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?

Consider three $$CPU$$-intensive process, which require $$10,20$$ and $$30$$ time units and arrive at times $$0,2$$ and $$6$$ respectively. How many context switches are needed if the operating system implements a shortest remaining time first scheduling algorithm? Do not count the context switches at time zero and at the end.

Consider three processes, all arriving at time zero, with total execution time of $$10,20,$$ and $$30$$ units, respectively. Each process spends the first $$20$$% of execution time doing $${\rm I}/O$$, the next $$70$$% of time doing computation, and the last $$10$$% of time doing $${\rm I}/O$$ again. The operating system uses a shortest remaining compute time first scheduling algorithm and scheduling a new process either when the running processes gets blocked on $${\rm I}/O$$ or when the running process finishes its compute burst. Assume that all $${\rm I}/O$$ operations can be overlapped as much as possible. For what percentage of time does the $$CPU$$ remain idle?

Consider this C code to swap two integers and these five statements:

void swap(int *px, int *py) 
{ 
    *px = *px - *py; 
    *py = *px + *py; 
    *px = *py - *px; 
}

S1: will generate a compilation error

S2: may generate a segmentation fault at runtime depending on the arguments passed

S3: correctly implements the swap procedure for all input pointers referring to integers stored in memory locations accessible to the process

S4: implements the swap procedure correctly for some but not all valid input pointers

S5: may add or subtract integers and pointers.