TTY(4)

NAME
tty - general terminal interface

SYNOPSIS
#include <sgtty.h>

DESCRIPTION
This section describes both a particular special file /dev/tty and the terminal drivers used for conversational computing.

Line disciplines.

The system provides different line disciplines for controlling communications lines. In this version of the system there are two disciplines available:

old The old (standard) terminal driver. This is used when using the standard shell sh(1) and for compatibility with other standard version 7 UNIX systems.

new A newer terminal driver, with features for job control; this must be used when using csh(1).

Line discipline switching is accomplished with the TIOCSETD ioctl: int ldisc = LDISC; ioctl(fildes, TIOCSETD, &ldisc); where LDISC is OTTYDISC for the standard tty driver and NTTYDISC for the new driver. The standard (currently old) tty driver is discipline 0 by convention. The current line discipline can be obtained with the TIOCGETD ioctl. Pending input is discarded when the line discipline is changed.

All of the low-speed asynchronous communications ports can use any of the available line disciplines, no matter what hardware is involved. The remainder of this section discusses the “old” and “new” disciplines.

Control terminal

When a terminal file is opened, it causes the process to wait until a connection is established. In practice, user programs seldom open these files; they are opened by init(8) and become a user's standard input and output file.

If a process which has no control terminal opens a terminal file, then that terminal file becomes the control terminal for that process. The control terminal is thereafter inherited by a child process during a fork(2), even if the control terminal is closed.

The file /dev/tty is, in each process, a synonym for a control terminal associated with that process. It is useful for programs that wish to be sure of writing messages on the terminal no matter how output has been redirected. It can also be used for programs that demand a file name for output, when typed output is desired and it is tiresome to find out which terminal is currently in use.

Process groups

Command processors such as csh(1) can arbitrate the terminal between different jobs by placing related jobs in a single process group and associating this process group with the terminal. A terminals associated process group may be set using the TIOCSPGRP ioctl(2): ioctl(fildes, TIOCSPGRP, &pgrp) or examined using TIOCGPGRP rather than TIOCSPGRP, returning the current process group in pgrp. The new terminal driver aids in this arbitration by restricting access to the terminal by processes which are not in the current process group; see Job access control below.

Modes

The terminal drivers have three major modes, characterized by the amount of processing on the input and output characters:

cooked
    The normal mode. In this mode lines of input are collected and input editing is done. The edited line is made available when it is completed by a newline or when an EOT (control-D, hereafter ^D) is entered. A carriage return is usually made synonymous with newline in this mode, and replaced with a newline whenever it is typed. All driver functions (input editing, interrupt generation, output processing such as delay generation and tab expansion, etc.) are available in this mode.

CBREAK
    This mode eliminates the character, word, and line editing input facilities, making the input character available to the user program as it is typed. Flow control, literal-next and interrupt processing are still done in this mode. Output processing is done.

RAW This mode eliminates all input processing and makes all input characters available as they are typed; no output processing is done either.

The style of input processing can also be very different when the terminal is put in non-blocking i/o mode; see fcntl(2). In this case a read(2) from the control terminal will never block, but rather return an error indication (EWOULDBLOCK) if there is no input available.

A process may also request a SIGIO signal be sent it whenever input is present. To enable this mode the FASYNC flag should be set using fcntl(2).

Input editing

A UNIX terminal ordinarily operates in full-duplex mode. Characters may be typed at any time, even while output is occurring, and are only lost when the system's character input buffers become completely choked, which is rare, or when the user has accumulated the maximum allowed number of input characters that have not yet been read by some program. Currently this limit is 256 characters. In the old terminal driver all the saved characters are thrown away when the limit is reached, without notice; the new driver simply refuses to accept any further input, and rings the terminal bell.

Input characters are normally accepted in either even or odd parity with the parity bit being stripped off before the character is given to the program. By clearing either the EVEN or ODD bit in the flags word it is possible to have input characters with that parity discarded (see the Summary below.)

In all of the line disciplines, it is possible to simulate terminal input using the TIOCSTI ioctl, which takes, as its third argument, the address of a character. The system pretends that this character was typed on the argument terminal, which must be the control terminal except for the super-user (this call is not in standard version 7 UNIX). The process must also have read access to the argument terminal.

