Opening a C++ crash dump does not show the right line in the call stack

Question

I see that when I open a C++ crash dump in Visual Studio, I find that the call stack points to - either the line from which it jumped to the next frame in that function, or sometimes the next line after the line from which it jumped to the next frame in that function. Why is that? What is the logic behind that?

TIA!

Answer 1

Basically the location of call is not recorded; the location of return is recorded. So the return location is displayed.

The call stack is extracted from the stack. When you call a functiom, the return location in your code where the instruction pointer is going to go when the function finishes is placed on the stack.

The debugger/call stack display software reverse engineers the data on the stack to work out where this return will be. Then pdb files are used to map the location of return to lines of code.

Two branches of one if clause could have different spots where you call a function, but both return at the exact same instruction. Determining which of the two where used to call the function is impractical, while knowing where the function returns to is easy and reliable. And that line is usually enough information to debug the problem.

On top of that, optimizations by the compiler break down the idea that you are runnimg C++ code line by line; you are actually writing code generated by C++ code. An instruction in the generated machine code may correspond to parts of multiple different C++ code lines.

Between the two, having the call stack frames pointing a line off is not rare. Sometimes it is estremely far off; and with identical comdat folding sometimes it is the wrong function entirely.