C++ function used to work, now returning 0xfdfdfdfd

Question

I have some code I wrote a few years ago. It has been working fine, but after a recent rebuild with some new, unrelated code elsewhere, it is no longer working. This is the code:

//myobject.h
...
inline  CMapStringToOb* GetMap(void)    {return (m_lpcMap);};
...

The above is accessed from the main app like so:

//otherclass.cpp
...
CMapStringToOb* lpcMap = static_cast<CMyObject*>(m_lpcBaseClass)->GetMap();
...

Like I said, this WAS working for a long time, but it's just decided to start failing as of our most recent build. I have debugged into this, and I am able to see that, in the code where the pointer is set, it is correctly setting the memory address to an actual value. I have even been able to step into the set function, write down the memory address, then move to this function, let it get 0xfdfdfdfd, and then manually get the memory address in the debugger. This causes the code to work. Now, from what I've read, 0xfdfdfdfd means guarding bytes or "no man's land", but I don't really understand what the implications of that are. Supposedly it also means an off by one error, but I don't understand how that could happen, if the code was working before.

Answer 1

Scenarios where "magic" happens almost always come back to memory corruption. I suspect that somewhere else in your code you've modified memory incorrectly, and it's resulting in this peculiar behavior. Try testing some different ways of entering this part of the code. Is the behavior consistent?

This could also be caused by an incorrectly built binary. Try cleaning and rebuilding your project.

Answer 2

I'm assuming from the Hungarian notation that you're using Visual Studio. Since you do know the address that holds the map pointer, start your program in the debugger and set a data breakpoint when that map pointer changes (the memory holding the map pointer, not the map pointed to). Then you'll find out exactly when it's getting overwritten.

Answer 3

0xfdfdfdfd typically implies that you have accessed memory that you weren't supposed to.

• There is a good chance the memory was allocated and subsequently freed. So you're using freed memory.
• static_cast can modify a pointer and you have an explicit cast to CMyObject and an implicit cast to CMapStringToOb. Check the validity of the pointer directly returned from GetMap().