Semaphores and Mutual Exclusion

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.

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.