Question: Propose your own problem which may encounter process synchronization and use semaphore to solve the problem

Solution:
Problem: The Dining Philosopher Problem

Descr iption: 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])

}

 

Code:

#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* philosopher(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, philosopher, &phil[i]); printf("Philosopher %d is thinking\n", i + 1); } for (i = 0; i < N; i++) pthread_join(thread_id[i], NULL); }