CSCI 3500: Studio 8

Signals

Signals are a mechanism for basic inter-process notification and event handling. Signals do not convey any data in themselves, but they do allow cooperating processes to invoke remote functions in other processes. In addition, signals are the mechanism central to handling certain basic functions you expect from an operating system, such as the ability to interrupt, suspend, or otherwise modify a currently running program.

In this studio, you will:

1. Send signals with keyboard shortcuts
2. Customize signal handling behavior by registering signal handlers

Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.

As you work through these exercises, please record your answers in a text file. When finished, submit your work by sending your text file and source code to dferry@slu.edu with the phrase Signal Handling in the subject line.

Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.

Required Exercises

1. As the answer to the first exercise, list the names of the people who worked together on this studio.

2. Download the program sleep.c. This program runs an infinte loop in a way that doesn't burn a lot of CPU cycles. Compile and run this program to see what it does. Use CTRL-C to stop the program.

   Copy and paste some program output as the answer to this exercise.

3. When you press CTRL-C on the keyboard you're actually generating a special signal called the interrupt signal, or SIGINT for short. The default behavior of this signal is to kill the recieving process, and since most programs don't handle this signal with customized behavior the result is normally that the process is killed.

   Open the manual page at man 7 signal. Scroll down to the list of standard signals. What numeric value does SIGINT have? What is the associated comment?

4. Open a second terminal. Start the sleep.c program in the first terminal and make a note of its PID. In the second terminal use the kill command documented at man 1 kill to send the SIGINT signal to the sleeping process. (Note- despite the name, the kill command can send any signal to a process, not just the kill signal!. The syntax for doing this is below:

   kill -s <signal number> <pid>

   What happens to the sleeper process?

5. Go back to man 7 signal and look through the list of standard signals. Pick another signal to send to a sleeping process. What signal did you pick? What happened? Copy and paste the results.

6. Now download the program call_sleeper.c. This program fork()s and exec()s a sleeper program, and then waits for it to finish. After the child returns it will print out a success message.

   Try running call_sleeper.c and then terminating the sleeper with CTRL-C. Which processes terminate: the sleeper, the caller, or both? Did the calling program print its success message?

7. We want to be able to interrupt the sleeper process without interrupting the caller process. To do so, we can define a custom signal handler that intercepts the SIGINT and ignores it. To do so, we can use the signal() system call that is documented at man 2 signal.

   Note: This is an newer function called sigaction() that is the newer, better way to do this. However, the interface is much more complicated (but flexible!) so we will use this function instead.

   First you will need to write the signal handler function. This function should have the signature:

   void <function_name> ( int signum )

   In the body of that function, print a message stating that your program is ignoring SIGINT. Then, in the main() function, make a call to signal(). The first parameter should be the signal number for SIGINT and the second should be the name of your signal handler function.

   Copy and paste your signal handling code.

8. Finally, run your new caller program and interrupt it with CTRL-C. If done correctly, your sleeper should terminate but your caller should simply print your statement that SIGINT is being ignored. Then, the caller will print a statement indicating that it successfully waited for the sleeper.

   Copy and paste your program output.

   NOTE: Once you handle SIGINT in this way, you will no longer be able to kill your process with CTRL-C. If you run into trouble you can still terminate your program with CTRL-\, which sends the SIGQUIT signal.

9. Signals are frequently used in computer systems to do asynchronous event handling- dealing with events that can't be predicted accurately. One example might be clicking the red X box on a GUI window to close a program. In this case the OS might send a signal to the GUI process to tell it to terminate. Give another situation where signal handling might be useful.

Optional Enrichment Exercises

1. One frequently used program that handles SIGINT with a custom handler is the terminal shell. Try pressing CTRL-C at an empty terminal- what happens? Why is this behavior important?

2. Linux provides two user-defined signals SIGUSR1 and SIGUSR2 specifically for implementing program functionality on top of signals. Try using the system call documented at man 2 kill to implement asynchronous program communication. For example, have one process wait in a loop until it recieves SIGUSR1, and then perform some useful work.