Advanced Programming in the UNIX® Environment

ptg10805159

Section 18.2 Overview 673

•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

areaccessible 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 thetcflushflush 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’rechanging the output attributes.)We can also tell the system to

discardeverything in the input queue when changing the terminal attributes.

(Wewant to do this if we’rechanging 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.Wecan think of this module as a box that sits between the

kernel’s generic read and write functions and the actual device driver (see Figure18.2).

read and write

functions

terminal

line discipline

terminal

device driver

user process

actual device

kernel

Figure 18.2 Te rminal line discipline

By isolating the canonical processing in a separate module, all terminal drivers can

support canonical processing consistently.We’ll return to this picturewhen we discuss

pseudo terminals in Chapter 19.