Starve_free_readers_writers_problem

int readcount; ( // Number of readers accessing resources // initial value = 0 )

semaphores used

semaphore mutex, read, write; 
// initial values of all semaphores = 1

  wait(semaphore){
    if(semaphore <= 0){
    // do nothing
    }
    else{
    semaphore--;
    }
  signal(semaphore){
      semaphore++
    }

//READERS CODE

reader(){
wait(read);
  wait(mutex);
    readcount++;
    if (readcount == 1)
      wait(write);
  signal(mutex);          
signal(read);

// reading is performed 

wait(mutex);
  readcount--;
  if(readcount == 0)
    signal(write);
signal(mutex);
}

//WRITERS CODE

  writer(){
    wait(read);
    wait(write);

     // writing is performed

    signal(write);  
    signal(read);
    }

Solution

read,write semaphores ensures that only one of them can access resource and mutex ensures readcount value change correctly. Here any number of of readers can read at a time and only one writer can write at a time.It is given turns so there will be no starvation for either readers or writers.

References

• Operating System Concepts by Abraham Silberschatz, Peter B. Galvin, Greg Gagne -PSK sir's Slides

• Wikipedia