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. Semaphore; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code34.c code34.c
   • A common strategy to avoid race conditions is to use semaphores.
   • The use of semaphores is important to prevent simultaneous access to system resources by separate processes or separate threads inside the same process.
   • Three system calls to create, use, and release semaphores:
     • semget - Returns an integer semaphore index that is assigned by the kernel
     • semop - Performs operations on the semaphore set
     • semctl - Performs control operations on the semaphore set
   • The program shows how to create a semaphore set and how to access the elements of that set. Does the followings:
     • Creates a unique key and creates a semaphore
     • Checks to make sure that the semaphore is created OK
     • Prints out the value of the semaphore at index 0 (should be 1)
     • Sets the semaphore (decrements the value of semaphore at index 0 to 0)
     • Prints out the value of the semaphore at index 0 (should be 0)
     • Unsets the semaphore (increments the value of semaphore at index 0 back to 1)
     • Prints out the value of the semaphore at index 0 (should be 1)
     • Removes the semaphore
   • Study the code.
   • Execute several times and observe that how the output changes.
   • Is there any possible race conditions? Explain.
4. Mutex; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code32.c code32.c
   • Several threads and shared data.
   • Mutex mechanism (pthread_mutex_lock) is used for concurrent executing.
   • Execute code several times and observe that how the execution order of the threads changes.