--- name: "tty" section: "4" platform: "openbsd" package: "man" version: "8.0-current-20261101" release: "OpenBSD 8.0-current (2026-11-01)" platforms: ["linux", "freebsd", "openbsd", "macos"] url: "https://new.carta.tech/tty/4" --- # tty(4) general terminal interface ## NAME **tty**, **cua** – general terminal interface ## SYNOPSIS **#include \** ## DESCRIPTION This section describes the interface to the terminal drivers in the system. ### 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 associated device and prepared the line for normal interactive use (see [getty(8)](/getty/8.md)). 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)](/ssh/1.md) 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)](/getty/8.md) 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 [getty(8)](/getty/8.md) 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)](/getty/8.md) process will deal with it properly, without having noticed the intervening dial-out action. For more information on dial-out, see [cu(1)](/cu/1.md). When an interactive user logs in, the system prepares the line to behave in a certain way (called a *line discipline*), described in [stty(1)](/stty/1.md) at the command level, and in [termios(4)](/termios/4.md) 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. ### 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 interface routines (such as [read(2)](/read/2.md) and [write(2)](/write/2.md)), 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)](/termios/4.md). 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 conjunction with a line discipline other than termios, and some may not be supported 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)](/ioctl/2.md) 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)](/termios/4.md) 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 underlying call that implements the [tcgetpgrp(3)](/tcgetpgrp/3.md) call. - TIOCSPGRP *int*, *\*tpgrp* Associate the terminal with the process group (as an integer) pointed to by *tpgrp*. This is the underlying call that implements the [tcsetpgrp(3)](/tcsetpgrp/3.md) 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)](/tcgetattr/3.md) call. - TIOCSETA *struct*, *termios*, *\*term* Set the termios state associated with the device immediately. This is the underlying call that implements the [tcsetattr(3)](/tcsetattr/3.md) 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)](/tcsetattr/3.md) 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 [tcsetattr(3)](/tcsetattr/3.md) 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)](/termios/4.md)) 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 *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)](/open/2.md) call: there is a specific ioctl called TIOCSCTTY to make a terminal the controlling terminal. In addition, a program can [fork(2)](/fork/2.md) and call the [setsid(2)](/setsid/2.md) 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. - 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. - TIOCCLRVERAUTH *void* Clear any verified auth status associated with this session. - TIOCCHKVERAUTH *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 TIOCSETVERAUTH. A zero return indicates 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 keyboard). - 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 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. - TIOCNXCL *void* Clear exclusive use of the terminal. Further opens are permitted. - 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)](/printf/9.md)) 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 redirect 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 represented 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 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 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*. - TIOCSTAT *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. ## FILES - */dev/tty* controlling terminal, if any ## SEE ALSO [cu(1)](/cu/1.md), [stty(1)](/stty/1.md), [tty(1)](/tty/1.md), [ioctl(2)](/ioctl/2.md), [pty(4)](/pty/4.md), [termios(4)](/termios/4.md), [ttys(5)](/ttys/5.md), [getty(8)](/getty/8.md) ## HISTORY A console typewriter device */dev/tty* and asynchronous communication interfaces */dev/tty\[0-5\]* first appeared in Version 1 AT&T UNIX. The cua support is inspired by similar support in SunOS.