth, long nHeight, long nFrameErrorNo)
{
   assert(nWidth < 4000);
   CCameraInstance *pThis = (CCameraInstance*)pContext;
   pThis->PaintFrame(pBuffer, nWidth, nHeight, nFrameErrorNo);
}

When the debugger breaks on the assert, nWidth has a big invalid value. However, nHeight is 320 (the width value) and nFrameErrorNo is 240 (the nHeight value).

How can the parameters get shift that way?

You might be seeing the aftershocks of Undefined Behavior. Check the code that executed _before_ this for any array bound violations, buffer overwrite or other such problems. — Masked Man, Oct 21 '13 at 15:27
Without the full code, I'm just making guesses here. But, I would bet my money that you are passing in an uninitialized value into the width in that function. — Caesar, Oct 21 '13 at 15:27
I'm still having this same problem after over a year. However, it's hard to reproduce. I'm using Visual Studio 2013. Any debugging tips? — Eric Pronovost, Mar 10 '15 at 17:08

score 0 · Answer 1 · edited May 23 '17 at 11:59

The shift could be caused by the hidden this pointer. http://www.learncpp.com/cpp-tutorial/8-8-the-hidden-this-pointer/

From the code you have pasted here

void CCameraInstance::VideoCallback(void *pContext, unsigned char *pBuffer, long nWidth, long nHeight, long nFrameErrorNo)

I can see the callback function is a member of class CCameraInstance

I'm not sure whether you are defining the function as a static function or a normal one. But in theory it should be a static function to avoid the this pointer. Using a C++ class member function as a C callback function

However, I had a problem with C++/CLI even if i have define the member function as static. The this pointer/Handle still exist.

I think you can try to define your function as

void CCameraInstance::VideoCallback(CCameraInstance* test,void *pContext, unsigned char *pBuffer, long nWidth, long nHeight, long nFrameErrorNo)

and have try.

If you are using C++/CLI it would be

void CCameraInstance::VideoCallback(CCameraInstance^ test,void *pContext, unsigned char *pBuffer, long nWidth, long nHeight, long nFrameErrorNo)

Callback parameters gets shifted

1 Answers1