  Application Programming Interfaces (API) are exposed to developers in order
to reuse software libraries. API directives are natural-language statements in
API documentation that make developers aware of constraints and guidelines
related to the usage of an API. This paper presents the design and the results
of an empirical study on the directives of API documentation of object-oriented
libraries. Its main contribution is to propose and extensively discuss a
taxonomy of 23 kinds of API directives.
