A file is shared between several reader and writer threads. Design a monitor to

Question

A file is shared between several reader and writer threads. Design a monitor to control the access of the file by the different threads so that the following constraints are satisfied: (i) at most one writer can be active on the file at a particular time. (ii) When a writer is writing to the file, no reader can read from the file. (iii) More than one reader can be reading from the file simultaneously. (iv) When a writer is waiting to write, no more new reader should be allowed to read. (v) When a writer is writing and some other writer is waiting to write, then the writer is given more preference over a reader waiting to read. The general structure of each reader and writer thread is shown in the following: Monitor FileControl { // Definition of the monitor class to be filled in by you … … } FileControl fc; // An instance of the monitor Writer Thread: Reader Thread: while (True) { while (True) { ... ... fc.WriterEntry(); fc.ReaderEntry(); Write (file); Read (file); fc.WriterExit(); fc.ReaderExit(); … … } } Fill in the pseudo-code for the monitor FileControl as shown above.

Explanation / Answer

4.>semaphore mutex=1,wrtcount=0,rsem=1;

//writer's program

do{

P(mutex);

rdcount++;

if (wrtcount==1) P(rSem);

V(mutex);

READ

P(mutex);

rdcount--;

if (wrtcount==0) V(rSem);

V(mutex);

}while(true)

//reader's program

do{

p(rsem);

Write//Critical section

v(rsem)

3.>

5.>

//rest can be inferred using the above

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