Showing deadlock, using Pthreads, on Dining Philosophers

Question

We have an assignment to show deadlock with the Dining Philosophers problem. We've coded it all and the code compiles but when running the code, one of the philosophers eventually eats. So doesn't that mean deadlock doesn't actually occur?

#include <pthread.h>
#include <stdio.h>

#define number_philo 5


pthread_mutex_t cutlery[number_philo];


void* philosopher (void* number)

{

    int my_num = *((int*)number);

    while (1)

    {


        printf("\n Philosopher %d is thinking.\n",my_num);


        pthread_mutex_lock (&cutlery[my_num]);
        printf("\nPhilosopher %d has left cutlery. \n",my_num);
        sleep(3);
        pthread_mutex_lock (&cutlery[(my_num + 1) %  number_philo]);
        printf("\nPhilosopher %d has right cutlery. \n",my_num);



        printf("\n Philosopher %d eating.\n", my_num);


        printf("\n Philosopher %d done.\n", my_num);


        pthread_mutex_unlock (&cutlery[(my_num + 1) % number_philo]);

        pthread_mutex_unlock (&cutlery[my_num]);
        printf("\nPhilosopher %d no longer has cutlery.\n", my_num);
    }

    return NULL;

}


int main ()

{

    int i;

    pthread_t phils[number_philo];

    void* return_val;


    for (i = 0; i < number_philo; i++)

        pthread_mutex_init (&cutlery[i], NULL);


    for (i = 0; i < number_philo; i++)

        pthread_create (&phils[i], NULL, philosopher, &i);


    for (i = 0; i < number_philo; i++)

        pthread_join (phils[i], &return_val);


    return 0;

}

And this is the output: output

Haven't you asked something similar a couple of hours ago? – Eugene Sh. Oct 20 '16 at 15:09 — Eugene Sh., Oct 20 '16 at 15:09
Same assignment, different premise – Hussam Shehab Oct 20 '16 at 15:14 — Hussam Shehab, Oct 20 '16 at 15:14

score 0 · Accepted Answer · answered Oct 21 '16 at 00:06

The problem is here:

  pthread_create (&phils[i], NULL, philosopher, &i);

You are passing a pointer to the same variable i to each thread, and those threads (and the main thread) are all accessing i in a racy way. You'll get multiple philosophers using the same number and some numbers not used at all.

You need to give each philosopher it's own variable to read, or wait for it to read i before going around the loop and changing i. An example of the former:

int phil_num[number_philo];

/* ... */

for (i = 0; i < number_philo; i++)
{
    phil_num[i] = i;
    pthread_create (&phils[i], NULL, philosopher, &phil_num[i]);
}

Showing deadlock, using Pthreads, on Dining Philosophers

1 Answers1