Sunday, April 24, 2011


Actually, the first thing is to translate it to C++. The sensors are crying out for inheritance. A little of this, a bit of that, ok, it compiles in C++. There are quite a few things that were warnings in C that are errors in C++, and some errors are caught by the linker instead of the compiler. These latter things are mostly caused by function overloading and its companion mechanism, name mangling.

With a clean toolchain, like Java, you can do overloading easily and cleanly. All the tools are designed to support it. However, with C++, only the compiler is C++. It has to translate C++ code with overloaded function names into something that the assembler and linker can understand. If you try to make an object with two symbols with the same name, the next step in the toolchain (in this case, the assembler) will complain. If you try to trick the assembler by putting the two functions in two different objects, then the linker gets confused and complains. So, it is up to the compiler. What happens is that the compiler, when it generates its assembly language output, gives the two overloaded functions different names. It tacks on extra stuff to the name depending on the types of the parameters it takes and on the return type. Then, when the function is called, the compiler figures out the types of the parameters, then tacks on the appropriate extra stuff to call the correct function. This is called name mangling. It's something that took me a long time to appreciate, and maybe I still don't. I don't like it, but at least I understand it.

The thing is then, when translating from C to C++, things like mismatched function prototypes between the header and the code used to be caught by the compiler. Now, the C++ compiler thinks the header describes one function and the mismatched body describes a completely different function. So, it happily compiles the one in the body, for which there is now no interface, and when it compiles another file that uses this header, it happily generates code to call a function which doesn't exist. So, you compile all 572 .cpp files and they all go through without an error, but then the linker says something totally opaque like:

cannot find symbol: __ZZZ__KQNNfoo_I_I_JJnncxa

and you are all like: The function is called foo(int,int)! Where did all that other stuff come from? That is the power and mystery of name mangling. The error will be reported in the object that tried to call foo(int,int), but the problem is in the header that defined foo(int,int) and the body which defined foo(int, int, struct bar*). Hopefully you can see the original name under the mangling and know or can find where that foo() function is. Then make the headers and bodies match.

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