Input characters are normally echoed by putting them in an output queue as they arrive. This may be disabled by clearing the ECHO bit in the flags word using the stty(3) call or the TIOCSETN or TIOCSETP ioctls (see the Summary below).

In cooked mode, terminal input is processed in units of lines. A program attempting to read will normally be suspended until an entire line has been received (but see the description of SIGTTIN in Modes above and FIONREAD in Summary below.) No matter how many characters are requested in the read call, at most one line will be returned. It is not, however, necessary to read a whole line at once; any number of characters may be requested in a read, even one, without losing information.

During input, line editing is normally done, with the character `#' logically erasing the last character typed and the character `@' logically erasing the entire current input line. These are often reset on crt's, with ^H replacing #, and ^U replacing @. These characters never erase beyond the beginning of the current input line or an ^D. These characters may be entered literally by preceding them with `\'; in the old teletype driver both the `\' and the character entered literally will appear on the screen; in the new driver the `\' will normally disappear.

The drivers normally treat either a carriage return or a newline character as terminating an input line, replacing the return with a newline and echoing a return and a line feed. If the CRMOD bit is cleared in the local mode word then the processing for carriage return is disabled, and it is simply echoed as a return, and does not terminate cooked mode input.

In the new driver there is a literal-next character ^V which can be typed in both cooked and CBREAK mode preceding any character to prevent its special meaning. This is to be preferred to the use of `\' escaping erase and kill characters, but `\' is (at least temporarily) retained with its old function in the new driver for historical reasons.

The new terminal driver also provides two other editing characters in normal mode. The word-erase character, normally ^W, erases the preceding word, but not any spaces before it. For the purposes of ^W, a word is defined as a sequence of non-blank characters, with tabs counted as blanks. Finally, the reprint character, normally ^R, retypes the pending input beginning on a new line. Retyping occurs automatically in cooked mode if characters which would normally be erased from the screen are fouled by program output.

Input echoing and redisplay

In the old terminal driver, nothing special occurs when an erase character is typed; the erase character is simply echoed. When a kill character is typed it is echoed followed by a new-line (even if the character is not killing the line, because it was preceded by a `\'!.)

The new terminal driver has several modes for handling the echoing of terminal input, controlled by bits in a local mode word.

Hardcopy terminals When a hardcopy terminal is in use, the LPRTERA bit is normally set in the local mode word. Characters which are logically erased are then printed out backwards preceded by `\' and followed by `/' in this mode.

Crt terminals When a crt terminal is in use, the LCRTBS bit is normally set in the local mode word. The terminal driver then echoes the proper number of erase characters when input is erased; in the normal case where the erase character is a ^H this causes the cursor of the terminal to back up to where it was before the logically erased character was typed. If the input has become fouled due to interspersed asynchronous output, the input is automatically retyped.

Erasing characters from a crt When a crt terminal is in use, the LCRTERA bit may be set to cause input to be erased from the screen with a “backspace-space-backspace” sequence when character or word deleting sequences are used. A LCRTKIL bit may be set as well, causing the input to be erased in this manner on line kill sequences as well.

Echoing of control characters If the LCTLECH bit is set in the local state word, then non-printing (control) characters are normally echoed as ^X (for some X) rather than being echoed unmodified; delete is echoed as ^?.

The normal modes for using the new terminal driver on crt terminals are speed dependent. At speeds less than 1200 baud, the LCRTERA and LCRTKILL processing is painfully slow, so stty(1) normally just sets LCRTBS and LCTLECH; at speeds of 1200 baud or greater all of these bits are normally set. Stty(1) summarizes these option settings and the use of the new terminal driver as “newcrt.”

Output processing

When one or more characters are written, they are actually transmitted to the terminal as soon as previously-written characters have finished typing. (As noted above, input characters are normally echoed by putting them in the output queue as they arrive.) When a process produces characters more rapidly than they can be typed, it will be suspended when its output queue exceeds some limit. When the queue has drained down to some threshold the program is resumed. Even parity is normally generated on output. The EOT character is not transmitted in cooked mode to prevent terminals that respond to it from hanging up; programs using raw or cbreak mode should be careful.

