Solutions

The system is in a safe state only if:
W = 0 and R >= 0, or W <= 1 and R = 0

This mutual exclusion problem can be solved by the use of blocking semaphores. The following files contain an example of solution using blocking semaphores.

• SemaphoreFile.java, implementation of class SemaphoreFile defining the semaphores for mutual exclusion
• Reader.java, implementation of class Reader simulating the behavior of a reader.
• Writer.java, implementation of class Writer simulating the behavior of a writer.
• ReadersWriters.java, implementation of class ReadersWriters used to launch the system.

The use of this solution can lead to the fact that writers will wait forever for the access if the number of readers is too high. In order to eliminate this problem, the addition of a variable writersWaiting can allow the system to know if writers are waiting for the access, and thus, not allow new readers to get access. Thus, the number of readers will decrease until a writer will be able to get the access. File below implements this concept:

• SemaphoreFile.java, implementation of class SemaphoreFile defining the semaphores for mutual exclusion