|TTY(4)||Device Drivers Manual||TTY(4)|
general terminal interface
This section describes the interface to the terminal drivers in the system.
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)). 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 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 cu(1).
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) at the command level, and in 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.
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
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
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). 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).
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);
TIOCGETAstruct termios *term
TIOCSETAstruct termios *term
TIOCSETAWstruct termios *term
TIOCSETAFstruct termios *term
TIOCNOTTYon 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 controlling terminal. This is the new and
preferred method for programs to lose their controlling terminal.
TIOCCHKVERAUTHcheck. The verified authentication status will expire after secs seconds. Only root may perform this operation.
TIOCSETVERAUTH. A zero return indicates success.
FREADbit as defined in
<sys/fcntl.h>, then all characters in the input queue are cleared. If it contains the
FWRITEbit, then all characters in the output queue are cleared. If the value of the integer is zero, then it behaves as if both the
FWRITEbits were set (i.e., clears both queues).
TIOCGWINSZstruct winsize *ws
TIOCSWINSZstruct winsize *ws
This call sets the terminal modem state to that represented by state. Not all terminals may support this.
TIOCGTSTAMPstruct timeval *timeval
TIOCSTSTAMPstruct tstamps *tstamps
TIOCM_CARare 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.
This call sets the serial port state to that represented by state. Not all serial ports may support this.
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.
|February 18, 2022||OpenBSD-current|