The terminal drivers provide necessary processing for cooked and CBREAK mode output including delay generation for certain special characters and parity generation. Delays are available after backspaces ^H, form feeds ^L, carriage returns ^M, tabs ^I and newlines ^J. The driver will also optionally expand tabs into spaces, where the tab stops are assumed to be set every eight columns. These functions are controlled by bits in the tty flags word; see Summary below.

The terminal drivers provide for mapping between upper and lower case on terminals lacking lower case, and for other special processing on deficient terminals.

Finally, in the new terminal driver, there is a output flush character, normally ^O, which sets the LFLUSHO bit in the local mode word, causing subsequent output to be flushed until it is cleared by a program or more input is typed. This character has effect in both cooked and CBREAK modes and causes pending input to be retyped if there is any pending input. An ioctl to flush the characters in the input and output queues TIOCFLUSH, is also available.

Upper case terminals and Hazeltines

If the LCASE bit is set in the tty flags, then all upper-case letters are mapped into the corresponding lower-case letter. The upper-case letter may be generated by preceding it by `\'. If the new terminal driver is being used, then upper case letters are preceded by a `\' when output. In addition, the following escape sequences can be generated on output and accepted on input:

for	`	|	~	{	}
use	\'	\!	\^	\(	\)

To deal with Hazeltine terminals, which do not understand that ~ has been made into an ASCII character, the LTILDE bit may be set in the local mode word when using the new terminal driver; in this case the character ~ will be replaced with the character ` on output.

Flow control

There are two characters (the stop character, normally ^S, and the start character, normally ^Q) which cause output to be suspended and resumed respectively. Extra stop characters typed when output is already stopped have no effect, unless the start and stop characters are made the same, in which case output resumes.

A bit in the flags word may be set to put the terminal into TANDEM mode. In this mode the system produces a stop character (default ^S) when the input queue is in danger of overflowing, and a start character (default ^Q) when the input has drained sufficiently. This mode is useful when the terminal is actually another machine or printer that obeys the conventions, or needs flow control to operate.

Line control and breaks

There are several ioctl calls available to control the state of the terminal line. The TIOCSBRK ioctl will set the break bit in the hardware interface causing a break condition to exist; this can be cleared (usually after a delay with sleep(3)) by TIOCCBRK. Break conditions in the input are reflected as a null character in RAW mode or as the interrupt character in cooked or CBREAK mode. The TIOCCDTR ioctl will clear the data terminal ready condition; it can be set again by TIOCSDTR.

When the carrier signal from the dataset drops (usually because the user has hung up his terminal) a SIGHUP hangup signal is sent to the processes in the distinguished process group of the terminal; this usually causes them to terminate (the SIGHUP can be suppressed by setting the LNOHANG bit in the local state word of the driver.) Access to the terminal by other processes is then normally revoked, so any further reads will fail, and programs that read a terminal and test for end-of-file on their input will terminate appropriately.

When using an ACU it is possible to ask that the phone line be hung up on the last close with the TIOCHPCL ioctl; this is normally done on the outgoing line.

Interrupt characters

There are several characters that generate interrupts in cooked and CBREAK mode; all are sent the processes in the control group of the terminal, as if a TIOCGPGRP ioctl were done to get the process group and then a killpg(2) system call were done, except that these characters also flush pending input and output when typed at a terminal ('a`'la TIOCFLUSH). The characters shown here are the defaults; the field names in the structures (given below) are also shown. The characters may be changed, although this is not often done.

^?  t_intrc (Delete) generates a SIGINT signal. This is the normal way to stop a process which is no longer interesting, or to regain control in an interactive program.

^\  t_quitc (FS) generates a SIGQUIT signal. This is used to cause a program to terminate and produce a core image, if possible, in the file core in the current directory.

^Z  t_suspc (EM) generates a SIGTSTP signal, which is used to suspend the current process group.

^Y  t_dsuspc (SUB) generates a SIGTSTP signal as ^Z does, but the signal is sent when a program attempts to read the ^Y, rather than when it is typed.

Job access control

When using the new terminal driver, if a process which is not in the distinguished process group of its control terminal attempts to read from that terminal its process group is sent a SIGTTIN signal. This signal normally causes the members of that process group to stop. If, however, the process is ignoring SIGTTIN, has SIGTTIN blocked, is an orphan process, or is in the middle of process creation using vfork(2)), it is instead returned an end-of-file. (An orphan process is a process whose parent has exited and has been inherited by the init(8) process.) Under older UNIX systems these processes would typically have had their input files reset to /dev/null, so this is a compatible change.

