Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left.

void barrier (void) { 
1: P(S); 
2: process_arrived++; 
3: V(S); 
4: while (process_arrived !=3); 
5: P(S); 
6: process_left++; 
7: if (process_left==3) { 
8: process_arrived = 0; 
9: process_left = 0; 
10: } 
11: V(S); 
} 

The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally.
Which one of the following rectifies the problem in the implementation?

The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x n y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore S.

void P (binary_semaphore *s) { 

   unsigned y; 
   unsigned *x = &(s->value); 
   
   do { 

     fetch-and-set x, y; 

   } while (y); 
} 

void V (binary_semaphore *s) { 

   S->value = 0; 

} 

Which one of the following is true?

Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:

while(1){
  W:
  Print '0';
  Print '0';
  X:
}

Process Q:

while(1){
  Y:
  Print '1';
  Print '1';
  Z:
}

Synchronization statements can be inserted only at points W, X, Y, and Z.

Which of the following will always lead to an output starting with 001100110011

Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:

while(1){
  W:
  Print '0';
  Print '0';
  X:
}

Process Q:

while(1){
  Y:
  Print '1';
  Print '1';
  Z:
}

Synchronization statements can be inserted only at points W, X, Y, and Z.

Which of the following will ensure that the output string never contains a substring of the form 01ⁿ0 or 10ⁿ1 where n is odd?