Question

pleasw answer as soon as possible . this is for my exam practice . please read the code and answer the question

In a system, there are multiple reader processes which read from a shared file and multiple writer processes which update a shared file. The following variables are shared among all processes: int readCounter; semaphore mutex, writeLock; Reader and writer processes are given in the following C++-like pseudo programs: Reader Process // Non-Critical Section P(mutex); Writer Process // Non-Critical Section P(writeLock); // - Start of Critical Section Write to Shared Data Base 1/- End of Critical Section - V(writeLock); readCounter++; if (readCounter P(writeLock); 1) V(mutex); 1/- Start of Critical Section - Read from Shared Data Base //-End of Critical Section - P(mutex); readCounter--; if (readCounter=0) V(writeLock); V(mutex);

Answer 1

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6. Initial values of the variables

• readCount is an integer variable used for couting the number of reader processes and
• mutex and writeBlock are binary semaphore variables which are initialized with 1
• readCount will increment as the reader processes are entering into the code
• mutex and writeBlock are used for atomic execution of the Critical Section and readCount update

7. When the writer process is executing the CS, the first reader process will do P(mutex) so that all the other reader processes will be waiting here since no more P(mutex) is possible. First reader will be updating the readCount and will be waiting on P(writeBlock) which is already decremented by the writer process. Once the writer process exit from the CS and do V(writeBlock), the first reader can enter into the CS. Before enter into the CS, first reader will do V(mutex) so that the other waiting reader can enter into the CS one by one.

Reader Process // Non-Critical Section P(mutex); readers will wait here readCounter++; if (readCounter 1) P(writeLock); First reader will wait Nere V(mutex); // - Start of Critical Section - Read from Shared Data Base Il-End of Critical Section - P(mutex); readCounter- if (readCounter ==0) V(writeLock); V(mutex);

8. When the first reader enter into the program, it set the readCounter as 1 and do P(writeBlock) so that writer cannot enter into the Writer Process. But other readers can enter into CS of Readers Process one by one using P(mutex) and V(mutex). When the last reader is exiting from the code, it do V(writeBlock) so that the writer process can enter into the CS of Writer Process.

9. Suppose a situation where the readers processes are continuing in the CS of the Reader Process, either the existing reader is not leaving the CS or new readers are entering into the CS. In such case, the writer process will never enter into the CS of the Writer Process since the value of writeBlock is 0.

10. Using another binary semaphore variable XYZ, which stops new readers, once a writer has come and waiting to enter into the CS of the writer process. So once all the reader processes exits which are inside the CS, the writer process is allowed inside the CS and hence no more starvation for the writer process.

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