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TTY(4)                   BSD Kernel Interfaces Manual                   TTY(4)

NAME
     tty, cua -- general terminal interface

SYNOPSIS
     #include <&lt;sys/ioctl.h>&gt;

DESCRIPTION
     This section describes the interface to the terminal drivers in the sys-
     tem.

   Terminal Special Files
     Each hardware terminal port (such as a serial port) on the system usually
     has a terminal special device file associated with it in the directory
     /dev/ (for example, /dev/tty03).  When a user logs into the system on one
     of these hardware terminal ports, the system has already opened the asso-
     ciated device and prepared the line for normal interactive use (see
     getty(8)).  There is also a special case of a terminal file that connects
     not to a hardware terminal port, but to another program on the other
     side.  These special terminal devices are called ptys and provide the
     mechanism necessary to give users the same interface to the system when
     logging in over a network (using ssh(1) or telnet(1) for example).  Even
     in these cases the details of how the terminal file was opened and set up
     is already handled by special software in the system.  Thus, users do not
     normally need to worry about the details of how these lines are opened or
     used.

     For hardware terminal ports, dial-out is supported through matching
     device nodes called calling units.  For instance, the terminal called
     /dev/tty03 would have a matching calling unit called /dev/cua03.  These
     two devices are normally differentiated by creating the calling unit
     device node with a minor number 128 greater than the dial-in device node.
     Whereas the dial-in device (the tty) normally requires a hardware signal
     to indicate to the system that it is active, the dial-out device (the
     cua) does not, and hence can communicate unimpeded with a device such as
     a modem, or with another system over a serial link.  This means that a
     process like getty(8) will wait on a dial-in device until a connection is
     established.  Meanwhile, a dial-out connection can be established on the
     dial-out device (for the very same hardware terminal port) without dis-
     turbing anything else on the system.  The getty(8) process does not even
     notice that anything is happening on the terminal port.  If a connecting
     call comes in after the dial-out connection has finished, the getty(8)
     process will deal with it properly, without having noticed the interven-
     ing dial-out action.  For more information on dial-out, see cu(1).

     When an interactive user logs in, the system prepares the line to behave
     in a certain way (called a line discipline), the particular details of
     which are described in stty(1) at the command level, and in termios(4) at
     the programming level.  A user may be concerned with changing settings
     associated with his particular login terminal and should refer to the
     preceding man pages for the common cases.  The remainder of this man page
     is concerned with describing details of using and controlling terminal
     devices at a low level, such as that possibly required by a program wish-
     ing to provide features similar to those provided by the system.

   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 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, although 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).

   Terminal File Operations
     All of the following operations are invoked using the ioctl(2) system
     call.  Refer to that man page for a description of the request and argp
     parameters.  In addition to the ioctl requests defined here, the specific
     line discipline in effect will define other requests specific to it
     (actually termios(4) defines them as function calls, not ioctl requests).
     The following section lists the available ioctl requests.  The name of
     the request, a description of its purpose, and the typed argp parameter
     (if any) are listed.  For example, the first entry says

           TIOCSETD int *ldisc

     and would be called on the terminal associated with file descriptor zero
     by the following code fragment:

             int ldisc;

             ldisc = TTYDISC;
             ioctl(0, TIOCSETD, &ldisc);

   Terminal File Request Descriptions
     TIOCSETD int *ldisc
                 Change to the new line discipline pointed to by ldisc.  The
                 available line disciplines currently available are:

                 TTYDISC     Termios interactive line discipline.
                 TABLDISC    Tablet line discipline.
                 SLIPDISC    Serial IP line discipline.
                 PPPDISC     Point-to-Point Protocol line discipline.
                 STRIPDISC   Starmode Radio IP line discipline.
                 NMEADISC    NMEA 0183 line discipline.
                 MSTSDISC    Meinberg Standard Time String line discipline.

     TIOCGETD int *ldisc
                 Return the current line discipline in the integer pointed to
                 by ldisc.

     TIOCSBRK void
                 Set the terminal hardware into BREAK condition.

     TIOCCBRK void
                 Clear the terminal hardware BREAK condition.

     TIOCSDTR void
                 Assert data terminal ready (DTR).

     TIOCCDTR void
                 Clear data terminal ready (DTR).

     TIOCGPGRP int *tpgrp
                 Return the current process group the terminal is associated
                 with in the integer pointed to by tpgrp.  This is the under-
                 lying call that implements the termios(4) tcgetpgrp() call.

     TIOCSPGRP int *tpgrp
                 Associate the terminal with the process group (as an integer)
                 pointed to by tpgrp.  This is the underlying call that imple-
                 ments the termios(4) tcsetpgrp() call.

     TIOCGETA struct termios *term
                 Place the current value of the termios state associated with
                 the device in the termios structure pointed to by term.  This
                 is the underlying call that implements the termios(4)
                 tcgetattr() call.

     TIOCSETA struct termios *term
                 Set the termios state associated with the device immediately.
                 This is the underlying call that implements the termios(4)
                 tcsetattr() call with the TCSANOW option.

     TIOCSETAW struct termios *term
                 First wait for any output to complete, then set the termios
                 state associated with the device.  This is the underlying
                 call that implements the termios(4) tcsetattr() call with the
                 TCSADRAIN option.

     TIOCSETAF struct termios *term
                 First wait for any output to complete, clear any pending
                 input, then set the termios state associated with the device.
                 This is the underlying call that implements the termios(4)
                 tcsetattr() call with the TCSAFLUSH option.

     TIOCOUTQ int *num
                 Place the current number of characters in the output queue in
                 the integer pointed to by num.

     TIOCSTI char *cp
                 Simulate typed input.  Pretend as if the terminal received
                 the character pointed to by cp.

