Question 3 -- 10 points

Behold the following monitor/condition variable calls:

Mon_t *mon_create() -- create and return a monitor.
void mon_enter(Mon_t *) -- enter a monitor.
void mon_exit(Mon_t *) -- exit a monitor.
Cv_t *cv_create() -- create and return a condition variable.
void cv_wait(Cv_t *, Mon_t *) -- wait on a condition variable.
void cv_signal(Cv_t *) -- signal on a condition variable.

Implement the following general semaphore calls (also provide a typedef for Gsem_t:

Gsem_t *gsem_create(int) -- create and a semaphore with a given value.
void gsem_P(Gsem_t *) -- Call P() on the semaphore.
void gsem_V(Gsem_t *) -- Call V() on the semaphore.

Don't worry about your #includes.

Answer

This is so straightforward it needs no explanation:

typedef struct {
  Mon_t *m;
  Cv_t *c;
  int val;
} Gsem_t;

Gsem_t *gsem_create(int i)
{
  Gsem_t *g;
  if (i < 0) return NULL;
  g = (Gsem_t *) malloc(sizeof(Gsem_t));

  g->m = mon_create();
  g->c = cv_create();
  g->val = i;
  return g;
}

void gsem_P(Gsem_t *g)
{
  mon_enter(g->m);
  g->val--;
  if (g->val < 0) cv_wait(g->c, g->m);
  mon_exit(g->m);
}


void gsem_V(Gsem_t *g)
{
  mon_enter(g->m);
  g->val++;
  if (g->val <= 0) cv_signal(g->c);
  mon_exit(g->m);
}

Grading

This was a very simple problem. Basically, you started at 10 points, and lost points for the following:

A: You omitted monitors entirely. 5 points.
B: You didn't use the monitors correctly. 3 points.
C: You use a while instead of an if in gsem_P(): 2 points.
D: You waited in gsem_V(): 2 points.
E: You decrement after calling wait() in gsem_P() (and it's protected by g->val < 0): 1 point.
F: You mixed up your < and <='s: 1 point.

If your answer didn't really make any sense, but you had the struct set up right, I gave you two points.

Question 3 -- 10 points

Answer

Grading

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