Section 18.2. Overview

18.2. Overview

Terminal I/O has two modes:

1. Canonical mode input processing. In this mode, terminal input is processed as lines. The terminal driver returns at most one line per read request.

2. Noncanonical mode input processing. The input characters are not assembled into lines.

If we don't do anything special, canonical mode is the default. For example, if the shell redirects standard input to the terminal and we use read and write to copy standard input to standard output, the terminal is in canonical mode, and each read returns at most one line. Programs that manipulate the entire screen, such as the vi editor, use noncanonical mode, since the commands may be single characters and are not terminated by newlines. Also, this editor doesn't want processing by the system of the special characters, since they may overlap with the editor commands. For example, the Control-D character is often the end-of-file character for the terminal, but it's also a vi command to scroll down one-half screen.

The Version 7 and older BSD-style terminal drivers supported three modes for terminal input: (a) cooked mode (the input is collected into lines, and the special characters are processed), (b) raw mode (the input is not assembled into lines, and there is no processing of special characters), and (c) cbreak mode (the input is not assembled into lines, but some of the special characters are processed). Figure 18.20 shows a POSIX.1 function that places a terminal in cbreak or raw mode.

POSIX.1 defines 11 special input characters, 9 of which we can change. We've been using some of these throughout the text: the end-of-file character (usually Control-D) and the suspend character (usually Control-Z), for example. Section 18.3 describes each of these characters.

We can think of a terminal device as being controlled by a terminal driver, usually within the kernel. Each terminal device has an input queue and an output queue, shown in Figure 18.1.

Figure 18.1. Logical picture of input and output queues for a terminal device

There are several points to consider from this picture.

• If echoing is enabled, there is an implied link between the input queue and the output queue.

• The size of the input queue, MAX_INPUT (see Figure 2.11), is finite. When the input queue for a particular device fills, the system behavior is implementation dependent. Most UNIX systems echo the bell character when this happens.

• There is another input limit, MAX_CANON, that we don't show here. This limit is the maximum number of bytes in a canonical input line.

• Although the size of the output queue is finite, no constants defining that size are accessible to the program, because when the output queue starts to fill up, the kernel simply puts the writing process to sleep until room is available.

• We'll see how the tcflush flush function allows us to flush either the input queue or the output queue. Similarly, when we describe the tcsetattr function, we'll see how we can tell the system to change the attributes of a terminal device only after the output queue is empty. (We want to do this, for example, if we're changing the output attributes.) We can also tell the system to discard everything in the input queue when changing the terminal attributes. (We want to do this if we're changing the input attributes or changing between canonical and noncanonical modes, so that previously entered characters aren't interpreted in the wrong mode.)

Most UNIX systems implement all the canonical processing in a module called the terminal line discipline. We can think of this module as a box that sits between the kernel's generic read and write functions and the actual device driver (see Figure 18.2).

Figure 18.2. Terminal line discipline

Note the similarity of this picture and the diagram of a stream shown in Figure 14.14. We'll return to this picture in Chapter 19, when we discuss pseudo terminals.

All the terminal device characteristics that we can examine and change are contained in a termios structure. This structure is defined in the header <termios.h>, which we use throughout this chapter:

    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 characters */    }; 

Roughly speaking, the input flags control the input of characters by the terminal device driver (strip eighth bit on input, enable input parity checking, etc.), the output flags control the driver output (perform output processing, map newline to CR/LF, etc.), the control flags affect the RS-232 serial lines (ignore modem status lines, one or two stop bits per character, etc.), and the local flags affect the interface between the driver and the user (echo on or off, visually erase characters, enable terminal-generated signals, job control stop signal for background output, etc.).

The type tcflag_t is big enough to hold each of the flag values and is often defined as an unsigned int or an unsigned long. The c_cc array contains all the special characters that we can change. NCCS is the number of elements in this array and is typically between 15 and 20 (since most implementations of the UNIX System support more than the 11 POSIX-defined special characters). The cc_t type is large enough to hold each special character and is typically an unsigned char.

Versions of System V that predated the POSIX standard had a header named <termio.h> and a structure named termio. POSIX.1 added an s to the names, to differentiate them from their predecessors.

Figures 18.3 through 18.6 list all the terminal flags that we can change to affect the characteristics of a terminal device. Note that even though the Single UNIX Specification defines a common subset that all platforms start from, all the implementations have their own additions. Most of these additions come from the historical differences between the systems. We'll discuss each of these flag values in detail in Section 18.5.

Figure 18.3. c_cflag terminal flags

Flag	Description	POSIX.1	FreeBSD 5.2.1	Linux 2.4.22	Mac OS X 10.3	Solaris 9
CBAUDEXT	extended baud rate					•
CCAR_OFLOW	DCD flow control of output		•		•
CCTS_OFLOW	CTS flow control of output		•		•	•
CDSR_OFLOW	DSR flow control of output		•		•
CDTR_IFLOW	DTR flow control of input		•		•
CIBAUDEXT	extended input baud rate					•
CIGNORE	ignore control flags		•		•
CLOCAL	ignore modem status lines	•	•	•	•	•
CREAD	enable receiver	•	•	•	•	•
CRTSCTS	enable hardware flow control		•	•	•	•
CRTS_IFLOW	RTS flow control of input		•		•	•
CRTSXOFF	enable input hardware flow control					•
CSIZE	character size mask	•	•	•	•	•
CSTOPB	send two stop bits, else one	•	•	•	•	•
HUPCL	hang up on last close	•	•	•	•	•
MDMBUF	same as CCAR_OFLOW		•		•
PARENB	parity enable	•	•	•	•	•
PAREXT	mark or space parity					•
PARODD	odd parity, else even	•	•	•	•	•

