Question

1. Write a program to simulate the Distributed Mutual Exclusion in ‘C’.

Answer 1

// Filename: peterson_spinlock.c 

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

int flag[2];  
const int MAX = 199; 
int ans = 0;
int turn; 

void lock_init() 
{ 

        flag[0] = flag[1] = 0; 
        turn = 0; 
} 
 
void lock(int self) 
{  
        flag[self] = 1; 
        turn = 1-self;  
        while (flag[1-self]==1 && turn==1-self) ; 
} 

void unlock(int self) 
{ 

        flag[self] = 0; 
} 

 
void* func(void *s)
{ 
        int i = 0; 
        int self = (int *)s; 
        printf("Thread Entered: %d\n", self); 

        lock(self); 
 
        for (i=0; i<MAX; i++) 
                ans++; 

        unlock(self); 
} 

// Driver code 
int main() 
{ 
        pthread_t p1, p2; 
        lock_init(); 
 
        pthread_create(&p1, NULL, func, (void*)0); 
        pthread_create(&p2, NULL, func, (void*)1); 

        pthread_join(p1, NULL); 
        pthread_join(p2, NULL); 

        printf("Actual Count: %d | Expected Count: %d\n", 
        ans, MAX*2); 

        return 0; 
} 

NoteApproch*

// Use Filename: peterson_spinlock.c
// Initialize lock by reseting the both the threads to acquire the locks. And, giving turn to one of them.
// Executed before entering critical section
// Set flag[self] = 1 saying you want to acquire lock
// But, first give the other thread the chance to
// acquire lock

// Wait until the other thread looses the desire
// to acquire lock or it is your turn to get the lock.
// Executed after leaving critical section
// You do not desire to acquire lock in future.
// This will allow the other thread to acquire the lock.
// A Sample function run by two threads created in main()
// Critical section (Only one thread can enter here at a time)

For Driver code
// Initialized the lock then fork 2 threads

// Create two threads (both run func)
// Wait for the threads to end.