Singleton.c

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#define NO_OF_THREADS 10

struct Singleton {
   char *Data;	
};


struct Singleton *singletonObjectPointer;


int addresses[NO_OF_THREADS];
sem_t sem;

void *runner(void *params); /* the thread */
struct Singleton *getInstance();

int main()
{
	int i;
	sem_init(&sem,0,1);
	pthread_t threadIds[NO_OF_THREADS];
	

	for (i=0; i < NO_OF_THREADS; i++){
		pthread_create(&threadIds[i], NULL, &runner, (void *)(i));
	} 

	/* Wait until all threads are done */
	for (i=0; i < NO_OF_THREADS; i++){
		pthread_join(threadIds[i], NULL);
	} 
	
	/* Control addresses. All of them should be same */
	int prevAddr=addresses[0];
	for (i=1; i < NO_OF_THREADS; i++){
		if(addresses[i]!=prevAddr){
			printf("Object is created more than once\n");
			return -1;
		}
		prevAddr=addresses[i];
	}
	for (i=0; i < NO_OF_THREADS; i++){
		printf("Singleton Addresses for each thread %x\n",addresses[i]);
	}
	printf("Successful\n");
	return 1;
}

/**
 * The thread will begin control in this function
 */
void *runner(void *params) 
{
	int i = (int)params;
	printf("Thread %d\n",i);
	struct Singleton *s = getInstance();
	addresses[i]=s;
	pthread_exit(0);
}
//Fill this method
struct Singleton *getInstance(){
	 if(singletonObjectPointer == NULL){ //Check if the object is null
        sem_wait(&sem); // Let only one thread to execute the if block at a time
        if (singletonObjectPointer == NULL){ // Second Check if the object is null
            singletonObjectPointer = (struct Singleton *)malloc(sizeof(struct Singleton)); 
            printf("---Address of singletonObjectPointer is %x\n",singletonObjectPointer);
            singletonObjectPointer->Data="This is object data";
        }
        sem_post(&sem); // Release the semaphore for another thread to execute for its own
    }
	return singletonObjectPointer;
}