Advanced Programming in the UNIX® Environment

ptg10805159

700 Te rminal I/O Chapter 18

Running the program from Figure18.16 gives us

$./a.out < /dev/console 2> /dev/null

fd 0: /dev/console

fd 1: /dev/ttys001

fd 2: not a tty

18.10 Canonical Mode

Canonical mode is simple: we issue a read, and the terminal driver returns when a line

has been entered. Several conditions cause the read to return.

•The read returns when the requested number of bytes have been read. Wedon’t

have to read a complete line. If we read a partial line, no information is lost; the

next read starts wherethe previous read stopped.

•The read returns when a line delimiter is encountered. Recall from Section 18.3

that the following characters areinterpreted as end of line in canonical mode:

NL, EOL, EOL2, and EOF.Also, recall from Section 18.5 that ifICRNLis set and

ifIGNCRis not set, then the CR character also terminates a line, since it acts just

like the NL character.

Of these five line delimiters, one(EOF)is discarded by the terminal driver when

it’s processed. The other four arereturned to the caller as the last character of

the line.

•The read also returns if a signal is caught and if the function is not automatically

restarted (Section 10.5).

Example —getpassFunction

We now examine the functiongetpass,which reads a passwordofsome type from the

user at a terminal. This function is called by thelogin( 1 )andcrypt( 1 )programs. To

read the password, the function must turn offechoing, but it can leave the terminal in

canonical mode, as whatever we type as the passwordforms a complete line.

Figure18.17 shows a typical implementation on a UNIX system.

Thereare several points to consider in this example.

•Instead of hard-wiring /dev/tty into the program, we call the function

ctermidto open the controlling terminal.

•Weread and write only to the controlling terminal and return an error if we

can’t open this device for reading and writing. Thereare other conventions to

use. The version ofgetpassin the GNU C library reads from standardinput

and writes to standarderror if the controlling terminal can’t be opened for

reading and writing. The Solaris version fails if it can’t open the controlling

terminal.