Opening a terminal device file

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 these files; they are opened by special programs and then become the standard input, standard output, and standard error of applications.

As described in open(), opening a terminal device file with the O_NONBLOCK flag clear causes the process to block until the terminal device is ready and available. If the CLOCAL mode is not set, this implies waiting until a connection is established. If CLOCAL mode is set in the terminal, or the O_NONBLOCK flag is specified when calling open(), the open() function returns a file descriptor without waiting for a connection to be established.