Advanced Programming in the UNIX® Environment

ptg10805159

672 Te rminal I/O Chapter 18

If we don’t do anything special, canonical mode is the default. For example, if the shell

redirects standardinput to the terminal and we usereadandwriteto copy standard

input to standardoutput, the terminal is in canonical mode, and eachreadreturns at

most one line. Programs that manipulate the entirescreen, such as thevieditor,use

noncanonical mode, since the commands may be single characters and arenot

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-Dcharacter is often the end-of-file character for the terminal, but it’s also avi

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 areprocessed), (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 areprocessed). Figure18.20 shows a POSIX.1 function that places a terminal in

cbreak or raw mode.

POSIX.1 defines 11special 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 Figure18.1.

output queue if echo enabled input queue

next character to

transmit to device

next character

written by process

next character

read from device

next character

read by process

MAX_INPUT

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

Thereare several points to consider from this picture.

•Ifechoing is enabled, there is an implied link between the input queue and the

output queue.

•The size of the input queue,MAX_INPUT(see Figure2.12), 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.