TTY(4) Kernel Interfaces Manual TTY(4)
NAME
tty, termios - terminals
DESCRIPTION
The tty driver family takes care of all user input and output. It governs the keyboard, the console, the serial lines, and pseudo ttys.
Input on any of these devices undergoes "input processing", and output undergoes "output processing" according to the standard termios ter-
minal interface.
Input processing
Each terminal device has an input queue. This queue is used to store preprocessed input characters, and to perform the backspacing and
erase functions. Some special characters like a newline make the contents of the queue available to a process reading from the terminal.
Characters up to and including the newline, or another so-called "line break", may be read by a process. The process need not read all
characters at once. An input line may be read byte by byte if one wants to. A line break just makes characters available for reading,
thats all.
When data is made available depends on whether the tty is in canonical mode or not. In canonical mode the terminal processes input line by
line. A line ends with a newline (NL), end-of-file (EOF), or end-of-line (EOL). Characters that have not been delimited by such a line
break may be erased one by one with the ERASE character or all at once with the KILL character. Once a line break is typed the characters
become available to a reading process and can no longer be erased. Once read they are removed from the input queue. Several lines may be
gathered in the input queue if no reader is present to read them, but a new reader will only receive one line. Two line breaks are never
returned in one read call. The input queue has a maximum length of MAX_CANON characters. Any more characters are discarded. One must use
ERASE or KILL to make the terminal functioning again if the input queue fills up. If nonblocking I/O is set then -1 is returned with errno
set to EAGAIN if the reader would otherwise be blocked.
In non-canonical mode (raw mode for short) all characters are immediately available to the reader in principle. One may however tune the
terminal to bursty input with the MIN and TIME parameters, see the raw I/O parameters section below. In raw mode no characters are dis-
carded if the input queue threatens to overflow if the device supports flow control.
Output processing
Characters written to a terminal device may undergo output processing, which is usually just inserting a carriage returns before newlines.
A writer may return before all characters are output if the characters can be stored in the output buffers. If not then the writer may be
blocked until space is available. If non-blocking I/O is set then only the count of the number of bytes that can be processed immediately
is returned. If no characters can be written at all then -1 is returned with errno set to EAGAIN.
Special characters
Some characters have special functions in some of the terminal modes. These characters are as follows, with the Minix defaults shown in
parentheses:
INTR (^?)
Special input character that is recognized if ISIG is set. (For ISIG and other flags see the various modes sections below.) It
causes a SIGINT signal to be sent to all processes in the terminal process group. (See the section on session leaders below.)
QUIT (^)
Special input character if ISIG is set. Causes a SIGQUIT signal to be sent to the terminal process group.
ERASE (^H)
Special input character if ICANON is set. Erases the last character in the current line.
KILL (^U)
Special input character if ICANON is set. Erases the entire line.
EOF (^D)
Special input character if ICANON is set. It is a line break character that is not itself returned to a reader. If EOF is typed with
no input present then the read returns zero, which normally causes the reader to assume that end-of-file is reached.
CR (^M)
Special input character if IGNCR or ICRNL is set. It is a carriage return ('
'). If IGNCR is set then CR is discarded. If ICRNL is
set and IGNCR is not set then CR is changed into an NL and has the same function as NL.
NL (^J)
Special input character if ICANON is set. It is both a newline ('
') and a line break.
Special output character if OPOST and ONLCR are set. A CR NL sequence is output instead of just NL. (Minix specific, but almost
mandatory on any UNIX-like system.)
TAB (^I)
Special character on output if OPOST and XTABS are set. It is transformed into the number of spaces necessary to reach a column posi-
tion that is a multiple of eight. (Only needed for terminals without hardware tabs.)
EOL (undefined)
Special input character if ICANON is set. It is an additional line break.
SUSP (^Z)
Special input character if job control is implemented and ISIG is set. It causes a SIGTSTP signal to be send to the terminal process
group. (Minix does not have job control.)