When using the new terminal driver with the LTOSTOP bit set in the local modes, a process is prohibited from writing on its control terminal if it is not in the distinguished process group for that terminal. Processes which are holding or ignoring SIGTTOU signals, which are orphans, or which are in the middle of a vfork(2) are excepted and allowed to produce output.

Summary of modes

Unfortunately, due to the evolution of the terminal driver, there are 4 different structures which contain various portions of the driver data. The first of these (sgttyb) contains that part of the information largely common between version 6 and version 7 UNIX systems. The second contains additional control characters added in version 7. The third is a word of local state peculiar to the new terminal driver, and the fourth is another structure of special characters added for the new driver. In the future a single structure may be made available to programs which need to access all this information; most programs need not concern themselves with all this state.

The basic ioctls use the structure defined in <sgtty.h>: struct sgttyb {
    char    sg_ispeed;    /* input speed */
    char    sg_ospeed;    /* output speed */
    char    sg_erase;    /* erase character */
    char    sg_kill;    /* kill character */
    int    sg_flags;    /* mode flags */
}; The sg_ispeed and sg_ospeed fields describe the input and output speeds of the device according to the following table, which corresponds to the lp(4) interface. If other hardware is used, impossible speed changes are ignored. Symbolic values in the table are as defined in <sys/ttydev.h>.

B0	0	(hang up modem)
B50	1	50 baud
B75	2	75 baud
B110	3	110 baud
B134	4	134.5 baud
B150	5	150 baud
B200	6	200 baud
B300	7	300 baud
B600	8	600 baud
B1200	9	1200 baud
B1800	10	1800 baud
B2400	11	2400 baud
B4800	12	4800 baud
B9600	13	9600 baud
EXTA	14	External A  (commonly 19200 baud)
EXTB	15	External B

Code conversion and line control required for IBM 2741's (134.5 baud) must be implemented by the user's program. The half-duplex line discipline required for the 202 dataset (1200 baud) is not supplied; full-duplex 212 datasets work fine.

The sg_erase and sg_kill fields of the argument structure specify the erase and kill characters respectively. (Defaults are # and @.)

The sg_flags field of the argument structure contains several bits that determine the system's treatment of the terminal:

ALLDELAY	0x0000ff00	Delay algorithm selection
BSDELAY	0x00008000	Select backspace delays (not implemented):
BS0	0x00000000
BS1	0x00008000
VTDELAY	0x00004000	Select form-feed and vertical-tab delays:
FF0	0x00000000
FF1	0x00004000
CRDELAY	0x00003000	Select carriage-return delays:
CR0	0x00000000
CR1	0x00001000
CR2	0x00002000
CR3	0x00003000
TBDELAY	0x00000c00	Select tab delays:
TAB0	0x00000000
TAB1	0x00000400
TAB2	0x00000800
XTABS	0x00000c00
NLDELAY	0x00000300	Select new-line delays:
NL0	0x00000000
NL1	0x00000100
NL2	0x00000200
NL3	0x00000300
EVENP	0x00000080	Even parity allowed on input (most terminals)
ODDP	0x00000040	Odd parity allowed on input
RAW	0x00000020	Raw mode: wake up on all characters, 8-bit interface
CRMOD	0x00000010	Map CR into LF; echo LF or CR as CR-LF
ECHO	0x00000008	Echo (full duplex)
LCASE	0x00000004	Map upper case to lower on input
CBREAK	0x00000002	Return each character as soon as typed
TANDEM	0x00000001	Automatic flow control

The delay bits specify how long transmission stops to allow for mechanical or other movement when certain characters are sent to the terminal. In all cases a value of 0 indicates no delay.

Backspace delays are currently ignored but might be used for Terminet 300's.

If a form-feed/vertical tab delay is specified, it lasts for about 2 seconds.

Carriage-return delay type 1 lasts about .08 seconds and is suitable for the Terminet 300. Delay type 2 lasts about .16 seconds and is suitable for the VT05 and the TI 700. Delay type 3 is suitable for the concept-100 and pads lines to be at least 9 characters at 9600 baud.

New-line delay type 1 is dependent on the current column and is tuned for Teletype model 37's. Type 2 is useful for the VT05 and is about .10 seconds. Type 3 is unimplemented and is 0.