     TIOCNOTTY void
                 This call is obsolete but left for compatibility.  In the
                 past, when a process that didn't have a controlling terminal
                 (see The Controlling Terminal in termios(4)) first opened a
                 terminal device, it acquired that terminal as its controlling
                 terminal.  For some programs this was a hazard as they didn't
                 want a controlling terminal in the first place, and this pro-
                 vided a mechanism to disassociate the controlling terminal
                 from the calling process.  It must be called by opening the
                 file /dev/tty and calling TIOCNOTTY on that file descriptor.

                 The current system does not allocate a controlling terminal
                 to a process on an open() call: there is a specific ioctl
                 called TIOCSCTTY to make a terminal the controlling terminal.
                 In addition, a program can fork() and call the setsid() sys-
                 tem call which will place the process into its own session -
                 which has the effect of disassociating it from the control-
                 ling terminal.  This is the new and preferred method for pro-
                 grams to lose their controlling terminal.

     TIOCSTOP void
                 Stop output on the terminal (like typing ^S at the keyboard).

     TIOCSTART void
                 Start output on the terminal (like typing ^Q at the key-
                 board).

     TIOCSCTTY void
                 Make the terminal the controlling terminal for the process
                 (the process must not currently have a controlling terminal).

     TIOCDRAIN void
                 Wait until all output is drained.

     TIOCEXCL void
                 Set exclusive use on the terminal.  No further opens are per-
                 mitted except by root.  Of course, this means that programs
                 that are run by root (or setuid) will not obey the exclusive
                 setting - which limits the usefulness of this feature.

     TIOCNXCL void
                 Clear exclusive use of the terminal.  Further opens are per-
                 mitted.

     TIOCFLUSH int *what
                 If the value of the int pointed to by what contains the FREAD
                 bit as defined in <sys/fcntl.h>, then all characters in the
                 input queue are cleared.  If it contains the FWRITE bit, then
                 all characters in the output queue are cleared.  If the value
                 of the integer is zero, then it behaves as if both the FREAD
                 and FWRITE bits were set (i.e., clears both queues).

     TIOCGWINSZ struct winsize *ws
                 Put the window size information associated with the terminal
                 in the winsize structure pointed to by ws.  The window size
                 structure contains the number of rows and columns (and pixels
                 if appropriate) of the devices attached to the terminal.  It
                 is set by user software and is the means by which most full-
                 screen oriented programs determine the screen size.

     TIOCSWINSZ struct winsize *ws
                 Set the window size associated with the terminal to be the
                 value in the winsize structure pointed to by ws (see above).

     TIOCCONS int *on
                 If on points to a non-zero integer, redirect kernel console
                 output (kernel printf()s) to this terminal.  If on points to
                 a zero integer, redirect kernel console output back to the
                 normal console.  This is usually used on workstations to re-
                 direct kernel messages to a particular window.

     TIOCMSET int *state
                 The integer pointed to by state contains bits that correspond
                 to modem state.  Following is a list of defined variables and
                 the modem state they represent:

                 TIOCM_LE   Line Enable.
                 TIOCM_DTR  Data Terminal Ready.
                 TIOCM_RTS  Request To Send.
                 TIOCM_ST   Secondary Transmit.
                 TIOCM_SR   Secondary Receive.
                 TIOCM_CTS  Clear To Send.
                 TIOCM_CAR  Carrier Detect.
                 TIOCM_CD   Carrier Detect (synonym).
                 TIOCM_RNG  Ring Indication.
                 TIOCM_RI   Ring Indication (synonym).
                 TIOCM_DSR  Data Set Ready.

                 This call sets the terminal modem state to that represented
                 by state.  Not all terminals may support this.

     TIOCMGET int *state
                 Return the current state of the terminal modem lines as rep-
                 resented above in the integer pointed to by state.

     TIOCMBIS int *state
                 The bits in the integer pointed to by state represent modem
                 state as described above; however, the state is OR-ed in with
                 the current state.

     TIOCMBIC int *state
                 The bits in the integer pointed to by state represent modem
                 state as described above; however, each bit which is on in
                 state is cleared in the terminal.

     TIOCGTSTAMP struct timeval *timeval
                 Return the (single) timestamp.

     TIOCSTSTAMP struct tstamps *tstamps
                 Chooses the conditions which will cause the current system
                 time to be immediately copied to the terminal timestamp stor-
                 age.  This is often used to determine exactly the moment at
                 which one or more of these events occurred, though only one
                 can be monitored.  Only TIOCM_CTS and TIOCM_CAR are honoured
                 in tstamps.ts_set and tstamps.ts_clr; these indicate which
                 raising and lowering events on the respective lines should
                 cause a timestamp capture.

     TIOCSFLAGS int *state
                 The bits in the integer pointed to by state contain bits that
                 correspond to serial port state.  Following is a list of
                 defined variables and the serial port state they represent:

                 TIOCFLAG_SOFTCAR  Ignore hardware carrier.
                 TIOCFLAG_CLOCAL   Set clocal on open.
                 TIOCFLAG_CRTSCTS  Set crtscts on open.
                 TIOCFLAG_MDMBUF   Set mdmbuf on open.

                 This call sets the serial port state to that represented by
                 state.  Not all serial ports may support this.

     TIOCGFLAGS int *state
                 Return the current state of the serial port as represented
                 above in the integer pointed to by state.

FILES
     /dev/tty  controlling terminal, if any

SEE ALSO
     cu(1), stty(1), tty(1), ioctl(2), pty(4), termios(4), ttys(5), getty(8)

HISTORY
     The cua support is inspired by similar support in SunOS.

BSD                            January 15, 2015                            BSD