Line Disciplines
The system provides different line disciplines for controlling communications lines. In this version of the system there are two disciplines available for use with terminals:
Line discipline switching is accomplished with the TIOCSETD ioctl:
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. Other disciplines may exist for special purposes, such as use of communications lines for network connections. 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.
The 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 getty(8) or rlogind(8C) 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.
A process can remove the association it has with its controlling terminal by opening the file /dev/tty and issuing an
This is often desirable in server processes.
Process Group
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 terminal's associated process group may be set using the TIOCSPGRP ioctl(2):
or examined using TIOCGPGRP, which returns 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:
The style of input processing can also be very different when the terminal is put in non-blocking I/O mode; see the FNDELAY flag described in 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 that a SIGIO signal be sent it whenever input is present and also whenever output queues fall below the low-water mark. 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 RAW mode, the terminal driver throws away all input and output without notice when the limit is reached. In CBREAK or cooked mode it refuses to accept any further input and, if in the new line discipline, 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.) If neither EVEN nor ODD is specified, both parity characters are accepted and the parity bit is stripped.
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).
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(3C) 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 Job access control and of FIONREAD in Summary, both 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 erase character sg_erase (by default, DELETE) logically erasing the last character typed and the sg_kill character (default, ^U: control-U) logically erasing the entire current input line. These characters never erase beyond the beginning of the current input line or an eof. These characters may be entered literally by preceding them with `\'; the `\' will normally be erased when the character is typed.
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 (normally ^V) which can be typed in both cooked and CBREAK mode preceding any character to prevent its special meaning to the terminal handler. This is to be preferred to the use of `\' escaping erase and kill characters, but `\' is retained with its old function in the new line discipline.
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
The terminal driver has several modes (not present in standard UNIX Version 7 systems) 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 use on CRT terminals are speed dependent. At speeds less than 1200 baud, the LCRTERA and LCRTKILL processing is painfully slow, and 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 generated on output if EVEN is specified (whether or not ODD is specified). If only ODD is specified, odd parity is generated. Zero parity is generated if neither EVEN nor ODD is specified. 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, and optionally convert newlines to carriage returns followed by newline. These functions are controlled by bits in the tty flags word; see Summary below.
The terminal drivers provide for mapping between uppercase and lowercase on terminals lacking lowercase, 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 or output queues, TIOCFLUSH, is also available.
Uppercase 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 `\'. Uppercase 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; 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 that obeys those conventions.
Hardware flow control is possible on NeXT 68040-based machines (see zs(4)).
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.
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 lines and dialups.
Interrupt Characters
There are several characters that generate interrupts in cooked and CBREAK mode; all are sent to 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.
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, or is in the middle of process creation using vfork(2)), the read will return -1 and set errno to EIO.
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 or which are in the middle of a vfork(2) are excepted and allowed to produce output. Terminal/window sizes. In order to accommodate terminals and workstations with variable-sized windows, the terminal driver provides a mechanism for obtaining and setting the current terminal size. The driver does not use this information internally, but only stores it and provides a uniform access mechanism. When the size is changed, a SIGWINCH signal is sent to the terminal's process group so that knowledgeable programs may detect size changes. This facility was added in 4.3BSD and is not available in earlier versions of the system.
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 added in 4BSD, 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.
Basic modes: sgtty.
The basic ioctls use the structure defined in <sgtty.h>:
struct sgttyb {
char sg_ispeed;
char sg_ospeed;
char sg_erase;
char sg_kill;
shortsg_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 DEC DH-11 interface. If other hardware is used, impossible speed changes are ignored. Symbolic values in the table are as defined in <sgtty.h>.
B0 0 (hang up dataphone) 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 19200 baud EXTB 15 38400 baud
Note: 19200 and 38400 baud aren't reliable on 68030-based machines.
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 DELETE and ^U.)