Tab delay type 1 is dependent on the amount of movement and is tuned to the Teletype model 37. Type 3, called XTABS, is not a delay at all but causes tabs to be replaced by the appropriate number of spaces on output.

Input characters with the wrong parity, as determined by bits 200 and 100, are ignored in cooked and CBREAK mode.

RAW disables all processing save output flushing with LFLUSHO; full 8 bits of input are given as soon as it is available; all 8 bits are passed on output. A break condition in the input is reported as a null character. If the input queue overflows in raw mode it is discarded; this applies to both new and old drivers.

CRMOD causes input carriage returns to be turned into new-lines; input of either CR or LF causes LF-CR both to be echoed (for terminals with a new-line function).

CBREAK is a sort of half-cooked (rare?) mode. Programs can read each character as soon as it is typed, instead of waiting for a full line; all processing is done except the input editing. Character erase, word erase, line kill, input reprint, and the special treatment of \ or EOT are disabled.

TANDEM mode causes the system to produce a stop character (default ^S) whenever the input queue is in danger of overflowing, and a start character (default ^Q) when the input queue has drained sufficiently. It is useful for flow control when the `terminal' is really another computer or printer which understands the conventions, or needs flow control to operate.

A large number of ioctl(2) calls apply to terminals. Some have the general form: #include <sgtty.h>

ioctl(fildes, code, arg) struct sgttyb *arg; The applicable codes are:

TIOCGETP
    Fetch the basic parameters associated with the terminal, and store it in the pointed-to sgttyb structure.

TIOCSETP
    Set the parameters according to the pointed-to sgttyb structure. The interface delays until output is quiescent, then throws away any unread characters, before changing the modes.

TIOCSETN
    Set the parameters like TIOCSETP but do not delay or flush input. Input is not preserved, however, when changing to or from RAW.

The following ioctl codes ignore arg.

TIOCEXCL
    Set “exclusive-use” mode. No further opens are permitted until the file has been closed.

TIOCNXCL
    Turn off “exclusive-use” mode.

TIOCHPCL
    When the file is closed for the last time, hang up the terminal. This is useful when the line is associated with an ACU used to place outgoing calls.

With the following codes arg is a pointer to an int.

TIOCGETD
    arg is a pointer to an int into which is placed the current line discipline number.

TIOCSETD
    arg is a pointer to an int whose value becomes the current line discipline number.

TIOCFLUSH
    If the int pointed to by arg has a zero value, all characters waiting in input or output queues are flushed. Otherwise, the value of the int is for the FREAD and FWRITE bits defined in <sys/file.h>; if the FREAD bit is set, all characters waiting in input queues are flushed, and if the FWRITE bit is set, all characters waiting in output queues are flushed. If the value of arg is zero then then input and output are also flushed. This is a common BSD4.3 usage that otherwise depends on zero at zero.

The remaining calls are not available in vanilla version 7 UNIX. In cases where arguments are required, they are described; arg should otherwise be given as 0.

TIOCSTI
    The argument is the address of a character that the system acts as if it was typed on the terminal.

TIOCSBRK
    The break bit is set in the terminal.

TIOCCBRK
    The break bit is cleared.

TIOCSDTR
    Data terminal ready is set.

TIOCCDTR
    Data terminal ready is cleared.

TIOCGPGRP
    Arg is the address of a word into which is placed the process group number of the control terminal.

TIOCSPGRP
    Arg is the address of a word (typically a process id) that becomes the process group for the control terminal.

FIONREAD
    Returns in the long integer whose address is arg the number of immediately readable characters from the argument unit. This works for files, pipes, and terminals, but not for multiplexed channels.

TIOCNOTTY
    If this terminal is the controlling terminal of the process, remove the process group and control terminal associations for the invoking process, and close all file descriptors this process has open on this terminal. The next terminal this process opens will become the new controlling terminal.

