The Linux Programming Interface

1300 Chapter 62

information. (Linux provides a vaguely similar feature in the form of the magic SysRq

key; see the kernel source file Documentation/sysrq.txt for details.)

System V derivatives provide the SWTCH character. This character is used to

switch shells under shell layers, a System V predecessor to job control.

Example program

Listing 62-1 shows the use of tcgetattr() and tcsetattr() to change the terminal

interrupt character. This program sets the interrupt character to be the character

whose numeric value is supplied as the program’s command-line argument, or dis-

ables the interrupt character if no command-line argument is supplied.

The following shell session demonstrates the use of this program. We begin by

setting the interrupt character to Control-L (ASCII 12), and then verify the change

with stty:

$ ./new_intr 12

$ stty

speed 38400 baud; line = 0;

intr = ^L;

We then start a process running sleep(1). We find that typing Control-C no longer

has its usual effect of terminating a process, but typing Control-L does terminate the

process.

$ sleep 10

^C Control-C has no effect; it is just echoed

Type Control-L to terminate sleep

We now display the value of the shell $? variable, which shows the termination status of

the last command:

$ echo $?

130

We see that the termination status of the process was 130. This indicates that the

process was killed by signal 130 – 128 = 2; signal number 2 is SIGINT.

Next, we use our program to disable the interrupt character.

$ ./new_intr

$ stty Verify the change

speed 38400 baud; line = 0;

intr = <undef>;

Now we find that neither Control-C nor Control-L generates a SIGINT signal, and we

must instead use Control-\ to terminate a program:

$ sleep 10

^C^L Control-C and Control-L are simply echoed

Type Control-\ to generate SIGQUIT

Quit

$ stty sane Return terminal to a sane state