What is "system call"

n. an instruction that interrupts the program being executed and passes control to the supervisor [syn: supervisor call instruction]

In computing, a system call is the programmatic way in which a computer program requests a service from the kernel of the operating system it is executed on. This may include hardware-related services (for example, accessing a hard disk drive), creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system.

In most systems, system calls are possible to be made only from userspace processes, while in some systems, OS/360 and successors for example, privileged system code also issues system calls.