c++ - How does "Edit and continue" work in Visual Studio?

Question

I have always found this to be a very useful feature in Visual Studio. For those who don't know about it, it allows you to edit code while you are debugging a running process, re-compile the code while the binary is still running and continue using the application seamlessly with the new code, without the need to restart it.

How is this feature implemented? If the code I am modifying is in a DLL loaded by the application, does the application simply unload the DLL and reload it again? This would seem to me like it would be prone to instability issues, so I assume it would be smarter than this. Any ideas?

Answer 1

My understanding is that when the app is compiled with support for Edit and Continue enabled, the compiler leaves extra room around the functions in the binary image to allow for adding additional code. Then the debugger can compile a new version of the function, replace the existing version (using the padding space as necessary), fix up the stack, set the instruction pointer, and keep going. That way you don't have to fix up any jump pointers, as long as you have enough padding.

Note that Edit and Continue doesn't usually work on code in libs/dlls, only with the main executable code.