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TTY(4)                     Kernel Interfaces Manual                     TTY(4)

       tty - general terminal interface

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

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

       As for terminals in general: all of the low-speed asynchronous communi-
       cations ports use the same general interface, no matter  what  hardware
       is  involved.   The remainder of this section discusses the common fea-
       tures of the interface.

       When a terminal 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 out-
       put  file.   The very first terminal file open in a process becomes the
       control terminal for that process.  The control terminal plays  a  spe-
       cial  role  in  handling quit or interrupt signals, as discussed below.
       The control terminal is inherited by a child  process  during  a  fork,
       even if the control terminal is closed.  The set of processes that thus
       share a control terminal is called a process group; all  members  of  a
       process  group  receive  certain  signals  together,  see DEL below and

       A terminal associated with one of these files  ordinarily  operates  in
       full-duplex mode.  Characters may be typed at any time, even while out-
       put 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  that  have
       not yet been read by some program.  Currently this limit is 256 charac-
       ters.  When the input limit is reached all  the  saved  characters  are
       thrown away without notice.

       Normally,  terminal  input  is processed in units of lines.  This means
       that a program attempting to read will be  suspended  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.   There  are
       special  modes,  discussed  below, that permit the program to read each
       character as typed without waiting for a full line.

       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 beyond 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 following escape sequences can  be  gener-
       ated on output and accepted on input:

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

       Certain  ASCII  control characters have special meaning.  These charac-
       ters are not passed to a reading program except in raw mode where  they
       lose  their  special  character.   Also, it is possible to change these
       characters from the default; see below.

       EOT    (Control-D) may be used to generate an end of file from a termi-
              nal.   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 standard end-of-file indication.

       DEL    (Rubout) is not passed to a program but generates  an  interrupt
              signal  which  is sent to all processes with the associated con-
              trol terminal.  Normally each such process is forced  to  termi-
              nate,  but  arrangements may be made either to ignore the signal
              or to receive a trap to an agreed-upon location.  See signal(2).

       FS     (Control-\ or control-shift-L) 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.

       DC3    (Control-S)  delays all printing on the terminal until something
              is typed in.

       DC1    (Control-Q) restarts  printing after DC3 without generating  any
              input to a program.

       When  the  carrier  signal  from the dataset drops (usually because the
       user has hung up his terminal) a hangup signal is sent to all processes
       with  the terminal as control terminal.  Unless other arrangements have
       been made, this signal causes  the  processes  to  terminate.   If  the
       hangup  signal is ignored, any read returns with an end-of-file indica-
       tion.  Thus programs that read a terminal and test for  end-of-file  on
       their input can terminate appropriately when hung up on.

       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
       exceeds 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 that respond to it from hanging up.

       Several  ioctl(2)  calls apply to terminals.  Most of them use the fol-
       lowing structure, defined in <sgtty.h>:

       struct sgttyb {
            char sg_ispeed;
            char sg_ospeed;
            char sg_erase;
            char sg_kill;
            int  sg_flags;

       The sg_ispeed and sg_ospeed fields describe the input and output speeds
       of  the  device  according to the following table, which corresponds to
       the DEC DH-11 interface.  If other hardware is used,  impossible  speed
       changes  are  ignored.   Symbolic values in the table are as defined in

       B0      0    (hang up dataphone)
       B50     1    50 baud
       B75     2    75 baud
       B110    3    110 baud
       B134    4    134.5 baud
       B150    5    150 baud
       B200    6    200 baud
       B300    7    300 baud
       B600    8    600 baud
       B1200   9    1200 baud
       B1800   10   1800 baud
       B2400   11   2400 baud
       B4800   12   4800 baud
       B9600   13   9600 baud
       EXTA    14   External A
       EXTB    15   External B

       In the current configuration, only 110, 150,  300  and  1200  baud  are
       really  supported  on  dial-up lines.  Code conversion and line control
       required for IBM 2741's (134.5 baud) must be implemented by the  user's
       program.   The half-duplex line discipline required for the 202 dataset
       (1200 baud) is not supplied; full-duplex 212 datasets work fine.

       The sg_erase and sg_kill fields of the argument structure  specify  the
       erase and kill characters respectively.  (Defaults are # and @.)

       The sg_flags field of the argument structure contains several bits that
       determine the system's treatment of the terminal:

       ALLDELAY 0177400 Delay algorithm selection
       BSDELAY  0100000 Select backspace delays (not implemented):
       BS0      0
       BS1      0100000
       VTDELAY  0040000 Select form-feed and vertical-tab delays:
       FF0      0
       FF1      0100000
       CRDELAY  0030000 Select carriage-return delays:
       CR0      0
       CR1      0010000
       CR2      0020000
       CR3      0030000
       TBDELAY  0006000 Select tab delays:
       TAB0     0
       TAB1     0001000
       TAB2     0004000
       XTABS    0006000
       NLDELAY  0001400 Select new-line delays:
       NL0      0
       NL1      0000400
       NL2      0001000
       NL3      0001400
       EVENP    0000200 Even parity allowed on input (most terminals)
       ODDP     0000100 Odd parity allowed on input
       RAW      0000040 Raw mode: wake up on all characters, 8-bit interface
       CRMOD    0000020 Map CR into LF; echo LF or CR as CR-LF
       ECHO     0000010 Echo (full duplex)
       LCASE    0000004 Map upper case to lower on input
       CBREAK   0000002 Return each character as soon as typed
       TANDEM   0000001 Automatic flow control

       The delay bits specify how long transmission stops to allow for mechan-
       ical  or  other movement when certain characters are sent to the termi-
       nal.  In all cases a value of 0 indicates no delay.

