3.5.2 An Alternative System of Tokens

The following listing shows an alternative implementation of the system of tokens. This implementation still uses four tokens, so no more than four diners attempt to eat at the same time. However, this implementation uses the sem_wait() and sem_post() semaphore routines to limit the number of eating philosophers. This version of the source file is called din_philo_fix2.c.

You must compiler din_philo_fix2.c with -lrt to link with the appropriate semaphore routines.

The following listing details din_philo_fix2.c:

    1	#include <pthread.h>
     2	#include <stdio.h>
     3	#include <unistd.h>
     4	#include <stdlib.h>
     5	#include <errno.h>
     6	#include <assert.h>
     7	#include <semaphore.h>
     8	
     9	#define PHILOS 5
    10	#define DELAY 5000
    11	#define FOOD 50
    12	
    13	void *philosopher (void *id);
    14	void grab_chopstick (int,
    15	                     int,
    16	                     char *);
    17	void down_chopsticks (int,
    18	                      int);
    19	int food_on_table ();
    20	int get_token ();
    21	void return_token ();
    22	
    23	pthread_mutex_t chopstick[PHILOS];
    24	pthread_t philo[PHILOS];
    25	pthread_mutex_t food_lock;
    26	int sleep_seconds = 0;
    27	sem_t num_can_eat_sem;
    28	
    29	
    30	int
    31	main (int argn,
    32	      char **argv)
    33	{
    34	    int i;
    35	
    36	    pthread_mutex_init (&food_lock, NULL);
    37	    sem_init(&num_can_eat_sem, 0, PHILOS - 1);
    38	    for (i = 0; i < PHILOS; i++)
    39	        pthread_mutex_init (&chopstick[i], NULL);
    40	    for (i = 0; i < PHILOS; i++)
    41	        pthread_create (&philo[i], NULL, philosopher, (void *)i);
    42	    for (i = 0; i < PHILOS; i++)
    43	        pthread_join (philo[i], NULL);    
    44	    return 0;
    45	}
    46	
    47	void *
    48	philosopher (void *num)
    49	{
    50	    int id;
    51	    int i, left_chopstick, right_chopstick, f;
    52	
    53	    id = (int)num;
    54	    printf ("Philosopher %d is done thinking and now ready to eat.\n", id);
    55	    right_chopstick = id;
    56	    left_chopstick = id + 1;
    57	
    58	    /* Wrap around the chopsticks. */
    59	    if (left_chopstick == PHILOS)
    60	        left_chopstick = 0;
    61	
    62	    while (f = food_on_table ()) {
    63	        get_token ();
    64	
    65	        grab_chopstick (id, right_chopstick, "right ");
    66	        grab_chopstick (id, left_chopstick, "left");
    67	
    68	        printf ("Philosopher %d: eating.\n", id);
    69	        usleep (DELAY * (FOOD - f + 1));
    70	        down_chopsticks (left_chopstick, right_chopstick);
    71	
    72	        return_token ();
    73	    }
    74	
    75	    printf ("Philosopher %d is done eating.\n", id);
    76	    return (NULL);
    77	}
    78	
    79	int
    80	food_on_table ()
    81	{
    82	    static int food = FOOD;
    83	    int myfood;
    84	
    85	    pthread_mutex_lock (&food_lock);
    86	    if (food > 0) {
    87	        food--;
    88	    }
    89	    myfood = food;
    90	    pthread_mutex_unlock (&food_lock);
    91	    return myfood;
    92	}
    93	
    94	void
    95	grab_chopstick (int phil,
    96	                int c,
    97	                char *hand)
    98	{
    99	    pthread_mutex_lock (&chopstick[c]);
   100	    printf ("Philosopher %d: got %s chopstick %d\n", phil, hand, c);
   101	}
   102	
   103	void
   104	down_chopsticks (int c1,
   105	                 int c2)
   106	{
   107	    pthread_mutex_unlock (&chopstick[c1]);
   108	    pthread_mutex_unlock (&chopstick[c2]);
   109	}
   110	
   111	
   112	int
   113	get_token ()
   114	{
   115	    sem_wait(&num_can_eat_sem);
   116	}
   117	
   118	void
   119	return_token ()
   120	{
   121	    sem_post(&num_can_eat_sem);
   122	}

This new implementation uses the semaphore num_can_eat_sem to limit the number of philosophers who can eat at the same time. The semaphore num_can_eat_sem is initialized to four, one less than the number of philosophers. Before attempting to eat, a philosopher calls get_token() which in turn calls sem_wait(&num_can_eat_sem). The call to sem_wait() causes the calling philosopher to wait until the semaphore's value is positive, then changes the semaphore's value by subtracting one from the value. When a philosopher is done eating, he calls return_token() which in turn calls sem_post(&num_can_eat_sem). The call to sem_post() changes the semaphore's value by adding one. The Thread Analyzer recognizes the calls to sem_wait() and sem_post(), and determines that not all philosophers attempt to eat concurrently.

If you run this new implementation of the program several times, you will find that it terminates normally each time and does not hang. You will also find that the Thread Analyzer does not report any actual or potential deadlocks, as the following screen-shot shows:

See Appendix A, Thread Analyzer User API for a listing of the threading and memory allocation APIs that the Thread Analyzer recognizes.