Terminals support read and write I/O operations, as well as a collection of terminal specific ioctl operations, to control input character interpretation and editing, and output format and delays. Terminal input

Terminals are handled according to the underlying communication characteristics such as baud rate and required delays, and a set of software parameters. Input modes

A terminal is in one of three possible modes: raw, cbreak, or cooked. In raw mode all input is passed through to the reading process immediately and without interpretation. In cbreak mode, the handler interprets input only by looking for characters that cause interrupts or output flow control; all other characters are made available as in raw mode. In cooked mode, input is processed to provide standard line-oriented local editing functions, and input is presented on a line-by-line basis. Interrupt characters

Interrupt characters are interpreted by the terminal handler only in cbreak and cooked modes, and 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. Line editing

When the terminal is in cooked 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 interpreted specially when a process is in cooked mode. The end of line character determines the end of an input record. The end of file character simulates an end of file occurrence on terminal input. 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.

Input characters may be echoed to the terminal as they are received. Non-graphic ASCII input characters may be echoed as a two-character printable representation, ``^character.'' 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, expanding tabs, and providing translations for upper-case only terminals. Terminal control operations

When a terminal is first opened it is initialized to a standard state and configured with a set of standard control, editing, and interrupt characters. A process may alter this configuration with certain control operations, specifying parameters in a standard structure:\(dg .FS \(dg The control interface described here is an internal interface only in 4.3BSD. Future releases will probably use a modified interface based on currently-proposed standards. .FE ._f struct ttymode { short tt_ispeed; /* input speed */ int tt_iflags; /* input flags */ short tt_ospeed; /* output speed */ int tt_oflags; /* output flags */ }; and ``special characters'' are specified with the ttychars structure, ._f struct ttychars { char tc_erasec; /* erase char */ char tc_killc; /* erase line */ char tc_intrc; /* interrupt */ char tc_quitc; /* quit */ char tc_startc; /* start output */ char tc_stopc; /* stop output */ char tc_eofc; /* end-of-file */ char tc_brkc; /* input delimiter (like nl) */ char tc_suspc; /* stop process signal */ char tc_dsuspc; /* delayed stop process signal */ char tc_rprntc; /* reprint line */ char tc_flushc; /* flush output (toggles) */ char tc_werasc; /* word erase */ char tc_lnextc; /* literal next character */ }; Terminal hardware support

The terminal handler allows a user to access basic hardware related functions; e.g. line speed, modem control, parity, and stop bits. A special signal, SIGHUP, is automatically sent to processes in a terminal's process group when a carrier transition is detected. This is normally associated with a user hanging up on a modem controlled terminal line. Structured devices

Structures 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. File systems are normally created in block devices. 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.