Here are some POSIX system calls:
s = kill(pid, signal): By using this system call a user sends a signal to a process. When the process is ready for the signal then signal handle is run and when it is not ready, then the process is terminated.
seconds = time(&seconds): It is used for getting elapsed time since Jan 1 , 1970.
size = brk(addr): It is used for setting size of data segment.
pid = getpid(): For returning process id of the caller.
pid = getpgrp(): For returning id of process group of the caller.
s = pause(): It is used for suspending caller until next signal.
s = pipe(&fd): Used for creating a pipe.
s = sync(): It is used for flushing all cache blocks.
uid = getuid(): It is used for retrieving uid of a caller.
s = rename(old, new): It is used for renaming a file.