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%sccs.include.redist.roff%
@(#)2.4.t 8.3 (Berkeley) 05/20/94
.Sh 2 "Terminals and Devices .Sh 3 "Terminals
Terminals support .Fn read and .Fn write I/O operations, as well as a collection of terminal specific .Fn ioctl operations, to control input character interpretation and editing, and output format and delays.
A terminal may be used as a controlling terminal (login terminal) for a login session. A controlling terminal is associated with a session (see section .Xr 1.1.4 ). A controlling terminal has a foreground process group, which must be a member of the session with which the terminal is associated. Members of the foreground process group are allowed to read from and write to the terminal and change the terminal settings; other process groups from the session may be stopped upon attempts to do these operations.
A session leader allocates a terminal as the controlling terminal for its session using the ioctl ioctl(fd, TIOCSCTTY, NULL); int fd; .Fn Only a session leader may acquire a controlling terminal. .Sh 4 "Terminal input
Terminals are handled according to the underlying communication characteristics such as baud rate and required delays, and a set of software parameters. These parameters are described in the termios structure maintained by the kernel for each terminal line:
| struct termios { |
| tcflag_t c_iflag; /* input flags */ |
| tcflag_t c_oflag; /* output flags */ |
| tcflag_t c_cflag; /* control flags */ |
| tcflag_t c_lflag; /* local flags */ |
| cc_t c_cc[NCCS]; /* control chars */ |
| long c_ispeed; /* input speed */ |
| long c_ospeed; /* output speed */ |
| }; |
Two general kinds of input processing are available, determined by whether the terminal device file is in canonical mode or noncanonical mode. Additionally, input characters are processed according to the c_iflag and c_lflag fields. Such processing can include echoing, which in general means transmitting input characters immediately back to the terminal when they are received from the terminal. Non-graphic ASCII input characters may be echoed as a two-character printable representation, ``^character.''
In canonical mode input processing, terminal input is processed in units of lines. A line is delimited by a newline character (NL), an end-of-file (EOF) character, or an end-of-line (EOL) character. Input is presented on a line-by-line basis. Using this mode means that a read request will not return until an entire line has been typed, or a signal has been received. Also, no matter how many bytes are requested in the read call, at most one line is returned. It is not, however, necessary to read a whole line at once; any number of bytes, even one, may be requested in a read without losing information.
When the terminal is in canonical mode, editing of an input line is performed. Editing facilities allow deletion of the previous character or word, or deletion of the current input line. In addition, a special character may be used to reprint the current input line after some number of editing operations have been applied. Certain other characters are also interpreted specially. Flow control is provided by stop output and start output control characters. Output may be flushed with the flush output character; and a literal character may be used to force literal input of the immediately following character in the input line.
In noncanonical mode input processing, input bytes are not assembled into lines, and erase and kill processing does not occur. All input is passed through to the reading process immediately and without interpretation. Signals may be enabled; here the handler interprets input only by looking for characters that cause interrupts or output flow control; all other characters are made available.
When interrupt characters are being interpreted by the terminal handler they cause a software interrupt to be sent to all processes in the process group associated with the terminal. Interrupt characters exist to send SIGINT and SIGQUIT signals, and to stop a process group with the SIGTSTP signal either immediately, or when all input up to the stop character has been read. .Sh 4 "Terminal output
On output, the terminal handler provides some simple formatting services. These include converting the carriage return character to the two character return-linefeed sequence, inserting delays after certain standard control characters, and expanding tabs. .Sh 3 "Structured devices
Structured devices are typified by disks and magnetic tapes, but may represent any random-access device. The system performs read-modify-write type buffering actions on block devices to allow them to be read and written in a totally random access fashion like ordinary files. Filesystems are normally mounted on block devices. .Sh 3 "Unstructured devices
Unstructured devices are those devices which do not support block structure. Familiar unstructured devices are raw communications lines (with no terminal handler), raster plotters, magnetic tape and disks unfettered by buffering and permitting large block input/output and positioning and formatting commands.