Question

In the class we mentioned briefly the readers-writers problem with writers priority. The problem can be solved in guarded commands as follows: void writer) void reader when writers-0 readers++ when (readers-0)&&(active writers0) active_writers++ /Iread //write [readers--] (writers-active writers- Here writers represents the number of threads that are either writing or waiting to write. The variable active_writers represents the number of threads (0 or 1) that are currently writing.

Answer 1

A reader-writer problem problem occurs when a seeker wants to access a file which is still under modification or, when one modifier want to change the data while other modifier is still using that file. For solving that, we use semaphores. Here, as given by your code, we are using three semaphores:

1. readers: who want to seek the file

2. writers: who want to modify the file

3. active_writers: currently modifying the file.

So, in reader function as we can see, we are checking for only one condition i.e if there are no writers who are modifying the file. If so, we add all the readers. All readers are added because we will not face any problem when it is in read mode. No changes can happen to the file while reading. While reading we can create a mutex lock on this file. Through this lock we can lock the writers to block access to this file. Once reading is finished the lock can be released and file can be closed for reading as readers job is done. Hence, fall-off of the readers.

In writers block we use the remaining semaphores i.e. writers and active_writers. We will increment all the writers who want to use the file. But, only one active_writer is given a chance to use this file as multiple writers can modify the file abnormally. So the condition for writing will be if, no readers are available and if there are no active_writers. Only then we will allow a writer to enter the file and make modifications. In this process, we can keep the file in mutex lock until modifications are done so that readers can have no access to the files. Once all modifications are done by all writers the lock can be released. We even decrement the active_writers to give chance to other writers who want to modify the data in the file.