Examples:

• Compile and run the code.
• Analyze the code and output.
• You should compile with -lpthread whenever necessary.

1. Signal; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code28.c code28.c
   • Signals are mechanisms for communicating with and manipulating processes.
   • A signal is a special message sent to a process. Signals are asynchronous; when a process receives a signal, it processes the signal immediately, without finishing the current function or even the current line of code.
   • Each signal type is specified by its signal number, but in programs, you usually refer to a signal by its name.
   • How to terminate the program? Break with Ctrl+Z, you will get

     [1]+ Stopped code28
     

     then kill the stopped process with

     kill %1 
     

2. Signal Handling; - http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code29.c code29.c
   • Even assigning a value to a global variable can be dangerous because the assignment may actually be carried out in two or more machine instructions, and a second signal may occur between them, leaving the variable in a corrupted state.
   • If you use a global variable to flag a signal from a signal-handler function, it should be of the special type sig_atomic_t.
   • Assignments to variables of this type are performed in a single instruction and therefore cannot be interrupted midway.
   • This program uses a signal-handler function to count the number of times that the program receives SIGUSR1, one of the signals reserved for application use.
3. Exercise: Write a C program which controls the POSIX interrupt signal (SIGINT).
   • Your program should continuously increase an integer value.
   • With each interrupt request (CTRL-C), your program should;
     • Display the current value of counter.
     • Toggle whether the counter should continue increasing, or stop.