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 discarded if the input queue threatens to overflow if the device supports flow control.
5 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 " (^\e)" 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 ('\er'). 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 ('\en') and a line break.
Special output character if OPOST and ONLCR are set. A CR NL sequence is output instead of just NL . (MINIX 3 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 position 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 3 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 happens 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 3. (Changes to START and STOP may be ignored under other termios implementations.) The REPRINT and LNEXT characters are MINIX 3 extensions that are commonly present in other implementations. \s-2POSIX\s+2 is unclear on whether IEXTEN, IGNCR and ICRNL should be active in non-canonical mode, but under MINIX 3 they are.
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.
5 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 3 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 '\e377', '\e0'. The character '\e377' is preceded by another '\e377' 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 '\e0', or if PARMRK is set as '\e377', '\e0', '\e0'. (Breaks are always ignored.)
OPOST Perform output processing. This flag is the "main switch" on output processing. All other flags are MINIX 3 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 ('\en'). 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.)
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.)
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 character 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 3 has no job control.)
5 "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.
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 3 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.
A pseudo terminal can be allocated by trying to open all the controlling devices /dev/pty nn 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.)
20 /dev/console System console.
/dev/ttyc[1-7] Virtual consoles. (0/1/7, 0/1/7)
/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.