struct sembuf lock_it;
struct sembuf unlock_it;
int a = 0;
int mutex;

void *thRoutine1(void *arg)
{
    while(1)
    {
        semop(mutex, & lock_it, 1);
        printf ("\nSemaphore locked by 'Routine 1'\n");
        fflush(stdout);
        a ++;
        printf ("\n 'Routine 1' unlock Semaphore");
        fflush(stdout);
        semop(mutex, & unlock_it, 1);
    }
}

void *thRoutine2(void *arg)
{
    while(1)
    {
        semop(mutex, & lock_it, 1);
        printf ("\nSemaphore locked by 'Routine 2'\n");
        fflush(stdout);
        a += 10000;
        printf ("\n 'Routine 1' unlock Semaphore");
        fflush(stdout);
        semop(mutex, & unlock_it, 1);
    }
}

void *thRead(void *arg)
{
    while(1)
    {
        semop(mutex, & lock_it, 1); // faccio un lock sulla risorsa
        printf ("\nSemaphore locked by 'Routine Read'\n");
        fflush(stdout);
        printf("\n          a = %d; ", a);
        fflush(stdout);
        printf ("\n 'Routine Read' unlock Semaphore");
        fflush(stdout);
        semop(mutex, & unlock_it, 1); // faccio un lock sulla risorsa
    }
}

int main()
{
pthread_t       th1, th2, thR;
key_t           unique_key;
union semun     options;
    unique_key = ftok(".", 'r');
    printf("\n Unique_key = %d", unique_key);
    fflush(stdout);
    if ( (mutex = semget(unique_key, 1, IPC_CREAT | IPC_EXCL | 0x0666)) == -1)
    {
        printf ("\nError");
        fflush(stdout);
        abort();
    }
    printf ("\n\nSemaphore ID = %d\n", mutex);
    fflush(stdout);
    options.val = 1;
    semctl(mutex, 0, SETVAL, options);
    // for lock operation
    lock_it.sem_num = 0;
    lock_it.sem_op  = -1;
    lock_it.sem_flg = IPC_NOWAIT;
    // for unlock operation
    unlock_it.sem_num = 0;
    unlock_it.sem_op  = 1;
    unlock_it.sem_flg = IPC_NOWAIT;
    pthread_create( & th1, NULL, thRoutine1, NULL);
    pthread_create( & th2, NULL, thRoutine2, NULL);
    pthread_create( & thR, NULL, thRead, NULL);
    while(1) ;
    return 0;
}