man/man4/tty.4

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.\"     @(#)tty.4	8.3 (Berkeley) 4/19/94
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.Dd $Mdocdate: August 16 2024 $
.Dt TTY 4
.Os
.Sh NAME
.Nm tty ,
.Nm cua
.Nd general terminal interface
.Sh SYNOPSIS
.In sys/ioctl.h
.Sh DESCRIPTION
This section describes the interface to the terminal drivers
in the system.
.Ss 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
.Pa /dev/
(for
example,
.Pa /dev/tty03 ) .
When a user logs into
the system on one of these hardware terminal ports, the system has already
opened the associated device and prepared the line for normal interactive
use (see
.Xr 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
.Em ptys
and provide the mechanism necessary to give users the same interface to the
system when logging in over a network (using
.Xr ssh 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.
.Pp
For hardware terminal ports, dial-out is supported through matching
device nodes called calling units.
For instance, the terminal called
.Pa /dev/tty03
would have a matching calling unit called
.Pa /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
.Em tty )
normally
requires a hardware signal to indicate to the system that it is active,
the dial-out device (the
.Em 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
.Xr 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 disturbing
anything else on the system.
The
.Xr 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
.Xr getty 8
process will deal with it properly, without having noticed the
intervening dial-out action.
For more information on dial-out, see
.Xr cu 1 .
.Pp
When an interactive user logs in, the system prepares the line to
behave in a certain way (called a
.Em "line discipline" ) ,
described in
.Xr stty 1
at the command level, and in
.Xr termios 4
at the programming level.
To change settings associated with a login terminal,
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 wishing to provide features similar to those provided
by the system.
.Ss 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
.Em "line discipline"
associated with it.
The
.Em "line discipline"
essentially glues the low level device driver code with the high
level generic interface routines (such as
.Xr read 2
and
.Xr 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
.Em "line discipline"
called the
.Dv 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
.Dv termios
line discipline is in effect, the terminal file behaves and is
operated according to the rules described in
.Xr termios 4 .
Refer to that man page for a full description of the terminal
semantics.
The operations described here
generally represent features common
across all
.Em "line disciplines" ,
although some of these calls may not
make sense in conjunction with a line discipline other than
.Dv termios ,
and some may not be supported by the underlying
hardware (or lack thereof, as in the case of ptys).
.Ss Terminal File Operations
All of the following operations are invoked using the
.Xr ioctl 2
system call.
Refer to that man page for a description of the
.Em request
and
.Em argp
parameters.
In addition to the ioctl
.Em requests
defined here, the specific line discipline
in effect will define other
.Em requests
specific to it (actually
.Xr termios 4
defines them as function calls, not ioctl
.Em requests ) .
The following section lists the available ioctl requests.
The name of the request, a description of its purpose, and the typed
.Em argp
parameter (if any)
are listed.
For example, the first entry says
.Pp
.D1 Em "TIOCSETD int *ldisc"
.Pp
and would be called on the terminal associated with
file descriptor zero by the following code fragment:
.Bd -literal
	int ldisc;

	ldisc = TTYDISC;
	ioctl(0, TIOCSETD, &ldisc);
.Ed
.Ss Terminal File Request Descriptions
.Bl -tag -width TIOCGWINSZ
.It Dv TIOCSETD Fa int *ldisc
Change to the new line discipline pointed to by
.Fa ldisc .
The available line disciplines currently available are:
.Pp
.Bl -tag -width TIOCGWINSZ -compact
.It TTYDISC
Termios interactive line discipline.
.It PPPDISC
Point-to-Point Protocol line discipline.
.It NMEADISC
NMEA 0183 line discipline.
.It MSTSDISC
Meinberg Standard Time String line discipline.
.El
.It Dv TIOCGETD Fa int *ldisc
Return the current line discipline in the integer pointed to by
.Fa ldisc .
.It Dv TIOCSBRK Fa void
Set the terminal hardware into BREAK condition.
.It Dv TIOCCBRK Fa void
Clear the terminal hardware BREAK condition.
.It Dv TIOCSDTR Fa void
Assert data terminal ready (DTR).