Figure 18.4. c_iflag terminal flags

Flag	Description	POSIX.1	FreeBSD 5.2.1	Linux 2.4.22	Mac OS X 10.3	Solaris 9
BRKINT	generate SIGINT on BREAK	•	•	•	•	•
ICRNL	map CR to NL on input	•	•	•	•	•
IGNBRK	ignore BREAK condition	•	•	•	•	•
IGNCR	ignore CR	•	•	•	•	•
IGNPAR	ignore characters with parity errors	•	•	•	•	•
IMAXBEL	ring bell on input queue full		•	•	•	•
INLCR	map NL to CR on input	•	•	•	•	•
INPCK	enable input parity checking	•	•	•	•	•
ISTRIP	strip eighth bit off input characters	•	•	•	•	•
IUCLC	map uppercase to lowercase on input			•		•
IXANY	enable any characters to restart output	XSI	•	•	•	•
IXOFF	enable start/stop input flow control	•	•	•	•	•
IXON	enable start/stop output flow control	•	•	•	•	•
PARMRK	mark parity errors	•	•	•	•	•

Figure 18.5. c_lflag terminal flags

Flag	Description	POSIX.1	FreeBSD 5.2.1	Linux 2.4.22	Mac OS X 10.3	Solaris 9
ALTWERASE	use alternate WERASE algorithm		•		•
ECHO	enable echo	•	•	•	•	•
ECHOCTL	echo control chars as ^(Char)		•	•	•	•
ECHOE	visually erase chars	•	•	•	•	•
ECHOK	echo kill	•	•	•	•	•
ECHOKE	visual erase for kill		•	•	•	•
ECHONL	echo NL	•	•	•	•	•
ECHOPRT	visual erase mode for hard copy		•	•	•	•
EXTPROC	external character processing		•		•
FLUSHO	output being flushed		•	•	•	•
ICANON	canonical input	•	•	•	•	•
IEXTEN	enable extended input char processing	•	•	•	•	•
ISIG	enable terminal-generated signals	•	•	•	•	•
NOFLSH	disable flush after interrupt or quit	•	•	•	•	•
NOKERNINFO	no kernel output from STATUS		•		•
PENDIN	retype pending input		•	•	•	•
TOSTOP	send SIGTTOU for background output	•	•	•	•	•
XCASE	canonical upper/lower presentation			•		•

Figure 18.6. c_oflag terminal flags

Flag	Description	POSIX.1	FreeBSD 5.2.1	Linux 2.4.22	Mac OS X 10.3	Solaris 9
BSDLY	backspace delay mask	XSI		•		•
CMSPAR	mark or space parity			•
CRDLY	CR delay mask	XSI		•		•
FFDLY	form feed delay mask	XSI		•		•
NLDLY	NL delay mask	XSI		•		•
OCRNL	map CR to NL on output	XSI	•	•		•
OFDEL	fill is DEL, else NUL	XSI		•		•
OFILL	use fill character for delay	XSI		•		•
OLCUC	map lowercase to uppercase on output			•		•
ONLCR	map NL to CR-NL	XSI	•	•	•	•
ONLRET	NL performs CR function	XSI	•	•		•
ONOCR	no CR output at column 0	XSI	•	•		•
ONOEOT	discard EOTs (^D) on output		•		•
OPOST	perform output processing	•	•	•	•	•
OXTABS	expand tabs to spaces		•		•
TABDLY	horizontal tab delay mask	XSI		•		•
VtdLY	vertical tab delay mask	XSI		•		•

Given all the options available, how do we examine and change these characteristics of a terminal device? Figure 18.7 summarizes the various functions defined by the Single UNIX Specification that operate on terminal devices. (All the functions listed are part of the base POSIX specification, except for tcgetsid, which is an XSI extension. We described tcgetpgrp, tcgetsid, and tcsetpgrp in Section 9.7.)

Figure 18.7. Summary of terminal I/O functions

Function	Description
tcgetattr	fetch attributes (termios structure)
tcsetattr	set attributes (termios structure)
cfgetispeed	get input speed
cfgetospeed	get output speed
cfsetispeed	set input speed
cfsetospeed	set output speed
tcdrain	wait for all output to be transmitted
tcflow	suspend transmit or receive
tcflush	flush pending input and/or output
tcsendbreak	send BREAK character
tcgetpgrp	get foreground process group ID
tcsetpgrp	set foreground process group ID
tcgetsid	get process group ID of session leader for controlling TTY (XSI extension)

Note that the Single UNIX Specification doesn't use the classic ioctl on terminal devices. Instead, it uses the 13 functions shown in Figure 18.7. The reason is that the ioctl function for terminal devices uses a different data type for its final argument, which depends on the action being performed. This makes type checking of the arguments impossible.

Although only 13 functions operate on terminal devices, the first two functions in Figure 18.7 (tcgetattr and tcsetattr) manipulate almost 70 different flags (see Figures 18.3 through 18.6). The handling of terminal devices is complicated by the large number of options available for terminal devices and trying to determine which options are required for a particular device (be it a terminal, modem, printer, or whatever).

The relationships among the 13 functions shown in Figure 18.7 are shown in Figure 18.8.

Figure 18.8. Relationships among the terminal-related functions

POSIX.1 doesn't specify where in the termios structure the baud rate information is stored; that is an implementation detail. Some systems, such as Linux and Solaris, store this information in the c_cflag field. BSD-derived systems, such as FreeBSD and Mac OS X, have two separate fields in the structure: one for the input speed and one for the output speed.