# tty(4)

    NAME
         tty, termios - terminals

    DESCRIPTION
         The tty driver family takes care  of  all  user  input  and  output.   It
         governs  the  keyboard,  the  console, the serial lines, and pseudo ttys.
         Input on any of these devices undergoes "input  processing",  and  output
         undergoes  "output processing" according to the standard termios terminal
         interface.

      Input processing
         Each terminal device has an input queue.  This queue  is  used  to  store
         preprocessed  input  characters, and to perform the backspacing and erase
         functions.  Some special characters like a newline make the  contents  of
         the  queue  available to a process reading from the terminal.  Characters
         up to and including the newline, or another so-called "line  break",  may
         be  read by a process.  The process need not read all characters at once.
         An input line may be read byte by byte if one wants  to.   A  line  break
         just makes characters available for reading, thats all.

         When data is made available depends on whether the tty  is  in  canonical
         mode  or  not.   In  canonical  mode the terminal processes input line by
         line.  A line ends with a newline (NL), end-of-file (EOF), or end-of-line
         (EOL).   Characters that have not been delimited by such a line break may
         be erased one by one with the ERASE character or all  at  once  with  the
         KILL  character.   Once  a  line  break  is  typed  the characters become
         available to a reading process and can no longer be  erased.   Once  read
         they  are removed from the input queue.  Several lines may be gathered in
         the input queue if no reader is present to read them, but  a  new  reader
         will  only  receive  one line.  Two line breaks are never returned in one
         read call.  The input queue has a maximum length of MAX_CANON characters.
         Any  more  characters  are discarded.  One must use ERASE or KILL to make
         the  terminal  functioning  again  if  the  input  queue  fills  up.   If
         nonblocking  I/O  is  set then -1 is returned with errno set to EAGAIN if
         the reader would otherwise be blocked.

         In non-canonical mode (raw mode for short) all characters are immediately
         available  to the reader in principle.  One may however tune the terminal
         to bursty input with the  MIN  and  TIME  parameters,  see  the  raw  I/O
         parameters section below.  In raw mode no characters are discarded if the
         input queue threatens to overflow if the device supports flow control.

      Output processing
         Characters written to a terminal device may  undergo  output  processing,
         which  is  usually  just inserting a carriage returns before newlines.  A
         writer may return before all characters are output if the characters  can
         be  stored  in the output buffers.  If not then the writer may be blocked
         until space is available.  If non-blocking I/O is set then only the count
         of the number of bytes that can be processed immediately is returned.  If
         no characters can be written at all then -1 is returned with errno set to
         EAGAIN.

      Special characters
         Some characters have special functions in some  of  the  terminal  modes.
         These  characters  are  as  follows,  with  the  Minix  defaults shown in
         parentheses:

         INTR (^?)
              Special input character that is recognized if  ISIG  is  set.   (For
              ISIG  and  other  flags  see  the various modes sections below.)  It
              causes a SIGINT signal to be sent to all processes in  the  terminal
              process group.  (See the section on session leaders below.)

         QUIT (^\)
              Special input character if ISIG is set.  Causes a SIGQUIT signal  to
              be sent to the terminal process group.

         ERASE (^H)
              Special input character if ICANON is set.  Erases the last character
              in the current line.

         KILL (^U)
              Special input character if ICANON is set.  Erases the entire line.

         EOF (^D)
              Special input character if ICANON  is  set.   It  is  a  line  break
              character  that is not itself returned to a reader.  If EOF is typed
              with no input present then the read  returns  zero,  which  normally
              causes the reader to assume that end-of-file is reached.

         CR (^M)
              Special input character if IGNCR or ICRNL is set.  It is a  carriage
              return  ('\r').   If IGNCR is set then CR is discarded.  If ICRNL is
              set and IGNCR is not set then CR is changed into an NL and  has  the
              same function as NL.

         NL (^J)
              Special input character if ICANON is set.   It  is  both  a  newline
              ('\n') and a line break.
              Special output character if OPOST  and  ONLCR  are  set.   A  CR  NL
              sequence  is output instead of just NL.  (Minix specific, but almost
              mandatory on any UNIX-like system.)

         TAB (^I)
              Special character on output if OPOST  and  XTABS  are  set.   It  is
              transformed  into  the  number of spaces necessary to reach a column
              position that is a multiple of eight.  (Only  needed  for  terminals
              without hardware tabs.)

         EOL (undefined)
              Special input character if ICANON is set.  It is an additional  line
              break.

         SUSP (^Z)
              Special input character if job control is implemented  and  ISIG  is
              set.   It causes a SIGTSTP signal to be send to the terminal process
              group.  (Minix does not have job control.)

