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When a terminal-device-file is opened, it normally causes the process to wait until a connection is established. In practice, application-programs seldom open such files; instead, at system-initialization time special-programs open terminal-device-files as the ``standard input'', ``standard output'' and ``standard error'' files (see Intro(S)).
Opening a terminal-device-file with the flag O_NONBLOCK clear on the open system call causes the process to block until the terminal-device is ready and available (see open(S)). The flag CLOCAL can also affect the open system call (see ``Control modes'' in termio(M)).