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Terminal Special Files

Each hardware terminal 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)). 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. 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 disturbing 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 intervening dial-out action. For more information on dial-out, see tip(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 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) 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 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) 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

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 are listed in ⟨sys/ttycom.h⟩ and currently 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 underlying 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 implements 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 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() 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() 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.

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. The winsize structure is defined in ⟨sys/ioctl.h⟩.

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 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.