프로세스 동기화의 고전 문제 2 철학가 식사 문제

#define N 5                /* number of philosophers */
 
void philosopher(int i)    /* i: philosopher number, from 0 to 4 */
{
    while (TRUE) {
        think( );              /* philosopher is thinking */
        take_fork(i);          /* take left fork */
        take_fork((i+1) % N);  /* take right fork; % is modulo operator */
        eat();                 /* yum-yum, spaghetti */
        put_fork(i);           /* Put left fork back on the table */
        put_fork((i+1) % N);   /* put right fork back on the table */
    }
}

#define N              5   /* number of philosophers */
#define LEFT           (i+N−1)%N /* number of i's left neighbor */
#define RIGHT          (i+1)%N /* number of i's right neighbor */
#define THINKING       0   /* philosopher is thinking */
#define HUNGRY         1   /* philosopher is trying to get forks */
#define EATING         2   /* philosopher is eating */
typedef int semaphore;     /* semaphores are a special kind of int */
int state[N];              /* array to keep track of everyone's state */
semaphore mutex = 1;       /* mutual exclusion for critical regions */
semaphore s[N];            /* one semaphore per philosopher */
 
void philosopher (int i)   /* i: philosopher number, from 0 to N−1 */
{
    while (TRUE) {         /* repeat forever */
        think();           /* philosopher is thinking */
        take_forks(i);     /* acquire two forks or block */
        eat();             /* yum-yum, spaghetti */
        put_forks(i);      /* put both forks back on table */
    }
}
 
void take_forks(int i)     /* i: philosopher number, from 0 to N−1 */
{
    down(&mutex);          /* enter critical region */
    state[i] = HUNGRY;     /* record fact that philosopher i is hungry */
    test(i);               /* try to acquire 2 forks */
    up(&mutex);            /* exit critical region */
    down(&s[i]);           /* block if forks were not acquired */
}
 
void put_forks(i)          /* i: philosopher number, from 0 to N−1 */
{
    down(&mutex);          /* enter critical region */
    state[i] = THINKING;   /* philosopher has finished eating */
    test(LEFT);            /* see if left neighbor can now eat */
    test(RIGHT);           /* see if right neighbor can now eat */
    up(&mutex);            /* exit critical region */
}
 
void test(i)               /* i: philosopher number, from 0 to N−1 */
{
  if (state[i] == HUNGRY && state[LEFT] != EATING && state[RIGHT] != EATING) {
    state[i] = EATING;
    up(&s[i]);
  }
}

Figure 2-33. A solution to the dining philosophers problem