`const char*`

As to the method names, I personally like `errmsg()` and `set_errmsg()` better but that's because I've drunk the Google Kool-Aid (Gool-Aid?) and liked it.

The philosophy behind it is that simple getters and setters are lower-cased (where 'simple' means 'simple (or no) observable side effects`) and more complex methods are camel-cased.