Don't do your own locking. Let the caller pass in state. Push locking to the caller. Don't have your own global state that would need hidden locks, but instead let the caller handle it with arguments.

That is similar how the STL does it, but not like stdio

I'm not sure I fully agree. - For simple libraries it's likely good advice. - But it encourages big locks and poor scaling. It may be right for desktop apps, but not necessarily for server code that needs to scale. For some things that's fine, but you don't want that for the big tree or hash table that your multi threaded server is built around on. - It avoids the problems of locks being non composable, that is the caller may need to know which order the locks need to be called, to avoid deadlock. Actually it doesn't avoid it, just pushes it to someone else. However if you make sure the library is always the leaf and never calls back the library locks will be generally at the bottom of the lock hierarchy.