PTY(4)                  GENIX Programmer's Manual                  PTY(4)



      NAME
           pty - pseudo-terminal device

      DESCRIPTION
           Ptys are controllers for terminals that exist only in software.  For
           each  pty  device,  there  exists  a corresponding software terminal
           (also known as the pty slave device).  Software terminals  act  like
           real terminals, so that programs running on them act normally.  How-
           ever, instead of communicating with real users, their input and out-
           put  go  through the process which owns the corresponding pty.  Ptys
           thus act similarly to pipes, but avoid the problems  of  pipes  (for
           example, interrupt characters work on ptys.)

           When a write is done to a pty, the data appears to have been "typed"
           into the corresponding software terminal, at which point the program
           running on that terminal can read the data  as  input.   The  normal
           terminal buffering mechanism is used, so that if too much is written
           to the pty too quickly, characters can be lost.  This can be avoided
           by using the TIOCCHKINP ioctl, which returns whether or not any pro-
           cess is sleeping waiting for input on the terminal.  Writing to  the
           pty  only  when input is desired (or when an interrupt character has
           to be typed) will prevent most overflow problems.

           When output is written to a software terminal, it is buffered  using
           the normal terminal buffer mechanism.  The data remains until a read
           is performed on the pty, at which time the data  is  returned.   The
           read  will  return  as  much data as currently exists, and thus will
           sleep only if the terminal buffer  is  empty.   The  FIONREAD  ioctl
           returns  how  much  data  is  ready  for reading on the pty, so that
           sleeps can be avoided.  If the terminal buffer fills up, the program
           on the terminal will sleep so that no data is lost.

           Ioctls performed on the pty  are  redirected  to  the  corresponding
           software  terminal.   This  allows  the owner of the pty to read the
           settings of the terminal and act accordingly.

           A pty is an exclusive-access device, so that it  can  be  opened  by
           only  one  user at a time.  When the pty is closed, the processes on
           the corresponding terminal are sent a SIGHUP signal and the terminal
           buffers  are  flushed.  Programs that use the software terminal when
           the corresponding pty is not in use will sleep until it is open.

      FILES
           /dev/ptyn The pty numbered n
           /dev/ttypn     The corresponding software terminal numbered n

      SEE ALSO
           tty(4)







      Printed 11/16/83                                                        1