.It Dv TIOCCDTR Fa void
Clear data terminal ready (DTR).
.It Dv TIOCGPGRP Fa int *tpgrp
Return the current process group the terminal is associated
with in the integer pointed to by
.Fa tpgrp .
This is the underlying call that implements the
.Xr tcgetpgrp 3
call.
.It Dv TIOCSPGRP Fa int *tpgrp
Associate the terminal with the process group (as an integer) pointed to by
.Fa tpgrp .
This is the underlying call that implements the
.Xr tcsetpgrp 3
call.
.It Dv TIOCGETA Fa struct termios *term
Place the current value of the termios state associated with the
device in the termios structure pointed to by
.Fa term .
This is the underlying call that implements the
.Xr tcgetattr 3
call.
.It Dv TIOCSETA Fa struct termios *term
Set the termios state associated with the device immediately.
This is the underlying call that implements the
.Xr tcsetattr 3
call with the
.Dv TCSANOW
option.
.It Dv TIOCSETAW Fa 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
.Xr tcsetattr 3
call with the
.Dv TCSADRAIN
option.
.It Dv TIOCSETAF Fa 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
.Xr tcsetattr 3
call with the
.Dv TCSAFLUSH
option.
.It Dv TIOCOUTQ Fa int *num
Place the current number of characters in the output queue in the
integer pointed to by
.Fa num .
.It Dv TIOCNOTTY Fa void
This call is obsolete but left for compatibility.
In the past, when a process that didn't have a controlling terminal
.Po
see
.Em The Controlling Terminal
in
.Xr termios 4
.Pc
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 provided a mechanism to disassociate
the controlling terminal from the calling process.
It
.Em must
be called by opening the file
.Pa /dev/tty
and calling
.Dv TIOCNOTTY
on that file descriptor.
.Pp
The current system does not allocate a controlling terminal to
a process on an
.Xr open 2
call: there is a specific ioctl called
.Dv TIOCSCTTY
to make a terminal the controlling
terminal.
In addition, a program can
.Xr fork 2
and call the
.Xr setsid 2
system call which will place the process into its own session - which
has the effect of disassociating it from the controlling terminal.
This is the new and preferred method for programs to lose their controlling
terminal.
.It Dv TIOCSETVERAUTH Fa int *secs
Indicate that the current user has successfully authenticated to this session.
Future authentication checks may then be bypassed by performing a
.Dv TIOCCHKVERAUTH
check.
The verified authentication status will expire after
.Fa secs
seconds.
Only root may perform this operation.
.It Dv TIOCCLRVERAUTH Fa void
Clear any verified auth status associated with this session.
.It Dv TIOCCHKVERAUTH Fa void
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
.Dv TIOCSETVERAUTH .
A zero return indicates success.
.It Dv TIOCSTOP Fa void
Stop output on the terminal (like typing ^S at the keyboard).
.It Dv TIOCSTART Fa void
Start output on the terminal (like typing ^Q at the keyboard).
.It Dv TIOCSCTTY Fa void
Make the terminal the controlling terminal for the process (the process
must not currently have a controlling terminal).
.It Dv TIOCDRAIN Fa void
Wait until all output is drained.
.It Dv TIOCEXCL Fa void
Set exclusive use on the terminal.
No further opens are permitted 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.
.It Dv TIOCNXCL Fa void
Clear exclusive use of the terminal.
Further opens are permitted.
.It Dv TIOCFLUSH Fa int *what
If the value of the int pointed to by
.Fa what
contains the
.Dv FREAD
bit as defined in
.In sys/fcntl.h ,
then all characters in the input queue are cleared.
If it contains the
.Dv 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
.Dv FREAD
and
.Dv FWRITE
bits were set (i.e., clears both queues).
.It Dv TIOCGWINSZ Fa struct winsize *ws
Put the window size information associated with the terminal in the
.Va winsize
structure pointed to by
.Fa 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.
.It Dv TIOCSWINSZ Fa struct winsize *ws
Set the window size associated with the terminal to be the value in
the
.Va winsize
structure pointed to by
.Fa ws
(see above).