STOP (^S)
Special input character if IXON is set. It suspends terminal output and is then discarded.
START (^Q)
Special output character if IXON is set. It starts terminal output if suspended and is then discarded. If IXANY is also set then any
other character also starts terminal output, but they are not discarded.
REPRINT (^R)
Special input character if IEXTEN and ECHO are set. Reprints the input queue from the last line break onwards. A reprint also hap-
pens automatically if the echoed input has been messed up by other output and ERASE is typed.
LNEXT (^V)
Special input character if IEXTEN is set. It is the "literal next" character that causes the next character to be input without any
special processing.
DISCARD (^O)
Special input character if IEXTEN is set. Causes output to be discarded until it is typed again. (Implemented only under Minix-vmd.)
All of these characters except CR, NL and TAB may be changed or disabled under Minix. (Changes to START and STOP may be ignored under
other termios implementations.) The REPRINT and LNEXT characters are Minix extensions that are commonly present in other implementations.
POSIX is unclear on whether IEXTEN, IGNCR and ICRNL should be active in non-canonical mode, but under Minix they are.
Terminal attributes
The attributes of a terminal, such as whether the mode should be canonical or non-canonical, are controlled by routines that use the
termios structure as defined in <termios.h>:
struct termios {
tcflag_t c_iflag; /* input modes */
tcflag_t c_oflag; /* output modes */
tcflag_t c_cflag; /* control modes */
tcflag_t c_lflag; /* local modes */
speed_t c_ispeed; /* input speed */
speed_t c_ospeed; /* output speed */
cc_t c_cc[NCCS]; /* control characters */
};
The types tcflag, speed_t and cc_t are defined in <termios.h> as unsigned integral types.
Input Modes
The c_iflag field contains the following single bit flags that control input processing:
ICRNL
Map CR to NL on input.
IGNCR
Ignore CR on input. This flag overrides ICRNL.
INLCR
Map NL to CR on input. This is done after the IGNCR check.
IXON Enable start/stop output control.
IXOFF
Enable start/stop input control. (Not implemented.)
IXANY
Allow any character to restart output. (Minix specific.)
ISTRIP
Strip characters to seven bits.
IGNPAR
Ignore characters with parity errors. (Not implemented.)
INPCK
Enable input parity checking. (Not implemented.)
PARMRK
Mark parity errors by preceding the faulty character with '377', ' '. The character '377' is preceded by another '377' to avoid
ambiguity. (Not implemented.)
BRKINT
Send the signal SIGINT to the terminal process group when receiving a break condition. (Not implemented.)
IGNBRK
Ignore break condition. If neither BRKINT or IGNBRK is set a break is input as a single ' ', or if PARMRK is set as '377', ' ',
' '. (Breaks are always ignored.)
Output Modes
The c_oflag field contains the following single bit flags that control output processing:
OPOST Perform output processing. This flag is the "main switch" on output processing. All other flags are Minix specific.
ONLCR Transform an NL to a CR NL sequence on output. Note that a key labeled "RETURN" or "ENTER" usually sends a CR. In line oriented
mode this is normally transformed into NL by ICRNL. NL is the normal UNIX line delimiter ('
'). On output an NL is transformed
into the CR NL sequence that is necessary to reach the first column of the next line. (This is a common output processing function
for UNIX-like systems, but not always separately switchable by an ONLCR flag.)
XTABS Transform a TAB into the number of spaces necessary to reach a column position that is a multiple of eight.
ONOEOT Discard EOT (^D) characters. (Minix-vmd only.)
Control Modes
The c_cflag field contains the following single bit flags and bit field for basic hardware control:
CLOCAL Ignore modem status lines.
CREAD Enable receiver. (The receiver is always enabled.)
CSIZE Number of bits per byte. CSIZE masks off the values CS5, CS6, CS7 and CS8 that indicate that 5, 6, 7 or 8 bits are used.
