使用P/Invoke使用char*参数调用C DLL函数

I've read other similar questions on this but they don't solve this particular problem. I have an old C-library with a touppercase function (as an example). This takes a char* and returns a char*. However, the pointer returned is a pointer to the same string (don't ask me I didn't write it).I've read other similar questions on this but t