Question

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A classical concurrent programming problem is called readers and writers. A data area is shared between several processes. Some processes, the writers, write to the area, the others, the readers, only read the data. At any given time only one reader can read the data or writer can write to the data. Also, no readers are allowed while a writer is executing.

Problem Statement: Devise suitable semaphores for this situation and design an outline of the reader and writer processes.

Answer 1

READER WRITER PROBLEM

In classical concurrent programming problem of reading and writing there are many processes in the system and a data area is shared among several processes.

The reader can only read the data and writer can write the data only one reader at a time can read the data or writer can write the data.If the two readers try accessing the same data areas or or two writer try to access the same data area then problems arises.the conditions are-

1) The writer should have exclusive control over critical section when writing so that no other reader or writer can perform any operation at the same time

2)The writer can access to the shared data area when no reader is waiting

3)Reader can access to the shared data area when no writer waiting

Two semaphore are used in this reading writing problems which are W and mutex and one variable read count is used

1) Semaphore W- This semaphore W is used by both reader and writer for entering in critical section and uses the wait and signal operations.

Critical section is data area where only one process can either perform read or write operation but only one process can gain entry into critical section at one time and one operation can be performed at one time.

2) Variable Readcount counts the number of processes reading in the critical section.

3) Mutex semaphore provides mutual exclusion when read count variable is updated means process first has to lock this mutex semaphore before changing readcount value.This semaphore is only used by reader.

There are two functions with semaphore

Wait() decrement semaphore value

Signal() increment semaphore value

semaphore can have either two value "0 or "1"

WRITER PROCESS

do

{

wait(W); //Writer request to enter critical section

//writing operation

Signal (W) //writer wants to leave critical section

} while(true);

1)Writer first request entry in critical section by wait(W) if W value less than 1 means value is 0 then can't perform write operationand if W value =1 then writer allowed to enter critical section and perform writing operation and if not allowed then it keep on waiting.

2) The writer after performing write operation gives signal(W) that write operation is done and other writer can either perform write or other reader can perform read operation.

INITIALIZATION

Mutex mutex;

Semaphore W=1;

Int readcount =1;

READER

do

{

wait(mutex) // reader want to enter critical section

Readcount ++;

If(readcount ==1)

{

Wait (W) ; // block write operation

}

Signal(mutex) // other reader can enter now in critical section

reading is performed

Wait(mutex) //reader want to leave critical section

Readcount --;

If(readcount==0)

{

Signal(W) // writer can now enters critical section

}

Signal(mutex)

}while(true); // reader leaves critical section.

1)The reader request entry in critical section by wait(mutex) is used in this statement this wait(mutex) is used for updating the value of read count that is reader uses mutual exclusion mutex for variables read count as read count variable can be updated by only one process at a time so after achieving the mutual exclusion by mutex it increment the read count variable readcount++ and make other reader to wait indicating  that read is to be performed.

2) If(read count =1)

This means reader is first reader so it makes the writer to wait by Wait(W) as in initialization value of W=1 but now the wait(W) decrement the value to 0 and 0 value means writer has to wait so that no other writer can write while first reader is reading  Means writer should wait so first reader blocks the writer.

3) Signal (Mutex) is used to signal  the other readers that one reader is now entering in critical section so other reader can now enter in critical section for their turn as signal(mutex) is giving signal to next readers so that next reader can update read count value.similarly all other reader will perform read operation.

4)Wait (mutex) indicates that now reader performed reading and want to leave the critical section so it lock again mutex for decrementing the read count value as wait(mutex) means it is telling other reader to wait so that other reader cannot change read count value while one reader is decrementing read count value.

5) After the reader perform reading then readcount will decrement as soon as all reader has performed reading then this readcount becomes 0 so it check for read count =0 if yes then it will give signal to writer to write  signal(W) so now writer can write.

6) Now signal(Mutex) means that reader has left the critical section and not need to control the variable read count so it is signalling the other process to continue and can change this read count variable.