Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
TTY(4)		       FreeBSD Kernel Interfaces Manual			TTY(4)

     tty, cua -- general terminal interface

     #include <sys/ioctl.h>

     This section describes the	interface to the terminal drivers in the sys-

   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

     For hardware terminal ports, dial-out is supported	through	matching de-
     vice 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 de-
     vice 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 im-
     plementing	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	ac-
     cording 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

   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 (ac-
     tually 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.
		 PPPDISC     Point-to-Point Protocol 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	tcgetpgrp(3) 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 tcsetpgrp(3)	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 tcgetattr(3) call.

     TIOCSETA struct termios *term
		 Set the termios state associated with the device immediately.
		 This is the underlying	call that implements the tcsetattr(3)
		 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 tcsetattr(3) call with the TCSADRAIN

     TIOCSETAF struct termios *term
		 First wait for	any output to complete,	clear any pending in-
		 put, then set the termios state associated with the device.
		 This is the underlying	call that implements the tcsetattr(3)
		 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.

     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(2) call: there	is a specific ioctl
		 called	TIOCSCTTY to make a terminal the controlling terminal.
		 In addition, a	program	can fork(2) and	call the setsid(2)
		 system	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.

     TIOCSETVERAUTH int	*secs
		 Indicate that the current user	has successfully authenticated
		 to this session.  Future authentication checks	may then be
		 bypassed by performing	a TIOCCHKVERAUTH check.	 The verified
		 authentication	status will expire after secs seconds.	Only
		 root may perform this operation.

		 Clear any verified auth status	associated with	this session.

		 Check the verified auth status	of this	session.  The calling
		 process must have the same real user ID and parent process as
		 the process which called TIOCSETVERAUTH.  A zero return indi-
		 cates success.

     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-

     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-

     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	(see printf(9))	to this	terminal.  If on points	to a
		 zero integer, redirect	kernel console output back to the nor-
		 mal console.  This is usually used on workstations to redi-
		 rect 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 de-
		 fined 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.

     TIOCSTAT void
		 Causes	the kernel to write a status message to	the terminal
		 that displays the current load	average, the name of the com-
		 mand in the foreground, its process ID, the symbolic wait
		 channel, the number of	user and system	seconds	used, the per-
		 centage of CPU	the process is getting,	and the	resident set
		 size of the process.

     /dev/tty  controlling terminal, if	any

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

     A console typewriter device /dev/tty and asynchronous communication in-
     terfaces /dev/tty[0-5] first appeared in Version 1	AT&T UNIX.  The	cua
     support is	inspired by similar support in SunOS.

FreeBSD	13.0		       September 7, 2019		  FreeBSD 13.0


Want to link to this manual page? Use this URL:

home | help