SWIG is a wrapper generator that is able to connect compiled libraries to a bunch of scripting languages. The process is mostly automatic, but to tackle some corner cases, you have to help the generator do the right thing. In my library, all functions would return an integer, which is an error code. A special function, following the same behavior as strerror_r, can be used to retrieve the meaning of a special error code. This is a pretty usual mechanism for C code. But that’s not the way scripting languages work. In their world, functions are rather supposed to raise exceptions.