The second structure associated with each terminal specifies characters that are special in both the old and new terminal interfaces: The following structure is defined in <sys/ioctl.h>, which is automatically included in <sgtty.h>: struct tchars {
    char    t_intrc;    /* interrupt */
    char    t_quitc;    /* quit */
    char    t_startc;    /* start output */
    char    t_stopc;    /* stop output */
    char    t_eofc;    /* end-of-file */
    char    t_brkc;    /* input delimiter (like nl) */
}; The default values for these characters are ^?, ^\, ^Q, ^S, ^D, and -1. A character value of -1 eliminates the effect of that character. The t_brkc character, by default -1, acts like a new-line in that it terminates a `line,' is echoed, and is passed to the program. The `stop' and `start' characters may be the same, to produce a toggle effect. It is probably counterproductive to make other special characters (including erase and kill) identical. The applicable ioctl calls are:

TIOCGETC
    Get the special characters and put them in the specified structure.

TIOCSETC
    Set the special characters to those given in the structure.

The third structure associated with each terminal is a local mode word; except for the LNOHANG bit, this word is interpreted only when the new driver is in use. The bits of the local mode word are:

LCRTBS	0x00000001	Backspace on erase rather than echoing erase
LPRTERA	0x00000002	Printing terminal erase mode
LCRTERA	0x00000004	Erase character echoes as backspace-space-backspace
LTILDE	0x00000008	Convert ~ to ` on output (for Hazeltine terminals)
LMDMBUF	0x00000010	Stop/start output when carrier drops
LLITOUT	0x00000020	Suppress output translations
LTOSTOP	0x00000040	Send SIGTTOU for background output
LFLUSHO	0x00000080	Output is being flushed
LNOHANG	0x00000100	Don't send hangup when carrier drops
LETXACK	0x00000200	Diablo style buffer hacking (unimplemented)
LCRTKIL	0x00000400	BS-space-BS erase entire line on line kill
LPASS8	0x00000800	Pass all 8 bits through on input, in any mode
LCTLECH	0x00001000	Echo input control chars as ^X, delete as ^?
LPENDIN	0x00002000	Retype pending input at next read or input character
LDECCTQ	0x00004000	Only ^Q restarts output after ^S, like DEC systems
LNOFLUSH	0x00008000	Don't flush output on signal

The applicable ioctl functions are:

TIOCLBIS
    arg is the address of a mask which is the bits to be set in the local mode word.

TIOCLBIC
    arg is the address of a mask of bits to be cleared in the local mode word.

TIOCLSET
    arg is the address of a mask to be placed in the local mode word.

TIOCLGET
    arg is the address of a word into which the current mask is placed.

Note that when LPASS8 is set, output delays are disabled.

The final structure associated with each terminal is the ltchars structure which defines interrupt characters for the new terminal driver. Its structure is: struct ltchars {
    char    t_suspc;    /* stop process signal */
    char    t_dsuspc;    /* delayed stop process signal */
    char    t_rprntc;    /* reprint line */
    char    t_flushc;    /* flush output (toggles) */
    char    t_werasc;    /* word erase */
    char    t_lnextc;    /* literal next character */
};

The default values for these characters are ^Z, ^Y, ^R, ^O, ^W, and ^V. A value of -1 disables the character.

The applicable ioctl functions are:

TIOCSLTC
    args is the address of a ltchars structure which defines the new local special characters.

TIOCGLTC
    args is the address of a ltchars structure into which is placed the current set of local special characters.

Note that due to the implementation of the literal next function, t_lnextc may not be set to be the "single shift" character (SS2) which has the octal representation 216.

Each terminal has provision for storage of the current terminal or window size in a winsize structure, with format:

struct winsize {
	unsigned short  ws_row; /* rows, in characters */
	unsigned short  ws_col; /* columns, in characters */
	unsigned short  ws_xpixel;      /* horizontal size, pixels */
	unsigned short  ws_ypixel;      /* vertical size, pixels */
};

A value of 0 in any field is interpreted as ``undefined;'' the entire structure is zeroed on final close.

The applicable ioctl functions are:

TIOCGWINSZ
    arg is a pointer to a struct winsize into which will be placed the current terminal or window size information.

TIOCSWINSZ
    arg is a pointer to a struct winsize which will be used to set the current terminal or window size information. If the new information is different than the old information, a SIGWINCH signal will be sent to the terminal's process group.

FILES
/dev/tty
/dev/tty*
/dev/console

SEE ALSO
csh(1), stty(1), ioctl(2), sigvec(2), stty(3C), getty(8), init(8)

BUGS
Half-duplex terminals are not supported.