# Operating System | Dining Philosopher Problem Using Semaphores

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 pickup 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 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 –

```
#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;

// 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
philospher, &phil[i]);

printf("Philosopher %d is thinking\n", i + 1);
}

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

}
```

Note – The below program may compile only with C compilers with semaphore and pthread library.

This article is contributed by Subham Biswas. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geekforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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