       Backspace delays are currently ignored but might be used  for  Terminet

       If  a  form-feed/vertical  tab delay is specified, it lasts for about 2

       Carriage-return delay type 1 lasts about .08 seconds  and  is  suitable
       for  the  Terminet  300.   Delay  type 2 lasts about .16 seconds and is
       suitable for the VT05 and the TI 700.  Delay type  3  is  unimplemented
       and is 0.

       New-line  delay  type 1 is dependent on the current column and is tuned
       for Teletype model 37's.  Type 2 is useful for the VT05  and  is  about
       .10 seconds.  Type 3 is unimplemented and is 0.

       Tab delay type 1 is dependent on the amount of movement and is tuned to
       the Teletype model 37.  Type 3, called XTABS, is not a delay at all but
       causes  tabs to be replaced by the appropriate number of spaces on out-

       Characters with the wrong parity, as determined by bits  200  and  100,
       are ignored.

       In raw mode, every character is passed immediately to the program with-
       out waiting until a full line has been typed.  No erase  or  kill  pro-
       cessing is done; the end-of-file indicator (EOT), the interrupt charac-
       ter (DEL) and the quit character (FS) are not treated specially.  There
       are  no  delays and no echoing, and no replacement of one character for
       another; characters are a full 8 bits for both input and output (parity
       is up to the program).

       Mode  020  causes  input  carriage returns to be turned into new-lines;
       input of either CR or LF causes LF-CR both to be echoed (for  terminals
       with a new-line function).

       CBREAK  is  a sort of half-cooked (rare?) mode.  Programs can read each
       character as soon as typed, instead of waiting for  a  full  line,  but
       quit  and  interrupt  work, and output delays, case-translation, CRMOD,
       XTABS, ECHO, and parity work normally.  On the other hand there  is  no
       erase or kill, and no special treatment of \ or EOT.

       TANDEM mode causes the system to produce a stop character (default DC3)
       whenever the input queue is in danger of overflowing, and a start char-
       acter  (default DC1) when the input queue has drained sufficiently.  It
       is useful for flow control when  the  `terminal'  is  actually  another
       machine that obeys the conventions.

       Several ioctl calls have the form:

       #include <&lt;sgtty.h>&gt;

       ioctl(fildes, code, arg)
       struct sgttyb *arg;

       The applicable codes are:

              Fetch  the parameters associated with the terminal, and store in
              the pointed-to structure.

              Set the parameters according to the pointed-to  structure.   The
              interface delays until output is quiescent, then throws away any
              unread characters, before changing the modes.

              Set the parameters but do not delay or flush  input.   Switching
              out of RAW or CBREAK mode may cause some garbage input.

       With the following codes the arg is ignored.

              Set ``exclusive-use'' mode: no further opens are permitted until
              the file has been closed.

              Turn off ``exclusive-use'' mode.

              When the file is closed for the last time, hang up the terminal.
              This  is  useful when the line is associated with an ACU used to
              place outgoing calls.

              All characters waiting in input or output queues are flushed.

       The following codes affect characters that are special to the  terminal
       interface.   The  argument  is  a  pointer  to the following structure,
       defined in &lt;sgtty.h&gt;:

       struct tchars {
            char t_intrc;       /* interrupt */
            char t_quitc;       /* quit */
            char t_startc; /* start output */
            char t_stopc;  /* stop output */
            char t_eofc;        /* end-of-file */
            char t_brkc;        /* input delimiter (like nl) */

       The default values for these characters are DEL, FS, DC1, DC3, EOT, and
       -1.   A  character value of -1 eliminates the effect of that character.
       The t_brkc character, by default -1, acts like a new-line  in  that  it
       terminates  a  `line,'  is  echoed,  and is passed to the program.  The
       `stop' and `start' characters may be the  same,  to  produce  a  toggle
       effect.  It is probably counterproductive to make other special charac-
       ters (including erase an kill) identical.

       The calls are:

              Change the various special characters  to  those  given  in  the

              Set the special characters to those given in the structure.


       getty(8), stty (1), signal(2), ioctl(2)

       Half-duplex terminals are not supported.

       The terminal handler has clearly entered the race for ever-greater com-
       plexity and generality.  It's still not complex and general enough  for
       TENEX fans.