         STOP (^S)
              Special input character if IXON is set.  It suspends terminal output
              and is then discarded.

         START (^Q)
              Special output character if IXON is set.  It starts terminal  output
              if  suspended  and is then discarded.  If IXANY is also set then any
              other character also  starts  terminal  output,  but  they  are  not
              discarded.

         REPRINT (^R)
              Special input character if IEXTEN and ECHO are  set.   Reprints  the
              input  queue  from  the  last  line  break  onwards.  A reprint also
              happens automatically if the echoed input  has  been  messed  up  by
              other output and ERASE is typed.

         LNEXT (^V)
              Special input character if IEXTEN is set.  It is the "literal  next"
              character  that  causes  the  next character to be input without any
              special processing.

         DISCARD (^O)
              Special input character if IEXTEN  is  set.   Causes  output  to  be
              discarded  until  it is typed again.  (Implemented only under Minix-
              vmd.)

         All of these characters except CR, NL and TAB may be changed or  disabled
         under  Minix.   (Changes  to  START  and  STOP may be ignored under other
         termios implementations.)  The REPRINT and  LNEXT  characters  are  Minix
         extensions  that are commonly present in other implementations.  POSIX is
         unclear on whether IEXTEN, IGNCR and  ICRNL  should  be  active  in  non-
         canonical mode, but under Minix they are.

      Terminal attributes
         The attributes of  a  terminal,  such  as  whether  the  mode  should  be
         canonical  or  non-canonical,  are  controlled  by  routines that use the
         termios structure as defined in <termios.h>:

              struct termios {
                  tcflag_t  c_iflag;       /* input modes */
                  tcflag_t  c_oflag;       /* output modes */
                  tcflag_t  c_cflag;       /* control modes */
                  tcflag_t  c_lflag;       /* local modes */
                  speed_t   c_ispeed;      /* input speed */
                  speed_t   c_ospeed;      /* output speed */
                  cc_t      c_cc[NCCS];    /* control characters */
              };

         The types tcflag, speed_t and cc_t are defined in <termios.h> as unsigned
         integral types.

      Input Modes
         The c_iflag field contains the following single bit  flags  that  control
         input processing:

         ICRNL
              Map CR to NL on input.

         IGNCR
              Ignore CR on input.  This flag overrides ICRNL.

         INLCR
              Map NL to CR on input.  This is done after the IGNCR check.

         IXON Enable start/stop output control.

         IXOFF
              Enable start/stop input control.  (Not implemented.)

         IXANY
              Allow any character to restart output.  (Minix specific.)

         ISTRIP
              Strip characters to seven bits.

         IGNPAR
              Ignore characters with parity errors.  (Not implemented.)

         INPCK
              Enable input parity checking.  (Not implemented.)

         PARMRK
              Mark parity errors by preceding the faulty  character  with  '\377',
              '\0'.   The  character '\377' is preceded by another '\377' to avoid
              ambiguity.  (Not implemented.)

         BRKINT
              Send the signal SIGINT to the terminal process group when  receiving
              a break condition.  (Not implemented.)

         IGNBRK
              Ignore break condition.  If neither BRKINT or IGNBRK is set a  break
              is  input  as  a  single  '\0', or if PARMRK is set as '\377', '\0',
              '\0'.  (Breaks are always ignored.)

      Output Modes
         The c_oflag field contains the following single bit  flags  that  control
         output processing:

         OPOST
              Perform output processing.  This flag is the "main switch" on output
              processing.  All other flags are Minix specific.

         ONLCR
              Transform an NL to a CR NL sequence on  output.   Note  that  a  key
              labeled  "RETURN"  or  "ENTER" usually sends a CR.  In line oriented
              mode this is normally transformed into  NL  by  ICRNL.   NL  is  the
              normal  UNIX  line delimiter ('\n').  On output an NL is transformed
              into the CR NL sequence that is necessary to reach the first  column
              of  the next line.  (This is a common output processing function for
              UNIX-like systems, but not always separately switchable by an  ONLCR
              flag.)

         XTABS
              Transform a TAB into the number  of  spaces  necessary  to  reach  a
              column position that is a multiple of eight.

         ONOEOT
              Discard EOT (^D) characters.  (Minix-vmd only.)

      Control Modes
         The c_cflag field contains the following single bit flags and  bit  field
         for basic hardware control:

         CLOCAL
              Ignore modem status lines.

         CREAD
              Enable receiver.  (The receiver is always enabled.)

         CSIZE
              Number of bits per byte.  CSIZE masks off the values CS5,  CS6,  CS7
              and CS8 that indicate that 5, 6, 7 or 8 bits are used.

