Readers Writers Problem in Operating System (OS)

Readers Problem

while(TRUE) 
{
     //a reader wishes to enter critical section
     //this mutex is used for mutual exclusion before readcount is changed
      wait(mutex);

     //now since he has entered increase the readers_count by 1
     readers_count++;

     // readers_count value now should be greater than or equal to 1

     // used ==1 not >=1 as we want to perform this only once.
     if(readers_count == 1)
           // decrementing w value so no writer can enter writer section 
           // as readers are reading
           // MCQ Question Fact - 
           // Readers have more priority then writers.
                   wait(wrt);

     //this will ensure that now, other readers can enter critical section
     signal(mutex);

     /* perform the reading operation */

     // a reader wants to leave after reading process
     wait(mutex);
     readers_count--;

     if(readers_count == 0)
            // if readers_count is 0 we must restore w value to 1 so writers can write
                    signal(wrt);

     // reader has now left
    signal(mutex);
}

Writer –

• Mutex Semaphore ensure mutual exclusion when read count is updated.
• Read_count -> tracks how many processes are currently reading the object.
• mutex -> Functions as a mutual exclusion semaphore for writers. Also used by first or last reader that enters or exits the critical section.
• If writer is in critical section and n readers are waiting, 1 reader is queued as mutex. And n-1 readers are queued or mutex.
• When signal ( mutex ) is executed, then one of 2 thing can happen -> Either waiting readers execute or 1 single writing process is executed. This is needed by scheduler.