TTY(IV)                      5/27/74                      TTY(IV)

     tty - general typewriter interface

     This  section  describes both a particular special file, and
     the general nature of the typewriter interface.

     The file /dev/tty is, in each process,  a  synonym  for  the
     control typewriter associated with that process.  It is use-
     ful for programs or Shell sequences which wish to be sure of
     writing  messages on the typewriter no matter how output has
     been redirected.  It can also be used for programs which de-
     mand  a  file  name for output, when typed output is desired
     and it is tiresome to find out which typewriter is currently
     in use.

     As  for  typewriters  in general: all of the low-speed asyn-
     chronous communications ports use the  same  general  inter-
     face, no matter what hardware is involved.  The remainder of
     this section discusses the common features of the interface;
     the  KL,  DC, and DH writeups(IV) describe peculiarities of
     the individual devices.

     When a typewriter file is opened, it causes the  process  to
     wait  until a connection is established.  In practice user's
     programs seldom open these files; they are  opened  by  init
     and  become  a user's input and output file.  The very first
     typewriter file open in a process becomes the control type-
     writer  for  that  process.   The control typewriter plays a
     special role in handling quit or interrupt signals, as  dis-
     cussed  below.   The  control  typewriter  is inherited by a
     child process during a fork.

     A terminal associated with one of these files ordinarily op-
     erates  in full-duplex mode.  Characters may be typed at any
     time, even while output is occurring, and are only lost when
     the  system's  character  input  buffers  become  completely
     choked, which is rare, or when the user has accumulated  the
     maximum  allowed  number  of input characters which have not
     yet been read by some program.  Currently this limit is  256
     characters.   When  the input limit is reached all the saved
     characters are thrown away without notice.

     These special files have a number  of  modes  which  can  be
     changed  by  use  of  the stty system call(II).  When first
     opened, the interface mode is 300 baud;  either  parity  ac-
     cepted; 10 bits/character (one stop bit); and newline action
     character.  Modes that can be changed by  stty  include  the
     interface  speed  (if  the  hardware permits); acceptance of
     even parity, odd parity, or both; a raw mode  in  which  all
     characters may be read one at a time; a carriage return (CR)
     mode in which CR is mapped into newline on input and  either
     CR  or  line  feed (LF) cause echoing of the sequence LF-CR;
     mapping of upper case letters into lower  case;  suppression
     of  echoing;  a variety of delays after function characters;

                              - 1 -

TTY(IV)                      5/27/74                      TTY(IV)

     and the printing of tabs as spaces.  See  getty(VIII)  for
     the way that terminal speed and type are detected.

     Normally,  typewriter  input is processed in units of lines.
     This means that a program attempting to read  will  be  sus-
     pended until an entire line has been typed.  Also, no matter
     how many characters are requested in the read call, at  most
     one  line  will be returned.  It is not however necessary to
     read a whole line at once; any number of characters  may  be
     requested in a read, even one, without losing information.

     During  input,  erase  and kill processing is normally done.
     By default, the character  `#'  erases  the  last  character
     typed, except that it will not erase beyond the beginning of
     a line or an EOT.  By default, the character `@'  kills  the
     entire  line up to the point where it was typed, but not be-
     yond an EOT.  Both these characters operate on  a  keystroke
     basis  independently  of any backspacing or tabbing that may
     have been done.  Either `@' or `#' may be entered  literally
     by preceding it by `\'; the erase or kill character remains,
     but the `\' disappears.  These two characters may be changed
     to others.

     When  desired,  all  upper-case  letters are mapped into the
     corresponding lower-case letter.  The upper-case letter  may
     be  generated by preceding it by `\'.  In addition, the fol-
     lowing escape sequences are generated on output and accepted
     on input:

            for    use
             `      \'
             |      \!
             ~      \^
             {      \(
             }      \)

     In raw mode, the program reading is awakened on each charac-
     ter.  No erase or kill processing is done; and the EOT, quit
     and interrupt characters are not treated specially.  The in-
     put parity bit is passed back to the reader, but  parity  is
     still generated for output characters.

     The  ASCII EOT (control-D) character may be used to generate
     an end of file from a typewriter.  When an EOT is  received,
     all the characters waiting to be read are immediately passed
     to the program, without waiting for a new-line, and the  EOT
     is  discarded.   Thus  if  there  are no characters waiting,
     which is to say the EOT occurred at the beginning of a line,
     zero  characters  will be passed back, and this is the stan-
     dard end-of-file indication.  The EOT  is  passed  back  un-
     changed in raw mode.

     When  the carrier signal from the dataset drops (usually be-
     cause the user has hung up his terminal) a hangup signal  is
     sent  to  all processes with the typewriter as control type-
     writer.  Unless other arrangements have been made, this sig-

                              - 2 -

TTY(IV)                      5/27/74                      TTY(IV)

     nal causes the processes to terminate.  If the hangup signal
     is ignored, any read returns with an end-of-file indication.
     Thus  programs  which read a typewriter and test for end-of-
     file on their input can terminate appropriately when hung up

     Two characters have a special meaning when typed.  The ASCII
     DEL character (sometimes called `rubout') is not passed to a
     program  but  generates an interrupt signal which is sent to
     all processes with the associated control typewriter.   Nor-
     mally each such process is forced to terminate, but arrange-
     ments may be made either to ignore the signal or to  receive
     a trap to an agreed-upon location.  See signal(II).

     The  ASCII  character  FS  generates  the  quit signal.  Its
     treatment is identical to the interrupt signal  except  that
     unless  a  receiving  process has made other arrangements it
     will not only be terminated but a core image  file  will  be
     generated.   If you find it hard to type this character, try
     control-\ or control-shift-L.

     When one or more characters are written, they  are  actually
     transmitted  to  the  terminal as soon as previously-written
     characters  have  finished  typing.   Input  characters  are
     echoed  by  putting them in the output queue as they arrive.
     When a process produces characters more  rapidly  than  they
     can be typed, it will be suspended when its output queue ex-
     ceeds some limit.  When the queue has drained down  to  some
     threshold  the  program  is  resumed.  Even parity is always
     generated on output.  The EOT character is  not  transmitted
     (except  in  raw mode) to prevent terminals which respond to
     it from hanging up.


     dc(IV), kl(IV), dh(IV), getty(VIII), stty (I, II),  gtty
     (I, II), signal(II)

     Half-duplex  terminals  are not supported.  On raw-mode out-
     put, parity should be transmitted as specified in the  char-
     acters written.

                              - 3 -