Provide an outline of a solution to the dining-philosophers problem using monito
ID: 3745154 • Letter: P
Question
Provide an outline of a solution to the dining-philosophers problem using monitors. Hence simulate the solution using any language. The C language version is here:
monitor DiningPhilosophers
{
enum {THINKING, HUNGRY, EATING} state[5];
condition self[5];
void pickup(int i) {
state[i] = HUNGRY;
test(i);
if (state[i] != EATING)
self[i].wait();
}
void putdown(int i) {
state[i] = THINKING;
test((i + 4) % 5);
test((i + 1) % 5);
}
void test(int i) {
if ((state[(i + 4) % 5] != EATING) &&
(state[i] == HUNGRY) &&
(state[(i + 1) % 5] != EATING)) {
state[i] = EATING;
self[i].signal();
}
}
initialization code() {
for (int i = 0; i < 5; i++)
state[i] = THINKING;
}
}
Explanation / Answer
#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 ",
phnum + 1, LEFT + 1, phnum + 1);
printf("Philosopher %d is Eating ", 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 ", 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 ",
phnum + 1, LEFT + 1, phnum + 1);
printf("Philosopher %d is thinking ", 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 ", i + 1);
}
for (i = 0; i < N; i++){
pthread_join(thread_id[i], NULL);
}
}
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