The sg_flags field of the argument structure contains several bits that determine the system's treatment of the terminal:
ALLDELAY 0177400 Delay algorithm selection BSDELAY 0100000 Select backspace delays (not implemented): BS0 0 BS1 0100000 VTDELAY 0040000 Select form-feed and vertical-tab delays: FF0 0 FF1 0040000 CRDELAY 0030000 Select carriage-return delays: CR0 0 CR1 0010000 CR2 0020000 CR3 0030000 TBDELAY 0006000 Select tab delays: TAB0 0 TAB1 0002000 TAB2 0004000 XTABS 0006000 NLDELAY 0001400 Select new-line delays: NL0 0 NL1 0000400 NL2 0001000 NL3 0001400 EVENP 0000200 Even parity allowed on input ODDP 0000100 Odd parity allowed on input RAW 0000040 Raw mode: wake up on all characters, 8-bit interface CRMOD 0000020 Map CR into LF; output LF as CR-LF ECHO 0000010 Echo (full duplex) LCASE 0000004 Map uppercase to lowercase on input and lowercase to uppercase on output CBREAK 0000002 Return each character as soon as typed TANDEM 0000001 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.
The flags for even and odd parity control parity checking on input and generation on output in cooked and CBREAK mode (unless LPASS8 or LPASS8OUT is enabled, see below). Even parity is generated on output if EVEN or both EVEN and ODD are specified. Odd parity is generated on output if only ODD is specified. Zero parity is generated on output if neither EVEN nor ODD is specified. Any parity is accepted on input if both EVEN and ODD are specified or if neither is specified. If only one of EVEN or ODD is specified, only that parity is accepted on input.
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 all data in the input and output queues are discarded; this applies to both new and old drivers.
CRMOD causes input carriage returns to be turned into new-lines, and output and echoed new-lines to be output as a carriage return followed by a line feed.
CBREAK is a sort of half-cooked (rare?) mode. Programs can read each character as soon as typed, instead of waiting for a full line; all processing is done except the input editing: character and word erase and line kill, input reprint, and the special treatment of \ and 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 which understands the conventions.
Note: The same ``stop'' and ``start'' characters are used for both directions of flow control; the t_stopc character is accepted on input as the character that stops output and is produced on output as the character to stop input, and the t_startc character is accepted on input as the character that restarts output and is produced on output as the character to restart input.
Basic ioctls
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:
With the following codes arg is ignored.
With the following codes arg is a pointer to an int.
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.
Tchars
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 ^C, ^\, ^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:
Local mode
The third structure associated with each terminal is a local mode word. The bits of the local mode word are:
| LCRTBS | 000001 | Backspace on erase rather than echoing erase |
| LPRTERA | 000002 | Printing terminal erase mode |
| LCRTERA | 000004 | Erase character echoes as backspace-space-backspace |
| LTILDE | 000010 | Convert ~ to ` on output (for Hazeltine terminals) |
| LMDMBUF | 000020 | Stop/start output when carrier drops |
| LLITOUT | 000040 | Suppress output translations |
| LTOSTOP | 000100 | Send SIGTTOU for background output |
| LFLUSHO | 000200 | Output is being flushed |
| LNOHANG | 000400 | Don't send hangup when carrier drops |
| LPASS8OUT | 001000 | Cooked mode with 8-bit output |
| (maintains tab expansion and newline to cr/lf mapping) | ||
| LCRTKIL | 002000 | BS-space-BS erase entire line on line kill |
| LPASS8 | 004000 | Pass all 8 bits through on input, in any mode |
| LCTLECH | 010000 | Echo input control chars as ^X, delete as ^? |
| LPENDIN | 020000 | Retype pending input at next read or input character |
| LDECCTQ | 040000 | Only ^Q restarts output after ^S |
| LNOFLSH | 100000 | Inhibit flushing of pending I/O when an interrupt character is typed. |
The applicable ioctl functions are:
Local special chars
The final control structure associated with each terminal is the ltchars structure which defines control 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:
Window/terminal sizes
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: