System calls - Definition and Overview

In computing, a system call is the mechanism used by an application program to request service from the operating system. System calls often use a special machine code instruction which causes the processor to change mode (e.g. to "supervisor mode" or "protected mode"). This allows the OS to perform restricted actions such as accessing hardware devices or the memory management unit.

On Posix and similar systems, popular system calls are for example: close, execve, fork, kill, open, read, wait, write.