general terminal interface
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)). 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), 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.
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 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
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
- Change to the new line discipline pointed to by
ldisc. The available line disciplines currently
- Termios interactive line discipline.
- Point-to-Point Protocol line discipline.
- NMEA 0183 line discipline.
- Meinberg Standard Time String line discipline.
- Return the current line discipline in the integer pointed to by ldisc.
- Set the terminal hardware into BREAK condition.
- Clear the terminal hardware BREAK condition.
- Assert data terminal ready (DTR).
- Clear data terminal ready (DTR).
- 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) call.
- Associate the terminal with the process group (as an integer) pointed to by tpgrp. This is the underlying call that implements the tcsetpgrp(3) call.
TIOCGETAstruct 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.
TIOCSETAstruct termios *term
- Set the termios state associated with the device immediately. This is the
underlying call that implements the
tcsetattr(3) call with the
TIOCSETAWstruct 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
TIOCSETAFstruct 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) call with the
- Place the current number of characters in the output queue in the integer pointed to by num.
- This call is obsolete but left for compatibility. In the past, when a
process that didn't have a controlling terminal (see
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
called by opening the file /dev/tty and calling
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
TIOCSCTTYto 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.
- Indicate that the current user has successfully authenticated to this
session. Future authentication checks may then be bypassed by performing a
TIOCCHKVERAUTHcheck. 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 indicates success.
- Stop output on the terminal (like typing ^S at the keyboard).
- Start output on the terminal (like typing ^Q at the keyboard).
- Make the terminal the controlling terminal for the process (the process must not currently have a controlling terminal).
- Wait until all output is drained.
- 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.
- Clear exclusive use of the terminal. Further opens are permitted.
- If the value of the int pointed to by what contains
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
- 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.
TIOCSWINSZstruct winsize *ws
- Set the window size associated with the terminal to be the value in the winsize structure pointed to by ws (see above).
- 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 normal console. This is usually used on workstations to redirect kernel messages to a particular window.
- 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:
- Line Enable.
- Data Terminal Ready.
- Request To Send.
- Secondary Transmit.
- Secondary Receive.
- Clear To Send.
- Carrier Detect.
- Carrier Detect (synonym).
- Ring Indication.
- Ring Indication (synonym).
- Data Set Ready.
This call sets the terminal modem state to that represented by state. Not all terminals may support this.
- Return the current state of the terminal modem lines as represented above in the integer pointed to by 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.
- 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.
TIOCGTSTAMPstruct timeval *timeval
- Return the (single) timestamp.
TIOCSTSTAMPstruct 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_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.
- 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:
- Ignore hardware carrier.
- Set clocal on open.
- Set crtscts on open.
- Set mdmbuf on open.
This call sets the serial port state to that represented by state. Not all serial ports may support this.
- Return the current state of the serial port as represented above in the integer pointed to by state.
- 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.
- controlling terminal, if any
cu(1), stty(1), tty(1), ioctl(2), pty(4), termios(4), ttys(5), getty(8)
The cua support is inspired by similar support in SunOS.