         CSTOPB
              Send two stop bits instead of one.  Two stop bits are normally  used
              at 110 baud or less.

         PARENB
              Enable parity generation.

         PARODD
              Generate odd parity if parity is generated, otherwise even parity.

         HUPCL
              Drop the modem control lines on the last close of the terminal line.
              (Not implemented.)

      Local Modes
         The c_lflag field contains the following single bit  flags  that  control
         various functions:

         ECHO Enable echoing of  input  characters.   Most  input  characters  are
              echoed  as they are.  Control characters are echoed as ^X where X is
              the letter used to say that the control character  is  CTRL-X.   The
              CR, NL and TAB characters are echoed with their normal effect unless
              they are escaped by LNEXT.

         ECHOE
              If ICANON and ECHO are set then echo ERASE and KILL as one  or  more
              backspace-space-backspace  sequences  to wipe out the last character
              or the entire line, otherwise they are echoed as they are.

         ECHOK
              If ICANON and ECHO are set and ECHOE is not set then  output  an  NL
              after  the  KILL  character.   (For hardcopy terminals it is best to
              unset ECHOE and to set ECHOK.)

         ECHONL
              Echo NL even if ECHO is not set, but ICANON is set.

         ICANON
              Canonical input.  This enables line oriented  input  and  erase  and
              kill processing.

         IEXTEN
              Enable implementation defined input extensions.

         ISIG Enable the signal characters INTR, QUIT and SUSP.

         NOFLSH
              Disable the flushing of the input and output queues that is normally
              done if a signal is sent.

         TOSTOP
              Send a SIGTTOU signal if job control is implemented and a background
              process tries to write.  (Minix has no job control.)

      Input and output speed
         The input and output speed are encoded into  the  c_ispeed  and  c_ospeed
         fields.   <termios.h> defines the symbols B0, B50, B75, B110, B134, B150,
         B200, B300, B600, B1200, B1800,  B2400,  B4800,  B9600,  B19200,  B38400,
         B57600  and B115200 as values used to indicate the given baud rates.  The
         zero baud rate, B0, if used for the input speed causes the input speed to
         be  equal to the output speed.  Setting the output speed to zero hangs up
         the line.  One should use  the  functions  cfgetispeed(),  cfgetospeed(),
         cfsetispeed()  and  cfsetospeed()  to  get  or  set  a speed, because the
         c_ispeed  and  c_ospeed  fields  may   not   be   visible   under   other
         implementations.   (The  c_ispeed  and c_ospeed fields and the B57600 and
         B115200 symbols are Minix specific.)

      Special characters
         The c_cc array contains the special characters that can be modified.  The
         array  has  length  NCCS  and  is  subscripted by the symbols VEOF, VEOL,
         VERASE, VINTR, VKILL, VMIN, VQUIT, VTIME, VSUSP, VSTART, VSTOP, VREPRINT,
         VLNEXT and VDISCARD.  All these symbols are defined in <termios.h>.  Some
         implementations may give the same values to the VMIN and VTIME subscripts
         and  the VEOF and VEOL subscripts respectively, and may ignore changes to
         START and STOP.  (Under Minix all special characters have their own  c_cc
         slot and can all be modified.)

      Raw I/O Parameters
         The MIN and TIME parameters can be used to adjust  a  raw  connection  to
         bursty  input.   MIN  represents  a  minimum number of bytes that must be
         received before a read call returns.  TIME  is  a  timer  of  0.1  second
         granularity that can be used to time out a read.  Setting either of these
         parameters to zero has special meaning, which leads to the following four
         possibilities:

         MIN > 0, TIME > 0
              TIME is an inter-byte timer that is started (and restarted)  when  a
              byte is received.  A read succeeds when either the minimum number of
              characters is received or the timer expires.  Note  that  the  timer
              starts  after  the first character, so the read returns at least one
              byte.

         MIN > 0, TIME = 0
              Now the timer is disabled, and a reader blocks indefinitely until at
              least MIN characters are received.

         MIN = 0, TIME > 0
              TIME is now a read timer that is started when a  read  is  executed.
              The  read  will  return if the read timer expires or if at least one
              byte is input.  (Note that a value of zero may be  returned  to  the
              reader.)

         MIN = 0, TIME = 0
              The bytes currently available are returned.  Zero is returned if  no
              bytes are available.

      User Level Functions
         Termios attributes are set or examined,  and  special  functions  can  be
         performed by using the functions described in termios(3).

