Advanced Programming in the UNIX® Environment

ptg10805159

298 Process Relationships Chapter 9

FreeBSD Linux Mac OS X Solaris

Method 8.0 3.2.0 10.6.8 10

openwithoutO_NOCTTY •••

TIOCSCTTY ioctlcommand • •••

Figure 9.8 How various implementations allocate controlling terminals

Thereare times when a program wants to talk to the controlling terminal, regardless

of whether the standardinput or standardoutput is redirected. The way a program

guarantees that it is talking to the controlling terminal is toopenthe file/dev/tty.

This special file is a synonym within the kernel for the controlling terminal. Naturally,

if the program doesn’t have a controlling terminal, theopenof this device will fail.

The classic example is thegetpass( 3 )function, which reads a password(with

terminal echoing turned off, of course). This function is called by thecrypt( 1 )program

and can be used in a pipeline. For example,

crypt < salaries | lpr

decrypts the filesalariesand pipes the output to the print spooler.Becausecrypt

reads its input file on its standardinput, the standardinput can’t be used to enter the

password. Also,cryptis designed so that we have to enter the encryption password

each time we run the program, to prevent us from saving the password in a file (which

could be a security hole).

Thereare known ways to break the encoding used by thecryptprogram. See

Garfinkel et al.[ 2003 ]for moredetails on encrypting files.

9.7 tcgetpgrp, tcsetpgrp,and tcgetsid Functions

We need a way to tell the kernel which process group is the foreground process group,

so that the terminal device driver knows where to send the terminal input and the

terminal-generated signals (Figure9.7).

#include <unistd.h>

pid_t tcgetpgrp(intfd);

Returns: process group ID of foreground process group if OK,−1 on error

int tcsetpgrp(intfd,pid_tpgrpid);

Returns: 0 if OK,−1 on error

The functiontcgetpgrpreturns the process group ID of the foreground process group

associated with the terminal open onfd.

If the process has a controlling terminal, the process can calltcsetpgrpto set the

foreground process group ID topgrpid.The value ofpgrpidmust be the process group

ID of a process group in the same session, andfdmust refer to the controlling terminal

of the session.