CSTOPB Send two stop bits instead of one. Two stop bits are normally used at 110 baud or less.
PARENB Enable parity generation.
PARODD Generate odd parity if parity is generated, otherwise even parity.
HUPCL Drop the modem control lines on the last close of the terminal line. (Not implemented.)
Local Modes
The c_lflag field contains the following single bit flags that control various functions:
ECHO Enable echoing of input characters. Most input characters are echoed as they are. Control characters are echoed as ^X where X is
the letter used to say that the control character is CTRL-X. The CR, NL and TAB characters are echoed with their normal effect
unless they are escaped by LNEXT.
ECHOE If ICANON and ECHO are set then echo ERASE and KILL as one or more backspace-space-backspace sequences to wipe out the last charac-
ter or the entire line, otherwise they are echoed as they are.
ECHOK If ICANON and ECHO are set and ECHOE is not set then output an NL after the KILL character. (For hardcopy terminals it is best to
unset ECHOE and to set ECHOK.)
ECHONL Echo NL even if ECHO is not set, but ICANON is set.
ICANON Canonical input. This enables line oriented input and erase and kill processing.
IEXTEN Enable implementation defined input extensions.
ISIG Enable the signal characters INTR, QUIT and SUSP.
NOFLSH Disable the flushing of the input and output queues that is normally done if a signal is sent.
TOSTOP Send a SIGTTOU signal if job control is implemented and a background process tries to write. (Minix has no job control.)
Input and output speed
The input and output speed are encoded into the c_ispeed and c_ospeed fields. <termios.h> defines the symbols B0, B50, B75, B110, B134,
B150, B200, B300, B600, B1200, B1800, B2400, B4800, B9600, B19200, B38400, B57600 and B115200 as values used to indicate the given baud
rates. The zero baud rate, B0, if used for the input speed causes the input speed to be equal to the output speed. Setting the output
speed to zero hangs up the line. One should use the functions cfgetispeed(), cfgetospeed(), cfsetispeed() and cfsetospeed() to get or set
a speed, because the c_ispeed and c_ospeed fields may not be visible under other implementations. (The c_ispeed and c_ospeed fields and
the B57600 and B115200 symbols are Minix specific.)
Special characters
The c_cc array contains the special characters that can be modified. The array has length NCCS and is subscripted by the symbols VEOF,
VEOL, VERASE, VINTR, VKILL, VMIN, VQUIT, VTIME, VSUSP, VSTART, VSTOP, VREPRINT, VLNEXT and VDISCARD. All these symbols are defined in
<termios.h>. Some implementations may give the same values to the VMIN and VTIME subscripts and the VEOF and VEOL subscripts respectively,
and may ignore changes to START and STOP. (Under Minix all special characters have there own c_cc slot and can all be modified.)
Raw I/O Parameters
The MIN and TIME parameters can be used to adjust a raw connection to bursty input. MIN represents a minimum number of bytes that must be
received before a read call returns. TIME is a timer of 0.1 second granularity that can be used to time out a read. Setting either of
these parameters to zero has special meaning, which leads to the following four possibilities:
MIN > 0, TIME > 0
TIME is an inter-byte timer that is started (and restarted) when a byte is received. A read succeeds when either the minimum number
of characters is received or the timer expires. Note that the timer starts after the first character, so the read returns at least
one byte.
MIN > 0, TIME = 0
Now the timer is disabled, and a reader blocks indefinitely until at least MIN characters are received.
MIN = 0, TIME > 0
TIME is now a read timer that is started when a read is executed. The read will return if the read timer expires or if at least one
byte is input. (Note that a value of zero may be returned to the reader.)
MIN = 0, TIME = 0
The bytes currently available are returned. Zero is returned if no bytes are available.
User Level Functions
Termios attributes are set or examined, and special functions can be performed by using the functions described in termios(2).
