Homework 3 Solution - CS423UG Fall 2005

1.

int boatLocation=0;
int boatCount=0;
ConditionVariable boatarrived, boatfull;

monitor Boat{

ArriveAtBoat(int location){
       while(true){
               if(boatlocation==location && boatcount < 3){
                       boatcount++;
                       if(boatcount < 3)
                               boatfull.wait();
                       else boatfull.broadcast();
                       return;
               }
               else boatarrived.wait();
       }

}

GetOffOfBoat(int location){
       boatcount--;
       if(boatcount==0){
               boatlocation=1-location;
               boatarrived.broadcast();
       }
}

2.

extern int N;
int turn=0;
semaphore mutex=1;
static semaphore everyonereached[2]={0,0};
static int count[2]={0,0};

void barrier(int processid){
       down(mutex);
       barrierturn(processed, turn);
}

void barrierturn(int processed, int thisturn){

       count[thisturn]++;
       if(count[thisturn]!=N){
               up(mutex);
               down(everyonereached[thisturn] );
       }
       else{
               count[thisturn]=0;
               for(i=1; i<=N-1; i++)
                       up(everyonereached[thisturn]);
               turn=1-turn; // global turn updated here: next barrier
will
                                       // use new turn value
               up(mutex);
}

3.

No matter what x is, A's needs for resource type 5 can never be satisfied
because there aren't enough instances of that resource type. The system is
unsafe.

4.

Order all accounts by their (unique) id (perhaps id=account number). Each
process locks accounts only in increasing order of the account id's. This
can never result in a deadlock since the Circular Wait condition will never
be satisfied.