The interface between an OS and programming languages

• interface is well-defined and documented using C headers files and online documentation

• well-defined, orthogonal set of system-calls

• system-calls receive a small number of fixed-sized parameters (integers and pointers that can be passed in CPU registers)

• system-calls return results through returned integer or reference parameters

• fixed-sized identifiers and parameters handled by the kernel

• variable-sized objects handled by user-level libraries

• collections - processes, memory, files, directories, users, networking

• consistent use of file-descriptors for all 'types' of files and devices