Line disciplines
     A terminal file is used like any other file in the system in that it can
     be opened, read, and written to using standard system calls.  For each
     existing terminal file, there is a software processing module called a
     line discipline is associated with it.  The line discipline essentially
     glues the low level device driver code with the high level generic inter-
     face routines (such as read(2) and write(2)), and is responsible for
     implementing the semantics associated with the device.  When a terminal
     file is first opened by a program, the default line discipline called the
     termios line discipline is associated with the file.  This is the primary
     line discipline that is used in most cases and provides the semantics
     that users normally associate with a terminal.  When the termios line
     discipline is in effect, the terminal file behaves and is operated
     according to the rules described in termios(4).  Please refer to that man
     page for a full description of the terminal semantics.  The operations
     described here generally represent features common across all line
     disciplines, however some of these calls may not make sense in conjunc-
     tion with a line discipline other than termios, and some may not be sup-
     ported by the underlying hardware (or lack thereof, as in the case of
     ptys).


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