Techniques for extending operating system subsystems are provided. The techniques involve receiving a library that provides functions that extend an application program interface of the subsystem of the operating system. The techniques dynamically add a set of local procedure call ports or system call numbers corresponding to functions provided by the library and provide a known local procedure call port for receiving requests to identify a dynamically added local procedure call port or system call number. When a function of the library is requested, the subsystem determines the dynamically added port or system call number corresponding to the function, and provides an indication of the determined port or system call number to a requester of the function. The requester of the function may then invoke the function using the indicated dynamically added port or system call number.