Explain with an example the need of Shift Alteration in critical section problem?

Consider processes P_i and P_j and consider the algorithm for P_i and P_j.

                           For P_i

                           do

   {

    while(turn!=i)

    critical section

    turn=j

    remainder section

   }whie(1);

For P_i

                           do

   {

    while(turn!=j)

    critical section

    turn=i

    remainder section

   }whie(1);

In the above code variable 'turn' is shared among the two processes. If turn=i after that process P_i is allowed to get into the critical section. This make sure that only one process at a time can be in its critical section. It doesn't satisfy the progress requirement and as a result we can do strict alteration with this code. That is if turn=0 and P₁ is ready to get into the critical section, P₁ can't enter into the critical section even though P₀ may be in its remainder section. therefore a new algorithm which stores the state of the process is implemented. The code is:

do

{

  flag[i]=true

  while(flag[j])

  critical section

  flag[i]=false

  remainder section

}whiel(1)

   Here P_i is setting its flag as true. Then it confirms whether P_j is present in the critical section. If so after that it waits until the process P_j comes out of the critical section. If state of P_j is set as false after that P_i directly enters into the critical section.