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Dining Philosopher Problem Using Semaphores
  • Difficulty Level : Easy
  • Last Updated : 01 Apr, 2021
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Prerequisite – Process Synchronization, Semaphores, Dining-Philosophers Solution Using Monitors
The Dining Philosopher Problem – The Dining Philosopher Problem states that K philosophers seated around a circular table with one chopstick between each pair of philosophers. There is one chopstick between each philosopher. A philosopher may eat if he can pick up the two chopsticks adjacent to him. One chopstick may be picked up by any one of its adjacent followers but not both. 
 

Semaphore Solution to Dining Philosopher –
Each philosopher is represented by the following pseudocode: 
 

process P[i]
 while true do
   {  THINK;
      PICKUP(CHOPSTICK[i], CHOPSTICK[i+1 mod 5]);
      EAT;
      PUTDOWN(CHOPSTICK[i], CHOPSTICK[i+1 mod 5])
   }

There are three states of the philosopher: THINKING, HUNGRY, and EATING. Here there are two semaphores: Mutex and a semaphore array for the philosophers. Mutex is used such that no two philosophers may access the pickup or putdown at the same time. The array is used to control the behavior of each philosopher. But, semaphores can result in deadlock due to programming errors.
Code – 
 

C




#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
 
#define N 5
#define THINKING 2
#define HUNGRY 1
#define EATING 0
#define LEFT (phnum + 4) % N
#define RIGHT (phnum + 1) % N
 
int state[N];
int phil[N] = { 0, 1, 2, 3, 4 };
 
sem_t mutex;
sem_t S[N];
 
void test(int phnum)
{
    if (state[phnum] == HUNGRY
        && state[LEFT] != EATING
        && state[RIGHT] != EATING) {
        // state that eating
        state[phnum] = EATING;
 
        sleep(2);
 
        printf("Philosopher %d takes fork %d and %d\n",
                      phnum + 1, LEFT + 1, phnum + 1);
 
        printf("Philosopher %d is Eating\n", phnum + 1);
 
        // sem_post(&S[phnum]) has no effect
        // during takefork
        // used to wake up hungry philosophers
        // during putfork
        sem_post(&S[phnum]);
    }
}
 
// take up chopsticks
void take_fork(int phnum)
{
 
    sem_wait(&mutex);
 
    // state that hungry
    state[phnum] = HUNGRY;
 
    printf("Philosopher %d is Hungry\n", phnum + 1);
 
    // eat if neighbours are not eating
    test(phnum);
 
    sem_post(&mutex);
 
    // if unable to eat wait to be signalled
    sem_wait(&S[phnum]);
 
    sleep(1);
}
 
// put down chopsticks
void put_fork(int phnum)
{
 
    sem_wait(&mutex);
 
    // state that thinking
    state[phnum] = THINKING;
 
    printf("Philosopher %d putting fork %d and %d down\n",
           phnum + 1, LEFT + 1, phnum + 1);
    printf("Philosopher %d is thinking\n", phnum + 1);
 
    test(LEFT);
    test(RIGHT);
 
    sem_post(&mutex);
}
 
void* philospher(void* num)
{
 
    while (1) {
 
        int* i = num;
 
        sleep(1);
 
        take_fork(*i);
 
        sleep(0);
 
        put_fork(*i);
    }
}
 
int main()
{
 
    int i;
    pthread_t thread_id[N];
 
    // initialize the semaphores
    sem_init(&mutex, 0, 1);
 
    for (i = 0; i < N; i++)
 
        sem_init(&S[i], 0, 0);
 
    for (i = 0; i < N; i++) {
 
        // create philosopher processes
        pthread_create(&thread_id[i], NULL,
                       philospher, &phil[i]);
 
        printf("Philosopher %d is thinking\n", i + 1);
    }
 
    for (i = 0; i < N; i++)
 
        pthread_join(thread_id[i], NULL);
}

Note – The below program may compile only with C compilers with semaphore and pthread library.
References – 
Solution of Dining Philosophers – cs.gordon.edu 
Solution of Dining Philosophers – cs.indiana.edu
 

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