System and device programming――thread and semaphore implements

A turnstile, also called a baffle gate, is a form of gate which allows one person to pass at a time.

When have k threads dividing into 2 phases,second must wait first finishes,can do this way:

1. typedef struct {

  int count;

  pthread_mutex_t mutex;

} Counter;

Counter *c;

int num_threads;

static void *

code (void *arg){

  int i;

  

  pthread_detach (pthread_self ());

  sleep(random() % 5);

  printf("%ld, 1\n", pthread_self());

  pthread_mutex_lock (&c->mutex);

  c->count++;

  if (c->count == num_threads) //last 1 waits all the other,including itself.to post k signals,then can let all sem_wait get signals.

    for (i=0;i<num_threads)

      sem_post(barrier);

  pthread_mutex_unlock (&c->mutex); //all  need this

  

  sem_wait(barrier);

  printf("%ld, 2\n", pthread_self());

  return  0;

}  

2.another good way:

typedef struct {

  int count;

  pthread_mutex_t mutex;

} Counter;

Counter *c;

int num_threads;

static void *

code (void *arg){

  pthread_detach (pthread_self ());

  sleep(random() % 5);

  printf("%ld, 1\n", pthread_self());

  pthread_mutex_lock (&c->mutex);

  c->count++;

  if (c->count == num_threads) 

    sem_post(barrier);

  pthread_mutex_unlock (&c->mutex);

  

  sem_wait(barrier); // turnstile   -|-

  sem_post(barrier);// one who gets barrier will release next,i.e.only one can enter each time

  printf("%ld, 2\n", pthread_self());

  return  0;

}  

For cyclic way

typedef struct {

  int count;

  pthread_mutex_t mutex;

} Counter;

Counter *c;

int num_threads;

static void *

code (void *arg){

int i, j, r;

  pthread_detach (pthread_self ());

  for (i=0;i<10;i++){

  r = random() % 5;

  sleep(r);

  printf("%ld slept %d  sec in phase 1\n", pthread_self(), r);

  pthread_mutex_lock (&c->mutex);

    c->count++;

    if (c->count == num_threads)

    for (j=0;j<num_threads;j++)

     sem_post(barrier1);

  pthread_mutex_unlock (&c->mutex);

    

  sem_wait(barrier1);

  

  printf("%ld phase 2\n", pthread_self());

  

  pthread_mutex_lock (&c->mutex);//phase 2 same way with previous

    c->count--;

    if (c->count == 0) 

    for (j=0;j<num_threads;j++)

     sem_post(barrier2);

  pthread_mutex_unlock (&c->mutex);

  

  sem_wait(barrier2);

}

  return  0;

}