A request from a program for an operating-system kernel service. In the Unix examples here, opening, reading, linking and unlinking files request controlled changes or access to operating system resources. The operating-system kernel checks the request and performs the permitted operation; an interface specified by POSIX does not by itself dictate every internal implementation detail.
A system call creating a child operating-system process from a parent. The Linux kernel documents copy-on-write for ordinary inherited private memory pages, with separate address spaces but shared open file descriptions for inherited file descriptors. [ref]
