TTY(4) BSD Kernel Interfaces Manual TTY(4)
NAME
tty -- general terminal interface
SYNOPSIS
#include <sys/ioctl.h>
DESCRIPTION
This section describes the interface to the terminal drivers in the system.
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 ter-minal terminal
minal 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 rlogin(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. Also, these lines are often used for dialing out of a system (through an out-calling
modem), but again the system provides programs that hide the details of accessing these terminal spe-cial special
cial files (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 is 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 is 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 seman-tics semantics
tics 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 gener-ally generally
ally represent features common across all line disciplines, however 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
TIOCSETD int *ldisc
and would be called on the terminal associated with file descriptor zero by the following code frag-ment: fragment:
ment:
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 PPP 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) tcgetattr() 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) tcsetattr() 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 TIOSCTTY 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 control-ling controlling
ling 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 exclu-sive exclusive
sive 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/file.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 inte-ger integer
ger 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 par-ticular particular
ticular 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, how-ever however
ever 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, how-ever however
ever each bit which is on in state is cleared in the terminal.
SEE ALSO
stty(1), ioctl(2), pty(4), termios(4), getty(8)
4th Berkeley Distribution August 14, 1992 4th Berkeley Distribution
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