System call

A system call allows communication between a program running in userland and the kernel. In UNIX all I/O is done through a system call except for shared memory which must be set up through a system call though.

UNIX has a list of system calls ranging from 0 through 313 (OpenBSD). In OpenBSD the syscalls 0 through 5 are:

syscall(2), exit(2), fork(2), read(2), write(2), open(2)

You can read more about these system calls by reading through section 2 of the UNIX manual.

Also see syscall.