Session Leaders and Process Groups
With the use of the setsid() function can a process become a session leader. A session leader forms a process group with a process group
id equal to the process id of the session leader. If a session leader opens a terminal device file then this terminal becomes the control-
ling tty of the session leader. Unless the terminal is already the controlling tty of another process, or unless the O_NOCTTY flag is used
to prevent the allocation of a controlling tty. The process group of the session leader is now remembered as the terminal process group
for signals sent by the terminal driver. All the children and grandchildren of the session leader inherit the controlling terminal and
process group until they themselves use setsid().
The controlling tty becomes inaccessible to the children of the session leader when the session leader exits, and a hangup signal is sent
to all the members of the process group. The input and output queues are flushed on the last close of a terminal and all attributes are
reset to the default state.
A special device /dev/tty is a synonym for the controlling tty of a process. It allows a process to reach the terminal even when standard
input, output and error are redirected. Opening this device can also be used as a test to see if a process has a controlling tty or not.
For Minix a special write-only device /dev/log exists for processes that want to write messages to the system console. Unlike the console
this device is still accessible when a session leader exits.
Minix-vmd also has a /dev/log device, but this device is read-write. All messages written to the log device or to the console when X11 is
active can be read from /dev/log. The system tries to preserve the log buffer over a reboot so that panic messages reappear in the log if
the system happens to crash.
Pseudo Terminals
Pseudo ttys allow a process such as a remote login daemon to set up a terminal for a remote login session. The login session uses a device
like /dev/ttyp0 for input and output, and the remote login daemon uses the device /dev/ptyp0 to supply input to or take output from the
login session and transfer this to or from the originating system. So the character flow may be: Local user input sent to the remote sys-
tem is written to /dev/ptyp0 by the remote login daemon, undergoes input processing and appears on /dev/ttyp0 as input to the login ses-
sion. Output from the login session to /dev/ttyp0 undergoes output processing, is read from /dev/ptyp0 by the remote login daemon and is
send over to the local system to be displayed for the user. (So there are only four data streams to worry about in a pseudo terminal.)
A pseudo terminal can be allocated by trying to open all the controlling devices /dev/ptynn one by one until it succeeds. Further opens
will fail once a pty is open. The process should now fork, the child should become session leader, open the tty side of the pty and start
a login session.
If the tty side is eventually closed down then reads from the pty side will return zero and writes return -1 with errno set to EIO. If the
pty side is closed first then a SIGHUP signal is sent to the session leader and further reads from the tty side return zero and writes
return -1 with errno set to EIO. (Special note: A line erase may cause up to three times the size of the tty input queue to be sent to
the pty reader as backspace overstrikes. Some of this output may get lost if the pty reader cannot accept it all at once in a single read
call.)
Backwards compatibility
The TIOCGETP, TIOCSETP, TIOCGETC and TIOCSETC ioctl functions that are used by the old sgtty terminal interface are still supported by the
terminal driver by emulation. Note that these old functions cannot control all termios attributes, so the terminal must be in a relatively
sane state to avoid problems.
FILES
The list below shows all devices that Minix and Minix-vmd have. Not all of these devices are configured in by default, as indicated by the
numbers (i/j/k, l/m/n) that tell the minimum, default and maximum possible number of these devices for Minix (i/j/k) and Minix-vmd (l/m/n).
/dev/console System console.
/dev/ttyc[1-7] Virtual consoles. (0/1/7, 0/1/7)
/dev/log Console log device.
/dev/tty0[0-3] Serial lines. (0/2/2, 4/4/4)
/dev/tty[p-w][0-f] Pseudo ttys. (0/0/64, 1/32/128)
/dev/pty[p-w][0-f] Associated pseudo tty controllers.
SEE ALSO
stty(1), termios(3), setsid(2), read(2), write(2).
BUGS
A fair number of flags are not implemented under Minix (yet). Luckily they are very limited utility and only apply to RS-232, not to the
user interface.
AUTHOR
Kees J. Bot (kjb@cs.vu.nl)
TTY(4)