.It Dv TIOCCONS Fa int *on
If
.Fa on
points to a non-zero integer, redirect kernel console output
.Po
see
.Xr printf 9
.Pc
to this terminal.
If
.Fa on
points to a zero integer, redirect kernel console output back to the normal
console.
This is usually used on workstations to redirect kernel messages
to a particular window.
.It Dv TIOCMSET Fa int *state
The integer pointed to by
.Fa state
contains bits that correspond to modem state.
Following is a list of defined variables and the modem state they represent:
.Pp
.Bl -tag -width TIOCMXCTS -compact
.It TIOCM_LE
Line Enable.
.It TIOCM_DTR
Data Terminal Ready.
.It TIOCM_RTS
Request To Send.
.It TIOCM_ST
Secondary Transmit.
.It TIOCM_SR
Secondary Receive.
.It TIOCM_CTS
Clear To Send.
.It TIOCM_CAR
Carrier Detect.
.It TIOCM_CD
Carrier Detect (synonym).
.It TIOCM_RNG
Ring Indication.
.It TIOCM_RI
Ring Indication (synonym).
.It TIOCM_DSR
Data Set Ready.
.El
.Pp
This call sets the terminal modem state to that represented by
.Fa state .
Not all terminals may support this.
.It Dv TIOCMGET Fa int *state
Return the current state of the terminal modem lines as represented
above in the integer pointed to by
.Fa state .
.It Dv TIOCMBIS Fa int *state
The bits in the integer pointed to by
.Fa state
represent modem state as described above; however, the state is OR-ed
in with the current state.
.It Dv TIOCMBIC Fa int *state
The bits in the integer pointed to by
.Fa state
represent modem state as described above; however, each bit which is on
in
.Fa state
is cleared in the terminal.
.It Dv TIOCGTSTAMP Fa struct timeval *timeval
Return the (single) timestamp.
.It Dv TIOCSTSTAMP Fa struct tstamps *tstamps
Chooses the conditions which will cause the current system time to be
immediately copied to the terminal timestamp storage.
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
.Dv TIOCM_CTS
and
.Dv TIOCM_CAR
are honoured in
.Va tstamps.ts_set
and
.Va tstamps.ts_clr ;
these indicate which raising and lowering events on the respective lines
should cause a timestamp capture.
.It Dv TIOCSFLAGS Fa int *state
The bits in the integer pointed to by
.Fa state
contain bits that correspond to serial port state.
Following is a list of defined variables and the serial port state they
represent:
.Pp
.Bl -tag -width TIOCFLAG_SOFTCAR -compact
.It TIOCFLAG_SOFTCAR
Ignore hardware carrier.
.It TIOCFLAG_CLOCAL
Set clocal on open.
.It TIOCFLAG_CRTSCTS
Set crtscts on open.
.It TIOCFLAG_MDMBUF
Set mdmbuf on open.
.El
.Pp
This call sets the serial port state to that represented by
.Fa state .
Not all serial ports may support this.
.It Dv TIOCGFLAGS Fa int *state
Return the current state of the serial port as represented
above in the integer pointed to by
.Fa state .
.It Dv TIOCSTAT Fa void
Causes the kernel to write a status message to the terminal that displays the
current load average,
the name of the command in the foreground,
its process ID,
the symbolic wait channel,
the number of user and system seconds used,
the percentage of CPU the process is getting,
and the resident set size of the process.
.El
.Sh FILES
.Bl -tag -width /dev/tty -compact
.It Pa /dev/tty
controlling terminal, if any
.El
.Sh SEE ALSO
.Xr cu 1 ,
.Xr stty 1 ,
.Xr tty 1 ,
.Xr ioctl 2 ,
.Xr pty 4 ,
.Xr termios 4 ,
.Xr ttys 5 ,
.Xr getty 8
.Sh HISTORY
A console typewriter device
.Pa /dev/tty
and asynchronous communication interfaces
.Pa /dev/tty[0-5]
first appeared in
.At v1 .
The cua support is inspired by similar support in SunOS.