Advanced Programming in the UNIX® Environment

ptg10805159

704 Te rminal I/O Chapter 18

Case C: MIN == 0, TIME > 0

TIME specifies a read timer that is started whenreadis called. (Comparethis

to case A, in which a nonzeroTIME represented an interbyte timer that was not

started until the first byte was received.) Thereadreturns when a single byte

is received or when the timer expires. If the timer expires,readreturns 0.

Case D: MIN == 0, TIME == 0

If some data is available,readreturns up to the number of bytes requested. If

no data is available,readreturns 0 immediately.

Realize in all these cases that MIN is only a minimum. If the program requests more

than MIN bytes of data, it’s possible to receive up to the requested amount. This also

applies to cases C and D, in which MIN is 0.

Figure18.19 summarizes the four cases for noncanonical input. In this figure,nbytes

is the thirdargument toread(the maximum number of bytes to return).

MIN > 0 MIN == 0

TIME > 0

A:readreturns[MIN,nbytes]

beforetimer expires;

readreturns [1, MIN)

if timer expires.

(TIME = interbyte timer.

Caller can block indefinitely.)

C:readreturns[1,nbytes]

beforetimer expires;

readreturns 0

if timer expires.

(TIME =readtimer.)

TIME == 0

B:readreturns[MIN,nbytes]

when available.

(Caller can block indefinitely.)

D:readreturns[0,nbytes]

immediately.

Figure 18.19 Four cases for noncanonical input

Be awarethat POSIX.1 allows the subscriptsVMINandVTIMEto have the same values asVEOF

andVEOL,respectively.Indeed, Solaris does this for backwardcompatibility with older

versions of System V.This creates a portability problem, however.Ingoing from

noncanonical to canonical mode, we must now restoreVEOFandVEOLas well. IfVMINequals

VEOFand we don’t restoretheir values, when we setVMINto its typical value of 1, the

end-of-file character becomes Control-A. The easiest way around this problem is to save the

entiretermiosstructurewhen going into noncanonical mode and restore it when going back

to canonical mode.

Example

The program in Figure18.20 defines thetty_cbreakandtty_rawfunctions that set

the terminal incbreak modeandraw mode.(The termscbreakandrawcome from the

Version 7 terminal driver.) Wecan reset the terminal to its original state (the state before

either of these functions was called) by calling the functiontty_reset.

If we’ve calledtty_cbreak, we need to calltty_resetbeforecallingtty_raw.

The same goes for callingtty_cbreakafter callingtty_raw.This improves the

chances that the terminal will be left in a usable state if we encounter any errors.