      Session Leaders and Process Groups
         With the use of the setsid() function can  a  process  become  a  session
         leader.   A  session leader forms a process group with a process group id
         equal to the process id of the session leader.  If a session leader opens
         a  terminal device file then this terminal becomes the controlling tty of
         the session leader.  Unless the terminal is already the  controlling  tty
         of  another  process,  or unless the O_NOCTTY flag is used to prevent the
         allocation of a controlling tty.  The process group of the session leader
         is  now  remembered as the terminal process group for signals sent by the
         terminal driver.  All the  children  and  grandchildren  of  the  session
         leader  inherit  the  controlling  terminal  and process group until they
         themselves use setsid().

         The controlling tty becomes inaccessible to the children of  the  session
         leader  when the session leader exits, and a hangup signal is sent to all
         the members of the process  group.   The  input  and  output  queues  are
         flushed  on  the last close of a terminal and all attributes are reset to
         the default state.

         A special device /dev/tty is a synonym  for  the  controlling  tty  of  a
         process.   It  allows  a process to reach the terminal even when standard
         input, output and error are redirected.  Opening this device can also  be
         used as a test to see if a process has a controlling tty or not.

         For Minix a special write-only device /dev/log exists for processes  that
         want  to  write  messages to the system console.  Unlike the console this
         device is still accessible when a session leader exits.

         Minix-vmd also has a /dev/log device, but this device is read-write.  All
         messages  written  to the log device or to the console when X11 is active
         can be read from /dev/log.  The system tries to preserve the  log  buffer
         over  a  reboot  so that panic messages reappear in the log if the system
         happens to crash.

      Pseudo Terminals
         Pseudo ttys allow a process such as a remote login daemon  to  set  up  a
         terminal  for  a  remote  login session.  The login session uses a device
         like /dev/ttyp0 for input and output, and the remote  login  daemon  uses
         the  device  /dev/ptyp0  to supply input to or take output from the login
         session and transfer this to or from  the  originating  system.   So  the
         character  flow  may  be:   Local user input sent to the remote system is
         written to  /dev/ptyp0  by  the  remote  login  daemon,  undergoes  input
         processing and appears on /dev/ttyp0  as  input  to  the  login  session.
         Output  from the login session to /dev/ttyp0 undergoes output processing,
         is read from /dev/ptyp0 by the remote login daemon and is  send  over  to
         the  local  system to be displayed for the user.  (So there are only four
         data streams to worry about in a pseudo terminal.)

         A pseudo terminal can be allocated by trying to open all the  controlling
         devices /dev/ptynn one by one until it succeeds.  Further opens will fail
         once a pty is open.  The process should now fork, the child should become
         session leader, open the tty side of the pty and start a login session.

         If the tty side is eventually closed down then reads from  the  pty  side
         will  return zero and writes return -1 with errno set to EIO.  If the pty
         side is closed first then a SIGHUP signal is sent to the  session  leader
         and further reads from the tty side return zero and writes return -1 with
         errno set to EIO.  (Special note:  A line erase may  cause  up  to  three
         times  the  size  of  the tty input queue to be sent to the pty reader as
         backspace overstrikes.  Some of this output  may  get  lost  if  the  pty
         reader cannot accept it all at once in a single read call.)

      Backwards compatibility
         The TIOCGETP, TIOCSETP, TIOCGETC and TIOCSETC ioctl  functions  that  are
         used  by  the  old  sgtty  terminal  interface are still supported by the
         terminal driver by emulation.   Note  that  these  old  functions  cannot
         control  all  termios attributes, so the terminal must be in a relatively
         sane state to avoid problems.

    FILES
         The list below shows all devices that Minix and Minix-vmd have.  Not  all
         of  these  devices  are  configured  in  by  default, as indicated by the
         numbers (i/j/k,  l/m/n)  that  tell  the  minimum,  default  and  maximum
         possible number of these devices for Minix (i/j/k) and Minix-vmd (l/m/n).

         /dev/console        System console.

         /dev/ttyc[1-7]      Virtual consoles.  (0/1/7, 0/1/7)

         /dev/log            Console log device.

         /dev/tty0[0-3]      Serial lines.  (0/2/2, 4/4/4)

         /dev/tty[p-w][0-f]  Pseudo ttys.  (0/0/64, 1/32/128)

         /dev/pty[p-w][0-f]  Associated pseudo tty controllers.

    SEE ALSO
         stty(1), termios(3), setsid(2), read(2), write(2).

    BUGS
         A fair number of flags are not implemented under  Minix  (yet).   Luckily
         they  are  very limited utility and only apply to RS-232, not to the user
         interface.

    AUTHOR
         Kees J. Bot (kjb@cs.vu.nl)