9.6. Controlling Terminal
Sessions and process groups have a few other characteristics.
A session can have a single controlling terminal. This is usually the terminal device (in the case of a terminal login) or pseudo-terminal device (in the case of a network login) on which we log in. The session leader that establishes the connection to the controlling terminal is called the controlling process. The process groups within a session can be divided into a single foreground process group and one or more background process groups. If a session has a controlling terminal, it has a single foreground process group, and all other process groups in the session are background process groups. Whenever we type the terminal's interrupt key (often DELETE or Control-C), this causes the interrupt signal be sent to all processes in the foreground process group. Whenever we type the terminal's quit key (often Control-backslash), this causes the quit signal to be sent to all processes in the foreground process group. If a modem (or network) disconnect is detected by the terminal interface, the hang-up signal is sent to the controlling process (the session leader).
These characteristics are shown in Figure 9.7.
Usually, we don't have to worry about the controlling terminal; it is established automatically when we log in.
POSIX.1 leaves the choice of the mechanism used to allocate a controlling terminal up to each individual implementation. We'll show the actual steps in Section 19.4.
Systems derived from UNIX System V allocate the controlling terminal for a session when the session leader opens the first terminal device that is not already associated with a session. This assumes that the call to open by the session leader does not specify the O_NOCTTY flag (Section 3.3).
BSD-based systems allocate the controlling terminal for a session when the session leader calls ioctl with a request argument of TIOCSCTTY (the third argument is a null pointer). The session cannot already have a controlling terminal for this call to succeed. (Normally, this call to ioctl follows a call to setsid, which guarantees that the process is a session leader without a controlling terminal.) The POSIX.1 O_NOCTTY flag to open is not used by BSD-based systems, except in compatibility-mode support for other systems.
There are times when a program wants to talk to the controlling terminal, regardless of whether the standard input or standard output is redirected. The way a program guarantees that it is talking to the controlling terminal is to open the 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, the open of this device will fail.
The classic example is the getpass(3) function, which reads a password (with terminal echoing turned off, of course). This function is called by the crypt(1) program and can be used in a pipeline. For example,
crypt < salaries | lpr
decrypts the file salaries and pipes the output to the print spooler. Because crypt reads its input file on its standard input, the standard input can't be used to enter the password. Also, crypt is 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).
There are known ways to break the encoding used by the crypt program. See Garfinkel et al. [2003] for more details on encrypting files.
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